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Mutual interaction of soil moisture state and atmospheric processes
Journal of
Hydrology ELSEVIER
Journal of Hydrology 184 (1996) 3-17
Mutual interaction of soil moisture state and atmospheric processes Dara Entekhabi a'*, Ignacio Rodriguez-Iturbe b, Fabio Castelli c a48-331 Ralph M. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA bDepartment of Civil Engineering, Texas A&M University, College Station, TX 77843, USA Clnstituto di Idraulica, Universiti~ degli Studi di Perugia, Perugia, Italy
Received 7 June 1995; accepted 29 October 1995
Abstract The purpose of this paper is to outline the pathways through which soil moisture and meteorological phenomena mutually influence one another at local, regional and global scales. This constitutes two-way land-atmosphere interaction, as meteorological phenomena both act as the forcing and react to the forcing by the soil moisture state. Land surface modification of the atmospheric environment and the atmospheric forcing of these land surface conditions form feedback loops which are significant factors in modulating the variability of the climatic system. The predictability and analysis of fluctuations and changes in regional hydrology and the atmospheric environment require an understanding of these feedback mechanisms and two-way land-atmosphere interaction. The dynamics of soil moisture at the land surface is governed by components with diverse time scales. Variability in both weather and climate are therefore influenced by the soil moisture state. In this paper, we present an overview of several studies on the mutual interaction of the soil moisture state and the atmospheric environment. We also outline possible directions for innovative investigations on the determination of soil moisture variation influences on the moist thermodynamics, energetics and dynamics of the overlying atmosphere.
1. Introduction Motions and disturbances in the atmospheric fluid at all scales are principally forced by the differential heating and friction at the surface. Even air column latent heating and radiative cooling, the remaining principal forcing mechanisms, are influenced by moisture availability that has its source at the surface. The input of heat into the atmosphere at the * Correspondingauthor. 0022-1694/96/$15.00 Copyright © 1996 Elsevier ScienceB.V. All rights reserved SSDI 0022-1694(95)02965-6
D. Entekhabi et al./Journal of Hydrology 184 (1996) 3-17
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Fig. 1. Conceptualdiagramof the pathwaysthroughwhich soil moistureaffectsand is mutuallyinfluencedby the overlying atmosphere(from Brubakerand Entekhabi, 1994). surface is achieved by turbulent flux and thermal radiation. Both mechanisms are strongly controlled by the availability of soil moisture. The heat generated by the radiative forcing of the surface is dissipated by the turbulent flux and thermal radiation. The partitioning between the two mechanisms is dependent on the surface temperature and the static stability of the near-surface air. Again, both factors are influenced by soil moisture. The turbulent flux itself is partitioned between sensible (dry) and latent (moist) heat flux; the relative partitioning is strongly controlled by soil moisture. Sensible heat flux is relatively less efficient than latent heat flux in dissipating heat; when there are (soil moisture) controls on evapotranspiration, the greater partitioning to sensible heat flux results in a rise of ground temperature. This in turn influences the dissipation of heat by radiative cooling at the surface. Capillary action controls the rate at which atmospheric evaporative demand and vegetation roots withdraw water from soil storage. Soil moisture also affects the thermal inertia and shortwave albedo of the surface. Fig. 1 is a conceptual diagram demonstrating the myriad of ways in which soil moisture anomalies affect the near-surface air. In this paper, we outline the role of land surface conditions on the development of atmospheric processes across a wide range of space and time scales. This includes weather phenomena such as the growth of squall lines, frontogenesis and local circulations. At the larger scales, we include climate variability, continental droughts and global feedback mechanisms. The land fraction of the Earth is small (30%) but its distribution into large contiguous areas and its distinctive hydrothermal inertia cause significant variations in regional climatic systems. It is nonetheless most important to note that the separation of atmospheric dynamical phenomena into weather and climate (depending on time scale) partially breaks down precisely because of the role of soil moisture. The reservoir of water in the soil column provides the thermal and moisture inertia that shifts the high-frequency fluctuations into
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TIME, IN YEARS Fig. 2. The propagation of a precipitation anomaly through the surface branch of the hydrologic cycle. Landatmosphere interaction and feedback mechanisms would extend the influence in the upward direction in the figure (based on McNab and Karl, 1989).
lower-frequency variability. Persistence and the action of feedback mechanisms may thus result in modes of variability that link weather anomalies and climatic departures from normal.
2. S o i l m o i s t u r e
and land-atmosphere
interaction
The blending of weather and climate anomalies may be illustrated using Fig. 2. An illustrative example of how land-atmosphere interaction may modulate climatic fluctuations may be posed using this conceptual figure from McNab and Karl (1989). The top curve in Fig. 2 represents typical fluctuations in the precipitation anomaly time-series. Over land regions, the surface runoff (cumulative hydrographs) responds to these anomalies with some delay. The next hydrologic reservoir, soil moisture, responds to the runoff anomalies with still more delay. The baseflow contribution to streamflow also follows the anomalies of soil moisture with some lag as well. Finally, the groundwater system fluctuations lag those of the streamflow and soil moisture. As the precipitation anomaly makes its way through components of the land hydrologic cycle (from storm runoff to soil moisture to streamflow to groundwater) its phase-lag with respect to the precipitation anomaly is increased and, owing to the storage associated with each component, the fluctuations are dampened significantly. The soil hydrology is acting essentially as a low-pass filter
6
D. Entekhabi et aL/Journal of Hydrology 184 (1996) 3-17
(Entekhabi and Rodriguez-Iturbe, 1994). The net result is an extension of meteorological drought (precipitation deficit) to delayed and prolonged hydrologic drought (soil moisture availability deficit). Fig. 2 demonstrates how an atmospheric forcing anomaly propagates down through the regional hydrologic system. An important effect that may be added to Fig. 2 is landatmosphere interaction. What would be the overall effect if the regional hydrology could affect the atmospheric forcing? That would correspond to the possibility of anomalies moving 'up' as well as 'down' in the figure. If soil moisture positively affected precipitation, then a soil moisture anomaly owing to an earlier precipitation anomaly would cause precipitation deficit currently and in the future, as a result of the lagging effect. Such a feedback mechanism would then lead to persistence and essentially locking of anomalous events until a large enough external fluctuation forces the system to regain its normal state. Of course, Fig. 2 may be extended to include spatial variations, anomalies and their interactions as well. Spatial patterns of soil moisture may exhibit interactions and feedbacks similar to those in time. The moving 'up' in Fig. 2, the influence of soil moisture variations on atmospheric phenomena, may involve moist thermodynamic or dynamic mechanisms. In this paper, we next turn to more detailed examination of each of these mechanisms. Many of the investigations on the role of land-atmosphere coupling and two-way interaction (in which both surface conditions and atmospheric states respond to each other) are based on numerical models that solve the meteorological Primitive Equations (conservation of mass, momentum and energy, Ideal Gas Law) together with various parameterizations of soil and vegetation control on infiltration and evapotranspiration. Although these numerical models form effective laboratories for testing basic mechanisms of land-atmosphere interaction and space-time variability, they are nonetheless limited to the extent that the parameterizations of physical processes are only approximations of the true soil physics and vegetation physiological action. The advantage of numerical laboratories is that cause and effect may be to some extent isolated and partial analysis on the role of soil moisture may be performed. The key disadvantage is that the results are conditional on the brevity of the parameterizations. In this respect, the results of numerical studies must be viewed in the context of the modeling assumption. In this paper, first the dynamical mechanisms at the local and regional scales, and then the dynamical mechanisms at continental and climate scales will be considered. The moist thermodynamic influences of soil moisture control on surface moisture and heat fluxes will then be outlined. Finally, some guidelines on the spatial and temporal sampling and characterization of soil moisture will be discussed. 2.1. Growth o f regional atmospheric dynamical disturbances and the soil moisture influence
The simplest form of local atmospheric dynamics forced by spatial variations in soil moisture is through the initiation and growth of thermally direct circulations. There have been a number of numerical studies on this phenomenon during the past decade; Segal and Arritt (1992) have provided a summary and comprehensive bibliography of these investigations. Basically, patches of dry and moist soils (or stressed and unstressed vegetation
D. Entekhabi et al,/Journal of Hydrology 184 (1996) 3-17
stands) create sharp gradients in sensible heat flux. This in turn forces a local circulation very much like a sea-breeze. The term land-breeze is often applied to these features. Landbreezes enhance the transport of heat from the surface through structured local thermally direct circulation. The land-breeze cells also affect the dryness and heat content of the near-surface air; they thus define the local evaporative environment. In this way a feedback mechanism may be established. There are a number of factors that significantly alter the effectiveness of soil moisture variations in forcing a land-breeze. First, the geometry of the patches determines the strength of the circulations. If the patches are large, then the converging surface winds do not result in a closed circulation pattern during the course of a diurnal cycle. If the patches are small or heterogeneous, then these circulations are disorganized and weak. Also, the present of a largescale synoptic (background) wind tends to destroy these cellular features. The numerical experiments using patches are thus specific to these test cases. Furthermore, conclusions based on two-dimensional models (height and one horizontal distance) will substantially change once the third dimension is included. Lynn et al. (1995) used a three-dimensional mesoscale atmospheric model to assess the importance of landscape patchiness on landatmosphere heat exchange. A number of mosaic patterns for wet and dry 'checkerboard' patches were considered with extremes of totally dry and saturated soil conditions in each of two surface types. Lynn et al. (1995) found that the land-breeze circulation induced heat flux in the two-dimensional model overestimates the impact of surface heterogeneity when compared with results from three-dimensional simulations. Enger and Tjernstrom (1991) performed similar numerical experiments using true surface topography and land use conditions. They analyzed the impact of a lake on the regional semi-arid climate near the Tunisian-Algerian border. Owing to the increased local moisture availability and development of strong land-breeze circulations, the precipitation in the region increases significantly. The impact of the lake is less pronounced when a strong synoptic wind is present. More fundamentally, however, spatial variation in soil moisture may trigger and affect the growth of squall lines and baroclinic disturbances. Sun and Ogura (1979) and Chang and Wetzel (1991) worked with numerical weather prediction models initialized with convective pre-storm environments in the atmosphere. They tested the growth of the disturbance with differing specifications of the surface conditions. Sun and Ogura (1979) concluded that the pre-storm convergence is significantly affected by horizontal temperature gradients. Similarly, Chang and Wetzel concluded that their test stationary front is strongly enhanced if there are strong gradients of soil moisture. Fast and McCorcle (1991) considered the changes in the characteristic of a moving front owing to variations in the surface conditions. They concluded that the evaporation of soil moisture significantly affected the boundary layer structure and in turn modified the spatial pattern and magnitude of the precipitation intensity field. Ziegler et al. (1995) reported that gradients in surface sensible heat flux estimated near 100 W m- 2 per 50 km is sufficiently large to form a dry-line which is an important pre-storm environment for major US Great Plains summer squall-line formation. Using numerical models, Lanicci et al. (1987) also found that spatial gradient in soil moisture (which also generally implies gradients in sensible heat flux and partitioning of net radiation) is an important contributing factor to severe storms in the same region.
8
D. Entekhabi et aL/Journal of Hydrology 184 (1996) 3-17
In all these simulation investigations, the soil moisture control on evapotranspiration, the resulting partitioning of turbulent flux more towards the sensible heat mechanism and the enhancement of horizontal temperature gradients affect the evolution of regional dynamical features. The precipitation intensity field, both in extent and magnitude, is affected by soil moisture variations. Blyth et al. (1994) included the role of vegetation in their numerical study of surface influences on pre-storm environments. They found that over a region in southern France, forest stands affect the near-surface atmospheric environment through unstressed evaporation from canopy interception. An equally important factor is the enhanced mass, heat and momentum transfer where rough elements of forest stands cover the surface. Castelli et al. (1996) similarly analyzed the role of soil moisture on frontogenesis. They, however, advanced one step further in closing the feedback loop between the land and the atmosphere at these scales. The soil moisture affects the distribution and intensity of the precipitation field. These precipitation fields in turn change the surface soil moisture state and its spatial patterns. Castelli et al. (1996) showed that this process constitutes a positive feedback mechanism whereby soil moisture and atmospheric dynamics mutually reinforce each other's spatial gradients.
2.2. Large-scale variability and climate: anomaly reinforcement by surface hydrologic processes As discussed in reference to Fig. 2, the hydrothermal inertia associated with surface and root-zone soil moisture affects climate time scales of variability. For instance, modeling studies show that the presence of the soil moisture reservoir adds to the persistence of anomalies in the near-surface air (Delworth and Manabe, 1989). Yang et al. (1994) demonstrated the impact of the persistence owing to the mutual coupling of surface and atmospheric processes on short-term weather prediction. Karl (1986), using monthly and seasonal precipitation and temperature records, reconstructed soil moisture anomaly fields for the USA. He demonstrated the increase in seasonal and monthly temperature forecast skill when soil moisture anomalies are used in the predictions. These studies demonstrate the possibility of feedback processes in land-atmosphere interaction that may reinforce the persistence of anomalies at different spatial as well as temporal scales. One of the earlier investigations on such phenomena is the modeling study of drought in the Sahel zone by Charney (1975). Reduced precipitation (possibly owing to large-scale air-sea interaction) results in the drying of near-surface soil moisture. Drier soils and desiccated vegetation have higher shortwave albedo values. Radiative forcing deficit in this low evaporation environment results in cooler temperatures and sinking air motion that in turn reinforces the precipitation anomaly which initiated the drying cycle in the first place. There have been a large number of studies on such positive feedback mechanisms between soil moisture and atmosphere in the Sahel based on the original investigation by Charney (1975). Synoptic observations by Namias (1988) also indicate that, over continental regions such as the US Great Plains, the atmospheric initiation of a hydrologic anomaly at the surface results in large-scale changes in soil moisture which in turn reinforce the atmospheric forcing anomaly. During the 1988 US drought, the soil water content over the
D. Entekhabi et al./Journal of Hydrology 184 (1996) 3-17
central Great Plains was significantly reduced. The elevated surface temperatures owing to controls on latent heat flux deepened the adiabatically mixed surface air layer and intensified the high-pressure ridge in the mid-continental region. Both these effects tend to inhibit precipitation. As a result of this positive feedback, the climatic anomaly is reinforced and it persists with even greater intensity. Escape from this cycle occurs whenever large-scale and synoptic factors are strong enough to overcome the positive feedback that locks low soil moisture anomalies with deficits in normal precipitation. This is essentially the outline of a sequence of temporal cause and effects. Owing to the positive feedback between soil moisture and the atmosphere, such anomalies expand in space as well. Karl (1983) provided observational evidence that the severity of droughts is related to their spatial extent. Important land-atmosphere interactions mediate these spatial and temporal linkages. Investigations of soil moisture influence on the persistence of climate anomalies (especially summer dry conditions) have also been based on general circulation models (GCMs). The results of experiments with these numerical laboratories are dependent on the realism of the parameterization used in the modeling. Especially when dealing with soil physics and vegetation transpiration, the role of process models and landscape heterogeneity are of highest concern. Whereas these numerical laboratories are not replacements for observation-based studies, they are valuable in performing partial analyses in a system where the many states are linked in complex ways. In this way, they may be valuable tools for developing insights and designing observation strategies. Walker and Rowntree (1977) and Bounoua and Krishnamurti (1993) modeled the West African semi-arid zone with contrasting wetness at the surface boundary. Rind (1982), Rowntree and Bolton (1983), Ogelsby and Erickson (1989) and Cook (1994) performed numerical GCM experiments on similar topics. In general, these studies using GCMs as numerical laboratories indicate that dry surface wetness anomalies tend to persist for longer periods than moist surface anomalies; the effect is more pronounced if the anomaly is initially present at the onset of the summer season. Georgakakos and Bae (1994) used estimated soil moisture from a conceptual rainfall-runoff model to construct long timeseries of surface wetness; they demonstrated that the frequency and duration of soil moisture anomalies in the Upper Mississippi basin are lower for wet anomalies than for dry anomalies. Dynamical feedbacks may be present which reinforce droughts and dry anomalies. Soil moisture and large-scale atmospheric processes mutually interact to create persistence in records of climate variability. The components of the land surface hydrology parameterization that are responsible for the enhancement of persistence in GCM model climates have been investigated as well. For example, the magnitude of potential evaporation (as a measure of the rate at which the atmosphere removes anomalies in soil moisture) is a major factor for moist climates. Scott et al. (1995) focused more on individual surface processes, and they identified components of the land surface hydrology that add to the time scales of GCM model climates. Dirmeyer (1994) focused on the impact of phonology and reaction to stress in vegetation on the initiation and prolongment of droughts. The dormancy of vegetation and strong seasonal cycle (and their phase difference) are shown to be important considerations in studying the transformation of meteorological droughts (a deficit in seasonal precipitation) to hydrologic droughts (persistent deficits in available soil moisture). Yeh et al. (1984)
10
D. Entekhabi et al./Journal of Hydrology 184 (1996) 3-17
introduced an added consideration in determining the impact of soil moisture anomalies on climate variability. They used GCM simulations to show that the presence of soil moisture anomalies where a large fraction of the precipitation is lost to runoff is less important than where precipitation and evaporation are the principal components of the surface hydrology. Beyond this, the surface temperature anomalies associated with soil moisture anomalies also have varying degrees of impact on the atmosphere depending on the latitude. Thermal wind relationship and its contribution to zonal circulation is the main mechanism through which this sensitivity exhibits itself. The next step for such investigations is to identify the exact mechanisms that constitute the positive feedback between soil moisture and the atmosphere in large numerical models. GCMs implicitly contain the major moist thermodynamic constraints on the climatic system; nonetheless it is rather difficult to decipher the model output and clearly pinpoint the pathways through which soil moisture affects the overlying atmosphere and vice versa. To understand why GCMs exhibit such response behavior between soil moisture and atmospheric anomalies, it is necessary to augment such large-scale numerical investigations with simple models based on the essential energetics and moist thermodynamics. Studies using the simple models provide valuable insight into the workings of the landocean-atmosphere climatic system and they help explain the more complex behavior evident in numerical GCM model climates. In the next section, we turn our focus to studies of soil moisture-atmosphere interaction using these simple moist thermodynamic and energy balance models and summarize some of the major findings. 2.3. Air moist thermodynamics influenced by soil moisture: origins of feedback mechanisms
The control of soil moisture on the surface exchanges of latent and sensible heat flux is the key mechanism through which feedback mechanisms between the land and atmosphere develop. The source of atmospheric moisture and precipitation is at the surface; deficits in moisture supply (evaporation) thus potentially reinforce deficits in subsequent precipitation simply through coupled water balance between the soil and atmospheric reservoirs. In terms of sensible heat flux, the added heating of the atmosphere owing to dry anomalies at the surface is amplified by radiative processes. Thus the moist thermodynamics of the near-surface air is strongly determined by soil moisture. The mechanism that involves moisture supply is precipitation recycling over continental regions. The precipitating water over large land areas is partially derived from evaporation over the region itself. Thus deficits in evaporation lead to deficits in precipitation which in turn dry the soil and further reduce the evaporation supply to the atmosphere. Brubaker et al. (1993) estimated the fraction of regional monthly precipitation derived from local evaporation based on atmospheric vapor flux observations. They provided estimates for this measure of positive land-atmosphere interaction for selected regions; Fig. 3 is an example of precipitation recycling estimates for the US Great Plains. Up to 40% of the precipitation is supplied by local evaporation during some seasons. Eltahir and Bras (1994) and Savenije (1995) analyzed the precipitation recycling over the Sahel and the Amazon basin using more detailed models of the atmospheric water balance. They also found that a large fraction of the regional precipitation is derived from local evaporation.
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To illustrate the nature and the consequences of this positive feedback mechanism between soil moisture and climate, Rodriguez-Iturbe et al. (1991a) and Entekhabi et al. (1992) set up a simple water balance equation for regional soil moisture and included the recycling of precipitation. When the system is forced by simple gaussian stochastic noise (representing large-scale forcing), the soil moisture state persists in two distinct statistical modes. Fig. 4 shows an example of such a non-gaussian bimodal probability distribution for soil moisture. This simple climatic system persists in one (dry) mode and it reinforces the dry anomaly, as low evaporation leads to low precipitation, dry soils and again low evaporation. Whenever the large-scale forcing is strong enough to force the escape of soil moisture climate from this positive feedback lock, the system changes to the second (wet) mode and persists there for some time. Similar positive feedback reinforces the increase in both precipitation and evaporation; Yeh et al. (1984) found a similar positive feedback that acts to reinforce positive soil moisture anomalies in a numerical climate model. Entekhabi et al. (1992) derived the distribution of escape times from each mode in their stochastic model of soil moisture evolution; droughts with a duration of several years to decades, such as those observed in the Sahel, are frequent. Rodriguez-Iturbe et al. (1991b) also analyzed the dynamic behavior of the soil moisture evolution. As a result of the nonlinear feedback mechanisms in the simple system, chaotic dynamics is observed. Beyond the simple moisture supply effect, soil moisture anomalies also exert some control on the heating of the overlying air column. Changes in the radiative divergence and static stability of the atmosphere are important factors and they significantly alter the moist thermodynamic conditions of the atmosphere. Sasamori (1970), Zdunkowski et al. (1975) and Raddatz (1993) investigated some of these effects with the use of numerical models of the planetary boundary layer. The magnitude and diurnal range of the heating profile and the growth of the turbulently mixed layer are significantly modified by soil
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RELATIVE SOIL SATURATION Fig. 4. Soil moisture probability distribution in a simple water balance model that includes the precipitation recycling mechanism for land-atmosphere interaction. A persistent dry (drought) model and a persistent wet mode are evident even thoughthe forcingis simple gaussian noise (from Rodriguez-Iturbeet al., 1990). moisture anomalies. These studies also used numerical models to demonstrate the role of surface heterogeneity in soil moisture (and soil texture in general) on the surface fluxes and growth of the turbulent mixed layer. The formation of clouds and both dry and precipitating convection are consequently affected as well (Segal et al., 1995). The moist thermodynamic conditions of the near-surface air are changed; these changes in turn modify the surface fluxes. In a further demonstration of the mutual influences of soil moisture state and the atmosphere, Brubaker and Entekhabi (1995) developed a set of coupled equations for the hydrothermal states in the soil and the turbulent boundary layer. They showed that the diurnal and seasonal range in the moisture and temperature states of the air layer are determined in large part by the soil moisture state. The model is used to provide quantitative measures for the relative strength of the pathways hitherto only qualitatively presented such as in Fig. 1. This simple analytic model of land-atmosphere was extended by Entekhabi and Brubaker (1995) to include random variations in the regional wind speed. The principal findings of this investigation are related to the delineation of exact pathways and mechanisms through which soil moisture affects climate variability at the large scale. For example, in Fig. 5 from that study the lagged cross-covariance of soil moisture and soil temperature is presented. Without land-atmosphere interaction, the mutual dependence of the two variables is severely curtailed. The balance as well as the covariability of heat and moisture at the surface are closely linked, and the link is through a series of pathways that include two-way interaction with the atmosphere. Entekhabi and Brubaker (1995) used the simple framework of the model to delineate these pathways that couple the
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14
D. Entekhabi et al.Hournal of Hydrology 184 (1996) 3-17
Mississippi basins. Georgakakos and Bae (1994) used these estimates to study the interannual variability in the record. They found that near-surface soil moisture may persist for up to one season, whereas, in the lower root-zone, water availability anomalies last for several seasons. The strong annual cycle in the region results in the soil column being almost saturated in June. This condition leads to large summertime evaporation and it also results in flood-prone surface conditions. In fact, Kunkel (1994) argued that a positive anomaly in such a condition in the Upper Mississippi contributed to the large precipitation anomalies and the great flood of 1993.
3. Discussion: observation and characterization of soil moisture space-time variability Given the critical role of soil moisture on surface moisture and energy balance and covariability with subsequent influences on the atmosphere, it is important to create the infrastructure to routinely and operationally estimate and observe this quantity. Topography, significant spatial heterogeneity in soil and vegetation properties, and the highly intermittent characteristic of precipitation fields result in large spatial variations in the soil moisture field. Entekhabi and Rodriguez-Iturbe (1994) analyzed the space and time scales of variability in the soil moisture field and identified the key roles of topography and precipitation variability in defining its statistical characteristics. There are some major obstacles to the monitoring of soil moisture. Fundamental among these is the very definition of soil moisture. As the soil-atmosphere interface is intermittently a source and sink for moisture, sharp vertical gradients in soil moisture develop at the surface as well. This factor complicates the very definition of soil moisture. Over what depth is soil moisture, the soil moisture that interacts with the atmosphere, defined? Given the sharp gradients near the surface, the dynamics of soil moisture variability will be very different depending on this depth. Another fundamental obstacle to the monitoring of soil water is the role of heterogeneity in the formation and evolution of soil moisture fields. Variations in soil type, geologic constraints, complex topography, preferential flow-paths formed by roots and fauna, and fluctuations in atmospheric forcing (e.g. spotty rainfall, aspect variations in exposure to solar radiation, etc.) all contribute to the heterogeneity in soil moisture fields over a large range of scales. As a result of this strong heterogeneity, point samples of soil moisture are not necessarily indicative of the regional value for this variable. These two fundamental obstacles have essentially inhibited the development of soil moisture monitoring programs. Important as this variable is, there are no extensive and reliable sources of soil moisture estimates available for the operational and research communities. Existing thought and approaches to this variable are inherently constrained in overcoming these and other equally effective obstacles. The applicability of data assimilation techniques to the estimation of soil moisture fields is not clear at this point. The degree to which they are feasible is dependent on the manner in which the governing relations implicitly relate the evolution of observed states to the regional soil moisture variable. Furthermore, the estimates of
D. Entekhabi et aL/Journal of Hydrology 184 (1996) 3-17
15
soil moisture based on the data assimilation contain the bias of the model used for assimilation. The distinction between field soil moisture and estimates derived from data assimilation must be made clear: model-based estimates and modeling values of soil moisture (in GCMs and mesoscale models) refer to the soil water variable in these models that is used principally to maintain consistent partitioning of incoming precipitation and radiation into infiltration, evaporation, runoff, turbulent and radiative energy fluxes over time. Comparison of GCM, mesoscale or model-assimilated soil moisture with a field-measured value is thus a problem that remains, up to this time, unresolved. The spatial heterogeneity in the horizontal domain and the sharp vertical gradients complicate the observation (and cross-validation with GCM and mesoscale numerical models) by in situ methods. Gravimetric techniques, neutron probe and time domain reflectometry are labor intensive. Therefore they cannot be deployed to operationally map the soil moisture field across large regions. Remote sensing techniques are limited by the problems associated with geophysical calibration. Low-frequency microwave remote sensing is the most promising tool to observe soil moisture fields directly (or as directly as possible) from space (Njoku and Entekhabi, 1996). The major shortcoming of this approach is that the emitted passive microwave radiation represents dielectric conditions in the top few centimeters of soil. As mentioned above, the sharp gradients of soil moisture near the soil-atmosphere boundary complicate the definition of soil moisture that is necessary for atmospheric studies. Entekhabi et al. (1994) developed a coupled modeling-remote sensing technique to infer soil moisture at larger depths. This poses a difficult inverse-problem. The use of active microwave radiation to achieve soil moisture averages over larger depths is severely limited by the noise introduced by microtopography and roughness of the vegetation cover. Furthermore, the lower frequencies (1 GHz or lower) are not protected for geophysical investigations and commercial transmission interference is present. Clearly, no one technique is suitable for application at the large scale and in an operational mode. Data assimilation techniques and combined modeling-observation programs are necessary to achieve the best results. Parallel to these observational considerations, significant efforts are necessary to add to our understanding of land-atmosphere interaction. By developing and expanding the knowledge about how soil moisture influences the surface and atmospheric processes, indirect methods for its characterization will follow. The clear and concise understanding of the physical linkages between the soil moisture state and atmospheric processes also brings gains to the characterization and predictability of atmospheric processes such as precipitation and climate variability. Controlled field experimentation (such as the FIFE and HAPEX micrometeorological experiments) and the use of simple models in conjunction with large numerical models are necessary to identify the physical mechanisms governing the mutual interaction of soil moisture and atmospheric processes. The wide range of time and space scales, the large natural variability inherent in the climatic system, and the complexity of the interactions add complications. The topic of investigation also crosses disciplinary barriers. These artificial barriers are rapidly disappearing with the recognition that the Earth sciences form one totality.
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References Bae, D.-H. and Georgakakos, K.P., 1994. Climate variability of soil water in the American Midwest, Part 1: Hydrologic modeling. J. Hydrol., 162: 379-390. Blyth, E.M., Dolman, A.J. and Noilhan, J., 1994. The effects of forest on mesoscale rainfall: an example from HAPEX-MOBILHY. J. Appl. Meteorol., 33: 445-464. Bounoua, L. and Krishnamurti, T.N., 1993. Influence of soil moisture on the Sahelian climate prediction: Part I. Meteorol. Atmos. Phys., 525: 183-203. Brubaker, K.L. and Entekhabi, D., 1994. Nonlinear dynamics of water and energy balance in land-atmosphere interaction, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering Technical Rep. No. 341,147 pp. Brubaker, K.L. and Entekhabi, D., 1995. An analytic approach to modeling land-atmosphere interaction. 1. Construct and equilibrium behavior. Water Resour. Res., 31(3): 619-632. Brubaker, K.L., Entekhabi, D. and Eagleson, P.S., 1991. Atmospheric water vapor transport: estimation of continental precipitation recycling and parameterization of a simple climate model, MIT Department of Civil Engineering, Ralph M. Parsons Laboratory Rep. No. 333, 166 pp. Brubaker, K.L., Entekhabi, D. and Eagleson, P.S., 1993. Estimation of continental precipitation recycling. J. Climate, 6(6): 1077-1089. Castelli, F., Rodriguez-Iturbe, I. and Entekhabi, D., 1996. An analytical framework for the modelling of the spatial interaction between the soil moisture and the atmosphere. J. Hydrol., 184: 19-34. Chang, J.-T. and Wetzel, P.J., 1991. Effects of spatial variations of soil moisture and vegetation on the evolution of the pre-storm environment. Mon. Weather Rev., 119: 1368-1390. Charney, J.G., 1975. Dynamics of deserts and drought in the Sahel. Q. J. R. Meteorol. Soc., 101: 193-202. Cook, K.H., 1994. Mechanisms by which surface drying perturbs tropical precipitation fields. J. Climate, 7(3): 400-413. Delworth, T. and Manabe, S., 1989. The influence of soil wetness on near surface atmospheric variability. J. Climate, 2(12): 1447-1462. Dirmeyer, P.A., 1994. Vegetation stress as a feedback mechanism in midlatitude drought. J. Climate, 7(10): 1463-1483. Eltahir, E. and Bras, R.L., 1994. Precipitation recycling over the Amazon basin. Q. J. R. Meteorol. Soc. Enger, L. and Tjernstrom, M., 1991. Estimating the effects on regional precipitation climate in semiarid region caused by an artificial lake using a mesoscale model. J. Appl. Meteorol., 30: 227-250. Entekhabi, D. and Brubaker, K.L., 1995. An analytic approach to modeling land-atmosphere interaction: 2. Stochastic extension. Water Resour. Res., 31(3): 635-643. Entekhabi, D. and Rodriguez-Iturbe, I., 1994. An analytic framework for the characterization of the space-time variability of soil moisture. Adv. Water Resour., 17(1-2): 35-46. Entekhabi, D., Rodriguez-Iturbe, I. and Bras, R.L., 1992. Variability in large-scale water balance with landsurface-atmosphere interaction. J. Climate, 5(8): 798-813. Entekhabi, D., Nakamura, H. and Njoku, E.G., 1994. Solving the inverse problem for soil moisture and temperature profiles by sequential assimilation of multifrequency remotely sensed observations. IEEE Trans. Geosci. Remote Sensing, 32(2): 438-448. Fast, J.D. and McCorcle, M.D., 1991. The effect of heterogeneous soil moisture on a summer baroclinic circulation in the United States. Mon. Weather Rev., 119: 2140-2167. Georgakakos, K.P. and Bae, D.-K, 1994. Climate variability of soil water in the American Midwest, Part 2: Spatio-temporal analysis. J. Hydrol., 162: 379-390. Huang, J. and van den Dool, H.M., 1993. Monthly precipitation temperature relations and temperature prediction over the United States. J. Climate, 6(6): 1111-1132. Karl, T.R., 1983. Some spatial characteristics of drought duration in the United States. J. Climate Appl. Meteorol., 23: 1356-1366. Karl, T.R., 1986. The relationship of soil moisture parameterizations to subsequent seasonal and monthly mean temperature in the United States. Mort. Weather Rev., 114: 675-686. Kunkel, K.E., 1994. A climatic prospective on the 1993 Flooding rains in the Upper Mississipi River Basin. Water Int., 19: 186-194.
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Lanicci, J.M., Carlson, T.B. and Warner, T.T., 1987. Sensitivity of the Great Plains severe-storm environment to soil moisture distribution. Mon. Weather Rev., 115: 2660-2673. Lynn, B.H., Rind, D. and Avissar, R., 1995. The importance of mesoscale circulations generated by subgrid-scale landscape heterogeneities in general circulation models. J. Climate, 8(2): 191-205. McNab, A.L. and Karl, T.R., 1989. Climate and droughts. US Geol. Surv. Water Supply Pap., 2375: 89-95. Namias, J., 1988. The 1988 summer drought over the Great Plains: a classic example of air-sea-land interaction. Trans. Am. Geophys. Union, 69: 1067. Njoku, E. and Entekhabi, D., 1996. Passive microwave remote sensing of soil moisture. J. Hydrol., 184: 101-129. Ogelsby, R.J. and Erickson, D.J., 1989. Soil moisture and the persistence of North American drought. J. Climate, 2(11): 1362-1380. Raddatz, R.L., 1993. Prairie agroclimate boundary-layer model: a simulation of the atmosphere/crop-soil interface. Atmos.-Ocean, 31(4): 399-419. Rind, D., 1982. The influence of ground moisture conditions in North America on summer climate as modelled by the GISS GCM. Mon. Weather Rev., 110: 1487-1494. Roads, J.O., Chen, S.-C., Guetter, A.K. and Georgakakos, K.P., 1994. Large-scale aspects of the United States hydrologic cycle. Bull. Am. Meteorol. Soc., 75(9): 1590-1610. Rodriguez-Iturbe, I., Entekhabi, D. and Bras, R.L., 1990. Nonlinear dynamics of soil moisture at climate scales: Stochastic analysis. MIT Center fro Global Change Science Rep. No. 5, 27 pp. Rodriguez-Iturbe, I., Entekhabi, D. and Bras, R.L., 1991a. Nonlinear dynamics of soil moisture at climate scales, 1. Stochastic analysis. Water Resour. Res., 27(8): 1899-1906. Rodriguez-Iturbe, I., Entekhabi, D., Lee, I-S. and Bras, R.L., 1991b. Nonlinear dynamics of soil moisture at climate scales, 2. Chaotic analysis. Water Resour. Res., 27(8): 1907-1916. Rowntree, P.R. and Bolton, J.A., 1983. Simulations of atmospheric response to soil moisture anomalies over Europe. Q. J. R. Meteorol. Soc., 109: 510-526. Sasamori, T., 1970. A numerical study of atmospheric and soil boundary layers. J. Atmos. Sci., 27: 1122-1137. Savenije, H.H.G., 1995. New definitions for moisture recycling and the relationship with land-use changes in the Sahel. J. Hydrol., 167: 57-78. Scott, R., Koster, R., Entekhabi, D. and Suarez, M., 1995. Effect of canopy interception reservoir on hydrologic persistence in a general circulation model. J. Climate, 8(7): 1917-1922. Segal, M. and Arritt, R.W., 1992. Non-classical mesoscale circulations caused by surface sensible heat flux gradients. Bull. Am. Meteorol. Soc., 73(10): 1593-1604. Segal, M., Arritt, R.W., Clark, C., Rabin, R. and Brown, J., 1995. Scaling evaluation of the effects of surface characteristics on potential for deep convection over uniform terrain. Mon. Weather Rev., 123: 383-400. Sun, W.-Y. and Ogura, Y., 1979. Boundary-layer forcing as a possible trigger to a squall-line formation. J. Atmos. Sci., 36: 235-254. Walker, J. and Rowntree, P.R., 1977. The effect of soil moisture on circulation and rainfall in a tropical model. Q. J. R. Meteorol. Soc., 103: 29-46. Yang, R., Fennessey, M.J. and Shukla, J., 1994. The influence of initial soil wetness on medium-range surface weather forecasts. Mon. Weather Rev., 122: 471-485. Yeh, T.-C., Wetherald, R.T. and Manabe, S., 1984. The effect of soil moisture on short-term climate and hydrology change: a numerical experiment. Mon. Weather Rev., 112: 474-490. Zdunkowski, W.G., Paegle, J. and Reilly, J.P., 1975. The effect of soil moisture upon the atmospheric and soil temperature near the air-soil interface. Arch. Meteorol. Geophys. Bioklimatol., Ser. AI, 24: 245-268. Zhao, W. and Khalil, M.A.K., 1993. The relationship between precipitation and temperature over the contiguous United States. J. Climate, 6(6): 1232-1236. Ziegler, C.L., Martin, W.J., Pielke, R.A. and Walko, R.L., 1995. A modeling study of the dryline. J. Atmos. Sci., 52(2).
Hydrology ELSEVIER
Journal of Hydrology 184 (1996) 3-17
Mutual interaction of soil moisture state and atmospheric processes Dara Entekhabi a'*, Ignacio Rodriguez-Iturbe b, Fabio Castelli c a48-331 Ralph M. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA bDepartment of Civil Engineering, Texas A&M University, College Station, TX 77843, USA Clnstituto di Idraulica, Universiti~ degli Studi di Perugia, Perugia, Italy
Received 7 June 1995; accepted 29 October 1995
Abstract The purpose of this paper is to outline the pathways through which soil moisture and meteorological phenomena mutually influence one another at local, regional and global scales. This constitutes two-way land-atmosphere interaction, as meteorological phenomena both act as the forcing and react to the forcing by the soil moisture state. Land surface modification of the atmospheric environment and the atmospheric forcing of these land surface conditions form feedback loops which are significant factors in modulating the variability of the climatic system. The predictability and analysis of fluctuations and changes in regional hydrology and the atmospheric environment require an understanding of these feedback mechanisms and two-way land-atmosphere interaction. The dynamics of soil moisture at the land surface is governed by components with diverse time scales. Variability in both weather and climate are therefore influenced by the soil moisture state. In this paper, we present an overview of several studies on the mutual interaction of the soil moisture state and the atmospheric environment. We also outline possible directions for innovative investigations on the determination of soil moisture variation influences on the moist thermodynamics, energetics and dynamics of the overlying atmosphere.
1. Introduction Motions and disturbances in the atmospheric fluid at all scales are principally forced by the differential heating and friction at the surface. Even air column latent heating and radiative cooling, the remaining principal forcing mechanisms, are influenced by moisture availability that has its source at the surface. The input of heat into the atmosphere at the * Correspondingauthor. 0022-1694/96/$15.00 Copyright © 1996 Elsevier ScienceB.V. All rights reserved SSDI 0022-1694(95)02965-6
D. Entekhabi et al./Journal of Hydrology 184 (1996) 3-17
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Fig. 1. Conceptualdiagramof the pathwaysthroughwhich soil moistureaffectsand is mutuallyinfluencedby the overlying atmosphere(from Brubakerand Entekhabi, 1994). surface is achieved by turbulent flux and thermal radiation. Both mechanisms are strongly controlled by the availability of soil moisture. The heat generated by the radiative forcing of the surface is dissipated by the turbulent flux and thermal radiation. The partitioning between the two mechanisms is dependent on the surface temperature and the static stability of the near-surface air. Again, both factors are influenced by soil moisture. The turbulent flux itself is partitioned between sensible (dry) and latent (moist) heat flux; the relative partitioning is strongly controlled by soil moisture. Sensible heat flux is relatively less efficient than latent heat flux in dissipating heat; when there are (soil moisture) controls on evapotranspiration, the greater partitioning to sensible heat flux results in a rise of ground temperature. This in turn influences the dissipation of heat by radiative cooling at the surface. Capillary action controls the rate at which atmospheric evaporative demand and vegetation roots withdraw water from soil storage. Soil moisture also affects the thermal inertia and shortwave albedo of the surface. Fig. 1 is a conceptual diagram demonstrating the myriad of ways in which soil moisture anomalies affect the near-surface air. In this paper, we outline the role of land surface conditions on the development of atmospheric processes across a wide range of space and time scales. This includes weather phenomena such as the growth of squall lines, frontogenesis and local circulations. At the larger scales, we include climate variability, continental droughts and global feedback mechanisms. The land fraction of the Earth is small (30%) but its distribution into large contiguous areas and its distinctive hydrothermal inertia cause significant variations in regional climatic systems. It is nonetheless most important to note that the separation of atmospheric dynamical phenomena into weather and climate (depending on time scale) partially breaks down precisely because of the role of soil moisture. The reservoir of water in the soil column provides the thermal and moisture inertia that shifts the high-frequency fluctuations into
D. Entekhabi et al./Journal of Hydrology 184 (1996) 3-17
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TIME, IN YEARS Fig. 2. The propagation of a precipitation anomaly through the surface branch of the hydrologic cycle. Landatmosphere interaction and feedback mechanisms would extend the influence in the upward direction in the figure (based on McNab and Karl, 1989).
lower-frequency variability. Persistence and the action of feedback mechanisms may thus result in modes of variability that link weather anomalies and climatic departures from normal.
2. S o i l m o i s t u r e
and land-atmosphere
interaction
The blending of weather and climate anomalies may be illustrated using Fig. 2. An illustrative example of how land-atmosphere interaction may modulate climatic fluctuations may be posed using this conceptual figure from McNab and Karl (1989). The top curve in Fig. 2 represents typical fluctuations in the precipitation anomaly time-series. Over land regions, the surface runoff (cumulative hydrographs) responds to these anomalies with some delay. The next hydrologic reservoir, soil moisture, responds to the runoff anomalies with still more delay. The baseflow contribution to streamflow also follows the anomalies of soil moisture with some lag as well. Finally, the groundwater system fluctuations lag those of the streamflow and soil moisture. As the precipitation anomaly makes its way through components of the land hydrologic cycle (from storm runoff to soil moisture to streamflow to groundwater) its phase-lag with respect to the precipitation anomaly is increased and, owing to the storage associated with each component, the fluctuations are dampened significantly. The soil hydrology is acting essentially as a low-pass filter
6
D. Entekhabi et aL/Journal of Hydrology 184 (1996) 3-17
(Entekhabi and Rodriguez-Iturbe, 1994). The net result is an extension of meteorological drought (precipitation deficit) to delayed and prolonged hydrologic drought (soil moisture availability deficit). Fig. 2 demonstrates how an atmospheric forcing anomaly propagates down through the regional hydrologic system. An important effect that may be added to Fig. 2 is landatmosphere interaction. What would be the overall effect if the regional hydrology could affect the atmospheric forcing? That would correspond to the possibility of anomalies moving 'up' as well as 'down' in the figure. If soil moisture positively affected precipitation, then a soil moisture anomaly owing to an earlier precipitation anomaly would cause precipitation deficit currently and in the future, as a result of the lagging effect. Such a feedback mechanism would then lead to persistence and essentially locking of anomalous events until a large enough external fluctuation forces the system to regain its normal state. Of course, Fig. 2 may be extended to include spatial variations, anomalies and their interactions as well. Spatial patterns of soil moisture may exhibit interactions and feedbacks similar to those in time. The moving 'up' in Fig. 2, the influence of soil moisture variations on atmospheric phenomena, may involve moist thermodynamic or dynamic mechanisms. In this paper, we next turn to more detailed examination of each of these mechanisms. Many of the investigations on the role of land-atmosphere coupling and two-way interaction (in which both surface conditions and atmospheric states respond to each other) are based on numerical models that solve the meteorological Primitive Equations (conservation of mass, momentum and energy, Ideal Gas Law) together with various parameterizations of soil and vegetation control on infiltration and evapotranspiration. Although these numerical models form effective laboratories for testing basic mechanisms of land-atmosphere interaction and space-time variability, they are nonetheless limited to the extent that the parameterizations of physical processes are only approximations of the true soil physics and vegetation physiological action. The advantage of numerical laboratories is that cause and effect may be to some extent isolated and partial analysis on the role of soil moisture may be performed. The key disadvantage is that the results are conditional on the brevity of the parameterizations. In this respect, the results of numerical studies must be viewed in the context of the modeling assumption. In this paper, first the dynamical mechanisms at the local and regional scales, and then the dynamical mechanisms at continental and climate scales will be considered. The moist thermodynamic influences of soil moisture control on surface moisture and heat fluxes will then be outlined. Finally, some guidelines on the spatial and temporal sampling and characterization of soil moisture will be discussed. 2.1. Growth o f regional atmospheric dynamical disturbances and the soil moisture influence
The simplest form of local atmospheric dynamics forced by spatial variations in soil moisture is through the initiation and growth of thermally direct circulations. There have been a number of numerical studies on this phenomenon during the past decade; Segal and Arritt (1992) have provided a summary and comprehensive bibliography of these investigations. Basically, patches of dry and moist soils (or stressed and unstressed vegetation
D. Entekhabi et al,/Journal of Hydrology 184 (1996) 3-17
stands) create sharp gradients in sensible heat flux. This in turn forces a local circulation very much like a sea-breeze. The term land-breeze is often applied to these features. Landbreezes enhance the transport of heat from the surface through structured local thermally direct circulation. The land-breeze cells also affect the dryness and heat content of the near-surface air; they thus define the local evaporative environment. In this way a feedback mechanism may be established. There are a number of factors that significantly alter the effectiveness of soil moisture variations in forcing a land-breeze. First, the geometry of the patches determines the strength of the circulations. If the patches are large, then the converging surface winds do not result in a closed circulation pattern during the course of a diurnal cycle. If the patches are small or heterogeneous, then these circulations are disorganized and weak. Also, the present of a largescale synoptic (background) wind tends to destroy these cellular features. The numerical experiments using patches are thus specific to these test cases. Furthermore, conclusions based on two-dimensional models (height and one horizontal distance) will substantially change once the third dimension is included. Lynn et al. (1995) used a three-dimensional mesoscale atmospheric model to assess the importance of landscape patchiness on landatmosphere heat exchange. A number of mosaic patterns for wet and dry 'checkerboard' patches were considered with extremes of totally dry and saturated soil conditions in each of two surface types. Lynn et al. (1995) found that the land-breeze circulation induced heat flux in the two-dimensional model overestimates the impact of surface heterogeneity when compared with results from three-dimensional simulations. Enger and Tjernstrom (1991) performed similar numerical experiments using true surface topography and land use conditions. They analyzed the impact of a lake on the regional semi-arid climate near the Tunisian-Algerian border. Owing to the increased local moisture availability and development of strong land-breeze circulations, the precipitation in the region increases significantly. The impact of the lake is less pronounced when a strong synoptic wind is present. More fundamentally, however, spatial variation in soil moisture may trigger and affect the growth of squall lines and baroclinic disturbances. Sun and Ogura (1979) and Chang and Wetzel (1991) worked with numerical weather prediction models initialized with convective pre-storm environments in the atmosphere. They tested the growth of the disturbance with differing specifications of the surface conditions. Sun and Ogura (1979) concluded that the pre-storm convergence is significantly affected by horizontal temperature gradients. Similarly, Chang and Wetzel concluded that their test stationary front is strongly enhanced if there are strong gradients of soil moisture. Fast and McCorcle (1991) considered the changes in the characteristic of a moving front owing to variations in the surface conditions. They concluded that the evaporation of soil moisture significantly affected the boundary layer structure and in turn modified the spatial pattern and magnitude of the precipitation intensity field. Ziegler et al. (1995) reported that gradients in surface sensible heat flux estimated near 100 W m- 2 per 50 km is sufficiently large to form a dry-line which is an important pre-storm environment for major US Great Plains summer squall-line formation. Using numerical models, Lanicci et al. (1987) also found that spatial gradient in soil moisture (which also generally implies gradients in sensible heat flux and partitioning of net radiation) is an important contributing factor to severe storms in the same region.
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In all these simulation investigations, the soil moisture control on evapotranspiration, the resulting partitioning of turbulent flux more towards the sensible heat mechanism and the enhancement of horizontal temperature gradients affect the evolution of regional dynamical features. The precipitation intensity field, both in extent and magnitude, is affected by soil moisture variations. Blyth et al. (1994) included the role of vegetation in their numerical study of surface influences on pre-storm environments. They found that over a region in southern France, forest stands affect the near-surface atmospheric environment through unstressed evaporation from canopy interception. An equally important factor is the enhanced mass, heat and momentum transfer where rough elements of forest stands cover the surface. Castelli et al. (1996) similarly analyzed the role of soil moisture on frontogenesis. They, however, advanced one step further in closing the feedback loop between the land and the atmosphere at these scales. The soil moisture affects the distribution and intensity of the precipitation field. These precipitation fields in turn change the surface soil moisture state and its spatial patterns. Castelli et al. (1996) showed that this process constitutes a positive feedback mechanism whereby soil moisture and atmospheric dynamics mutually reinforce each other's spatial gradients.
2.2. Large-scale variability and climate: anomaly reinforcement by surface hydrologic processes As discussed in reference to Fig. 2, the hydrothermal inertia associated with surface and root-zone soil moisture affects climate time scales of variability. For instance, modeling studies show that the presence of the soil moisture reservoir adds to the persistence of anomalies in the near-surface air (Delworth and Manabe, 1989). Yang et al. (1994) demonstrated the impact of the persistence owing to the mutual coupling of surface and atmospheric processes on short-term weather prediction. Karl (1986), using monthly and seasonal precipitation and temperature records, reconstructed soil moisture anomaly fields for the USA. He demonstrated the increase in seasonal and monthly temperature forecast skill when soil moisture anomalies are used in the predictions. These studies demonstrate the possibility of feedback processes in land-atmosphere interaction that may reinforce the persistence of anomalies at different spatial as well as temporal scales. One of the earlier investigations on such phenomena is the modeling study of drought in the Sahel zone by Charney (1975). Reduced precipitation (possibly owing to large-scale air-sea interaction) results in the drying of near-surface soil moisture. Drier soils and desiccated vegetation have higher shortwave albedo values. Radiative forcing deficit in this low evaporation environment results in cooler temperatures and sinking air motion that in turn reinforces the precipitation anomaly which initiated the drying cycle in the first place. There have been a large number of studies on such positive feedback mechanisms between soil moisture and atmosphere in the Sahel based on the original investigation by Charney (1975). Synoptic observations by Namias (1988) also indicate that, over continental regions such as the US Great Plains, the atmospheric initiation of a hydrologic anomaly at the surface results in large-scale changes in soil moisture which in turn reinforce the atmospheric forcing anomaly. During the 1988 US drought, the soil water content over the
D. Entekhabi et al./Journal of Hydrology 184 (1996) 3-17
central Great Plains was significantly reduced. The elevated surface temperatures owing to controls on latent heat flux deepened the adiabatically mixed surface air layer and intensified the high-pressure ridge in the mid-continental region. Both these effects tend to inhibit precipitation. As a result of this positive feedback, the climatic anomaly is reinforced and it persists with even greater intensity. Escape from this cycle occurs whenever large-scale and synoptic factors are strong enough to overcome the positive feedback that locks low soil moisture anomalies with deficits in normal precipitation. This is essentially the outline of a sequence of temporal cause and effects. Owing to the positive feedback between soil moisture and the atmosphere, such anomalies expand in space as well. Karl (1983) provided observational evidence that the severity of droughts is related to their spatial extent. Important land-atmosphere interactions mediate these spatial and temporal linkages. Investigations of soil moisture influence on the persistence of climate anomalies (especially summer dry conditions) have also been based on general circulation models (GCMs). The results of experiments with these numerical laboratories are dependent on the realism of the parameterization used in the modeling. Especially when dealing with soil physics and vegetation transpiration, the role of process models and landscape heterogeneity are of highest concern. Whereas these numerical laboratories are not replacements for observation-based studies, they are valuable in performing partial analyses in a system where the many states are linked in complex ways. In this way, they may be valuable tools for developing insights and designing observation strategies. Walker and Rowntree (1977) and Bounoua and Krishnamurti (1993) modeled the West African semi-arid zone with contrasting wetness at the surface boundary. Rind (1982), Rowntree and Bolton (1983), Ogelsby and Erickson (1989) and Cook (1994) performed numerical GCM experiments on similar topics. In general, these studies using GCMs as numerical laboratories indicate that dry surface wetness anomalies tend to persist for longer periods than moist surface anomalies; the effect is more pronounced if the anomaly is initially present at the onset of the summer season. Georgakakos and Bae (1994) used estimated soil moisture from a conceptual rainfall-runoff model to construct long timeseries of surface wetness; they demonstrated that the frequency and duration of soil moisture anomalies in the Upper Mississippi basin are lower for wet anomalies than for dry anomalies. Dynamical feedbacks may be present which reinforce droughts and dry anomalies. Soil moisture and large-scale atmospheric processes mutually interact to create persistence in records of climate variability. The components of the land surface hydrology parameterization that are responsible for the enhancement of persistence in GCM model climates have been investigated as well. For example, the magnitude of potential evaporation (as a measure of the rate at which the atmosphere removes anomalies in soil moisture) is a major factor for moist climates. Scott et al. (1995) focused more on individual surface processes, and they identified components of the land surface hydrology that add to the time scales of GCM model climates. Dirmeyer (1994) focused on the impact of phonology and reaction to stress in vegetation on the initiation and prolongment of droughts. The dormancy of vegetation and strong seasonal cycle (and their phase difference) are shown to be important considerations in studying the transformation of meteorological droughts (a deficit in seasonal precipitation) to hydrologic droughts (persistent deficits in available soil moisture). Yeh et al. (1984)
10
D. Entekhabi et al./Journal of Hydrology 184 (1996) 3-17
introduced an added consideration in determining the impact of soil moisture anomalies on climate variability. They used GCM simulations to show that the presence of soil moisture anomalies where a large fraction of the precipitation is lost to runoff is less important than where precipitation and evaporation are the principal components of the surface hydrology. Beyond this, the surface temperature anomalies associated with soil moisture anomalies also have varying degrees of impact on the atmosphere depending on the latitude. Thermal wind relationship and its contribution to zonal circulation is the main mechanism through which this sensitivity exhibits itself. The next step for such investigations is to identify the exact mechanisms that constitute the positive feedback between soil moisture and the atmosphere in large numerical models. GCMs implicitly contain the major moist thermodynamic constraints on the climatic system; nonetheless it is rather difficult to decipher the model output and clearly pinpoint the pathways through which soil moisture affects the overlying atmosphere and vice versa. To understand why GCMs exhibit such response behavior between soil moisture and atmospheric anomalies, it is necessary to augment such large-scale numerical investigations with simple models based on the essential energetics and moist thermodynamics. Studies using the simple models provide valuable insight into the workings of the landocean-atmosphere climatic system and they help explain the more complex behavior evident in numerical GCM model climates. In the next section, we turn our focus to studies of soil moisture-atmosphere interaction using these simple moist thermodynamic and energy balance models and summarize some of the major findings. 2.3. Air moist thermodynamics influenced by soil moisture: origins of feedback mechanisms
The control of soil moisture on the surface exchanges of latent and sensible heat flux is the key mechanism through which feedback mechanisms between the land and atmosphere develop. The source of atmospheric moisture and precipitation is at the surface; deficits in moisture supply (evaporation) thus potentially reinforce deficits in subsequent precipitation simply through coupled water balance between the soil and atmospheric reservoirs. In terms of sensible heat flux, the added heating of the atmosphere owing to dry anomalies at the surface is amplified by radiative processes. Thus the moist thermodynamics of the near-surface air is strongly determined by soil moisture. The mechanism that involves moisture supply is precipitation recycling over continental regions. The precipitating water over large land areas is partially derived from evaporation over the region itself. Thus deficits in evaporation lead to deficits in precipitation which in turn dry the soil and further reduce the evaporation supply to the atmosphere. Brubaker et al. (1993) estimated the fraction of regional monthly precipitation derived from local evaporation based on atmospheric vapor flux observations. They provided estimates for this measure of positive land-atmosphere interaction for selected regions; Fig. 3 is an example of precipitation recycling estimates for the US Great Plains. Up to 40% of the precipitation is supplied by local evaporation during some seasons. Eltahir and Bras (1994) and Savenije (1995) analyzed the precipitation recycling over the Sahel and the Amazon basin using more detailed models of the atmospheric water balance. They also found that a large fraction of the regional precipitation is derived from local evaporation.
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To illustrate the nature and the consequences of this positive feedback mechanism between soil moisture and climate, Rodriguez-Iturbe et al. (1991a) and Entekhabi et al. (1992) set up a simple water balance equation for regional soil moisture and included the recycling of precipitation. When the system is forced by simple gaussian stochastic noise (representing large-scale forcing), the soil moisture state persists in two distinct statistical modes. Fig. 4 shows an example of such a non-gaussian bimodal probability distribution for soil moisture. This simple climatic system persists in one (dry) mode and it reinforces the dry anomaly, as low evaporation leads to low precipitation, dry soils and again low evaporation. Whenever the large-scale forcing is strong enough to force the escape of soil moisture climate from this positive feedback lock, the system changes to the second (wet) mode and persists there for some time. Similar positive feedback reinforces the increase in both precipitation and evaporation; Yeh et al. (1984) found a similar positive feedback that acts to reinforce positive soil moisture anomalies in a numerical climate model. Entekhabi et al. (1992) derived the distribution of escape times from each mode in their stochastic model of soil moisture evolution; droughts with a duration of several years to decades, such as those observed in the Sahel, are frequent. Rodriguez-Iturbe et al. (1991b) also analyzed the dynamic behavior of the soil moisture evolution. As a result of the nonlinear feedback mechanisms in the simple system, chaotic dynamics is observed. Beyond the simple moisture supply effect, soil moisture anomalies also exert some control on the heating of the overlying air column. Changes in the radiative divergence and static stability of the atmosphere are important factors and they significantly alter the moist thermodynamic conditions of the atmosphere. Sasamori (1970), Zdunkowski et al. (1975) and Raddatz (1993) investigated some of these effects with the use of numerical models of the planetary boundary layer. The magnitude and diurnal range of the heating profile and the growth of the turbulently mixed layer are significantly modified by soil
12
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RELATIVE SOIL SATURATION Fig. 4. Soil moisture probability distribution in a simple water balance model that includes the precipitation recycling mechanism for land-atmosphere interaction. A persistent dry (drought) model and a persistent wet mode are evident even thoughthe forcingis simple gaussian noise (from Rodriguez-Iturbeet al., 1990). moisture anomalies. These studies also used numerical models to demonstrate the role of surface heterogeneity in soil moisture (and soil texture in general) on the surface fluxes and growth of the turbulent mixed layer. The formation of clouds and both dry and precipitating convection are consequently affected as well (Segal et al., 1995). The moist thermodynamic conditions of the near-surface air are changed; these changes in turn modify the surface fluxes. In a further demonstration of the mutual influences of soil moisture state and the atmosphere, Brubaker and Entekhabi (1995) developed a set of coupled equations for the hydrothermal states in the soil and the turbulent boundary layer. They showed that the diurnal and seasonal range in the moisture and temperature states of the air layer are determined in large part by the soil moisture state. The model is used to provide quantitative measures for the relative strength of the pathways hitherto only qualitatively presented such as in Fig. 1. This simple analytic model of land-atmosphere was extended by Entekhabi and Brubaker (1995) to include random variations in the regional wind speed. The principal findings of this investigation are related to the delineation of exact pathways and mechanisms through which soil moisture affects climate variability at the large scale. For example, in Fig. 5 from that study the lagged cross-covariance of soil moisture and soil temperature is presented. Without land-atmosphere interaction, the mutual dependence of the two variables is severely curtailed. The balance as well as the covariability of heat and moisture at the surface are closely linked, and the link is through a series of pathways that include two-way interaction with the atmosphere. Entekhabi and Brubaker (1995) used the simple framework of the model to delineate these pathways that couple the
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14
D. Entekhabi et al.Hournal of Hydrology 184 (1996) 3-17
Mississippi basins. Georgakakos and Bae (1994) used these estimates to study the interannual variability in the record. They found that near-surface soil moisture may persist for up to one season, whereas, in the lower root-zone, water availability anomalies last for several seasons. The strong annual cycle in the region results in the soil column being almost saturated in June. This condition leads to large summertime evaporation and it also results in flood-prone surface conditions. In fact, Kunkel (1994) argued that a positive anomaly in such a condition in the Upper Mississippi contributed to the large precipitation anomalies and the great flood of 1993.
3. Discussion: observation and characterization of soil moisture space-time variability Given the critical role of soil moisture on surface moisture and energy balance and covariability with subsequent influences on the atmosphere, it is important to create the infrastructure to routinely and operationally estimate and observe this quantity. Topography, significant spatial heterogeneity in soil and vegetation properties, and the highly intermittent characteristic of precipitation fields result in large spatial variations in the soil moisture field. Entekhabi and Rodriguez-Iturbe (1994) analyzed the space and time scales of variability in the soil moisture field and identified the key roles of topography and precipitation variability in defining its statistical characteristics. There are some major obstacles to the monitoring of soil moisture. Fundamental among these is the very definition of soil moisture. As the soil-atmosphere interface is intermittently a source and sink for moisture, sharp vertical gradients in soil moisture develop at the surface as well. This factor complicates the very definition of soil moisture. Over what depth is soil moisture, the soil moisture that interacts with the atmosphere, defined? Given the sharp gradients near the surface, the dynamics of soil moisture variability will be very different depending on this depth. Another fundamental obstacle to the monitoring of soil water is the role of heterogeneity in the formation and evolution of soil moisture fields. Variations in soil type, geologic constraints, complex topography, preferential flow-paths formed by roots and fauna, and fluctuations in atmospheric forcing (e.g. spotty rainfall, aspect variations in exposure to solar radiation, etc.) all contribute to the heterogeneity in soil moisture fields over a large range of scales. As a result of this strong heterogeneity, point samples of soil moisture are not necessarily indicative of the regional value for this variable. These two fundamental obstacles have essentially inhibited the development of soil moisture monitoring programs. Important as this variable is, there are no extensive and reliable sources of soil moisture estimates available for the operational and research communities. Existing thought and approaches to this variable are inherently constrained in overcoming these and other equally effective obstacles. The applicability of data assimilation techniques to the estimation of soil moisture fields is not clear at this point. The degree to which they are feasible is dependent on the manner in which the governing relations implicitly relate the evolution of observed states to the regional soil moisture variable. Furthermore, the estimates of
D. Entekhabi et aL/Journal of Hydrology 184 (1996) 3-17
15
soil moisture based on the data assimilation contain the bias of the model used for assimilation. The distinction between field soil moisture and estimates derived from data assimilation must be made clear: model-based estimates and modeling values of soil moisture (in GCMs and mesoscale models) refer to the soil water variable in these models that is used principally to maintain consistent partitioning of incoming precipitation and radiation into infiltration, evaporation, runoff, turbulent and radiative energy fluxes over time. Comparison of GCM, mesoscale or model-assimilated soil moisture with a field-measured value is thus a problem that remains, up to this time, unresolved. The spatial heterogeneity in the horizontal domain and the sharp vertical gradients complicate the observation (and cross-validation with GCM and mesoscale numerical models) by in situ methods. Gravimetric techniques, neutron probe and time domain reflectometry are labor intensive. Therefore they cannot be deployed to operationally map the soil moisture field across large regions. Remote sensing techniques are limited by the problems associated with geophysical calibration. Low-frequency microwave remote sensing is the most promising tool to observe soil moisture fields directly (or as directly as possible) from space (Njoku and Entekhabi, 1996). The major shortcoming of this approach is that the emitted passive microwave radiation represents dielectric conditions in the top few centimeters of soil. As mentioned above, the sharp gradients of soil moisture near the soil-atmosphere boundary complicate the definition of soil moisture that is necessary for atmospheric studies. Entekhabi et al. (1994) developed a coupled modeling-remote sensing technique to infer soil moisture at larger depths. This poses a difficult inverse-problem. The use of active microwave radiation to achieve soil moisture averages over larger depths is severely limited by the noise introduced by microtopography and roughness of the vegetation cover. Furthermore, the lower frequencies (1 GHz or lower) are not protected for geophysical investigations and commercial transmission interference is present. Clearly, no one technique is suitable for application at the large scale and in an operational mode. Data assimilation techniques and combined modeling-observation programs are necessary to achieve the best results. Parallel to these observational considerations, significant efforts are necessary to add to our understanding of land-atmosphere interaction. By developing and expanding the knowledge about how soil moisture influences the surface and atmospheric processes, indirect methods for its characterization will follow. The clear and concise understanding of the physical linkages between the soil moisture state and atmospheric processes also brings gains to the characterization and predictability of atmospheric processes such as precipitation and climate variability. Controlled field experimentation (such as the FIFE and HAPEX micrometeorological experiments) and the use of simple models in conjunction with large numerical models are necessary to identify the physical mechanisms governing the mutual interaction of soil moisture and atmospheric processes. The wide range of time and space scales, the large natural variability inherent in the climatic system, and the complexity of the interactions add complications. The topic of investigation also crosses disciplinary barriers. These artificial barriers are rapidly disappearing with the recognition that the Earth sciences form one totality.
16
D. Entekhabi et aL/Journal of Hydrology 184 (1996) 3-17
References Bae, D.-H. and Georgakakos, K.P., 1994. Climate variability of soil water in the American Midwest, Part 1: Hydrologic modeling. J. Hydrol., 162: 379-390. Blyth, E.M., Dolman, A.J. and Noilhan, J., 1994. The effects of forest on mesoscale rainfall: an example from HAPEX-MOBILHY. J. Appl. Meteorol., 33: 445-464. Bounoua, L. and Krishnamurti, T.N., 1993. Influence of soil moisture on the Sahelian climate prediction: Part I. Meteorol. Atmos. Phys., 525: 183-203. Brubaker, K.L. and Entekhabi, D., 1994. Nonlinear dynamics of water and energy balance in land-atmosphere interaction, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering Technical Rep. No. 341,147 pp. Brubaker, K.L. and Entekhabi, D., 1995. An analytic approach to modeling land-atmosphere interaction. 1. Construct and equilibrium behavior. Water Resour. Res., 31(3): 619-632. Brubaker, K.L., Entekhabi, D. and Eagleson, P.S., 1991. Atmospheric water vapor transport: estimation of continental precipitation recycling and parameterization of a simple climate model, MIT Department of Civil Engineering, Ralph M. Parsons Laboratory Rep. No. 333, 166 pp. Brubaker, K.L., Entekhabi, D. and Eagleson, P.S., 1993. Estimation of continental precipitation recycling. J. Climate, 6(6): 1077-1089. Castelli, F., Rodriguez-Iturbe, I. and Entekhabi, D., 1996. An analytical framework for the modelling of the spatial interaction between the soil moisture and the atmosphere. J. Hydrol., 184: 19-34. Chang, J.-T. and Wetzel, P.J., 1991. Effects of spatial variations of soil moisture and vegetation on the evolution of the pre-storm environment. Mon. Weather Rev., 119: 1368-1390. Charney, J.G., 1975. Dynamics of deserts and drought in the Sahel. Q. J. R. Meteorol. Soc., 101: 193-202. Cook, K.H., 1994. Mechanisms by which surface drying perturbs tropical precipitation fields. J. Climate, 7(3): 400-413. Delworth, T. and Manabe, S., 1989. The influence of soil wetness on near surface atmospheric variability. J. Climate, 2(12): 1447-1462. Dirmeyer, P.A., 1994. Vegetation stress as a feedback mechanism in midlatitude drought. J. Climate, 7(10): 1463-1483. Eltahir, E. and Bras, R.L., 1994. Precipitation recycling over the Amazon basin. Q. J. R. Meteorol. Soc. Enger, L. and Tjernstrom, M., 1991. Estimating the effects on regional precipitation climate in semiarid region caused by an artificial lake using a mesoscale model. J. Appl. Meteorol., 30: 227-250. Entekhabi, D. and Brubaker, K.L., 1995. An analytic approach to modeling land-atmosphere interaction: 2. Stochastic extension. Water Resour. Res., 31(3): 635-643. Entekhabi, D. and Rodriguez-Iturbe, I., 1994. An analytic framework for the characterization of the space-time variability of soil moisture. Adv. Water Resour., 17(1-2): 35-46. Entekhabi, D., Rodriguez-Iturbe, I. and Bras, R.L., 1992. Variability in large-scale water balance with landsurface-atmosphere interaction. J. Climate, 5(8): 798-813. Entekhabi, D., Nakamura, H. and Njoku, E.G., 1994. Solving the inverse problem for soil moisture and temperature profiles by sequential assimilation of multifrequency remotely sensed observations. IEEE Trans. Geosci. Remote Sensing, 32(2): 438-448. Fast, J.D. and McCorcle, M.D., 1991. The effect of heterogeneous soil moisture on a summer baroclinic circulation in the United States. Mon. Weather Rev., 119: 2140-2167. Georgakakos, K.P. and Bae, D.-K, 1994. Climate variability of soil water in the American Midwest, Part 2: Spatio-temporal analysis. J. Hydrol., 162: 379-390. Huang, J. and van den Dool, H.M., 1993. Monthly precipitation temperature relations and temperature prediction over the United States. J. Climate, 6(6): 1111-1132. Karl, T.R., 1983. Some spatial characteristics of drought duration in the United States. J. Climate Appl. Meteorol., 23: 1356-1366. Karl, T.R., 1986. The relationship of soil moisture parameterizations to subsequent seasonal and monthly mean temperature in the United States. Mort. Weather Rev., 114: 675-686. Kunkel, K.E., 1994. A climatic prospective on the 1993 Flooding rains in the Upper Mississipi River Basin. Water Int., 19: 186-194.
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Lanicci, J.M., Carlson, T.B. and Warner, T.T., 1987. Sensitivity of the Great Plains severe-storm environment to soil moisture distribution. Mon. Weather Rev., 115: 2660-2673. Lynn, B.H., Rind, D. and Avissar, R., 1995. The importance of mesoscale circulations generated by subgrid-scale landscape heterogeneities in general circulation models. J. Climate, 8(2): 191-205. McNab, A.L. and Karl, T.R., 1989. Climate and droughts. US Geol. Surv. Water Supply Pap., 2375: 89-95. Namias, J., 1988. The 1988 summer drought over the Great Plains: a classic example of air-sea-land interaction. Trans. Am. Geophys. Union, 69: 1067. Njoku, E. and Entekhabi, D., 1996. Passive microwave remote sensing of soil moisture. J. Hydrol., 184: 101-129. Ogelsby, R.J. and Erickson, D.J., 1989. Soil moisture and the persistence of North American drought. J. Climate, 2(11): 1362-1380. Raddatz, R.L., 1993. Prairie agroclimate boundary-layer model: a simulation of the atmosphere/crop-soil interface. Atmos.-Ocean, 31(4): 399-419. Rind, D., 1982. The influence of ground moisture conditions in North America on summer climate as modelled by the GISS GCM. Mon. Weather Rev., 110: 1487-1494. Roads, J.O., Chen, S.-C., Guetter, A.K. and Georgakakos, K.P., 1994. Large-scale aspects of the United States hydrologic cycle. Bull. Am. Meteorol. Soc., 75(9): 1590-1610. Rodriguez-Iturbe, I., Entekhabi, D. and Bras, R.L., 1990. Nonlinear dynamics of soil moisture at climate scales: Stochastic analysis. MIT Center fro Global Change Science Rep. No. 5, 27 pp. Rodriguez-Iturbe, I., Entekhabi, D. and Bras, R.L., 1991a. Nonlinear dynamics of soil moisture at climate scales, 1. Stochastic analysis. Water Resour. Res., 27(8): 1899-1906. Rodriguez-Iturbe, I., Entekhabi, D., Lee, I-S. and Bras, R.L., 1991b. Nonlinear dynamics of soil moisture at climate scales, 2. Chaotic analysis. Water Resour. Res., 27(8): 1907-1916. Rowntree, P.R. and Bolton, J.A., 1983. Simulations of atmospheric response to soil moisture anomalies over Europe. Q. J. R. Meteorol. Soc., 109: 510-526. Sasamori, T., 1970. A numerical study of atmospheric and soil boundary layers. J. Atmos. Sci., 27: 1122-1137. Savenije, H.H.G., 1995. New definitions for moisture recycling and the relationship with land-use changes in the Sahel. J. Hydrol., 167: 57-78. Scott, R., Koster, R., Entekhabi, D. and Suarez, M., 1995. Effect of canopy interception reservoir on hydrologic persistence in a general circulation model. J. Climate, 8(7): 1917-1922. Segal, M. and Arritt, R.W., 1992. Non-classical mesoscale circulations caused by surface sensible heat flux gradients. Bull. Am. Meteorol. Soc., 73(10): 1593-1604. Segal, M., Arritt, R.W., Clark, C., Rabin, R. and Brown, J., 1995. Scaling evaluation of the effects of surface characteristics on potential for deep convection over uniform terrain. Mon. Weather Rev., 123: 383-400. Sun, W.-Y. and Ogura, Y., 1979. Boundary-layer forcing as a possible trigger to a squall-line formation. J. Atmos. Sci., 36: 235-254. Walker, J. and Rowntree, P.R., 1977. The effect of soil moisture on circulation and rainfall in a tropical model. Q. J. R. Meteorol. Soc., 103: 29-46. Yang, R., Fennessey, M.J. and Shukla, J., 1994. The influence of initial soil wetness on medium-range surface weather forecasts. Mon. Weather Rev., 122: 471-485. Yeh, T.-C., Wetherald, R.T. and Manabe, S., 1984. The effect of soil moisture on short-term climate and hydrology change: a numerical experiment. Mon. Weather Rev., 112: 474-490. Zdunkowski, W.G., Paegle, J. and Reilly, J.P., 1975. The effect of soil moisture upon the atmospheric and soil temperature near the air-soil interface. Arch. Meteorol. Geophys. Bioklimatol., Ser. AI, 24: 245-268. Zhao, W. and Khalil, M.A.K., 1993. The relationship between precipitation and temperature over the contiguous United States. J. Climate, 6(6): 1232-1236. Ziegler, C.L., Martin, W.J., Pielke, R.A. and Walko, R.L., 1995. A modeling study of the dryline. J. Atmos. Sci., 52(2).