Poseidon for rainfall estimation over oceans

ELSEVIER

Potential of Simultaneous Dual-Frequency Radar Altimeter Measurements from TOPEX/Poseidon for Rainfall Estimation over Oceans S. M. Bhandari* and A. K. Varma* T O P E X / Poseidon- a joint U,S. (NASA) and French (CNES) mission called Ocean Topography Experiment-carries onboard a dual frequency (5.3 GHz and 13.6 GHz) radar altimeter providing simultaneous measurements of radar backscatter coefficient over the global oceans since August 1992. In the present work, we have used the concept of differential attenuation of the radar signal due to rain at two widely separated frequencies to estimate rainfall. Simultaneously available passive microwave radiometric measurements from TOPEX/Poseidon itself have also been used to delineate and quantify rain over oceanic regions surrounding India. Based on a reasonably good correlation between rainfall inferred from radiometric measurements and the difference Oa ° between radar backscatter coej~cients at 5 GHz and 13 GHz, significant rain events are isolated during the course of the 1993 South-West (SW) monsoon season over the Indian region. Monthly maps of these rain events from altimeter based analysis clearly bring out the nature of rainfall activity associated with SW monsoon. The results from Oa ° also compare very well with the operational quantitative precipitation estimates available from INSAT-VHRR analyses. The advantages and limitations of radar altimeter data are discussed in terms of those of the other current and future rain measurement systems. Synergistic application of the present technique with visible/IR and microwave techniques hold promise for more precise rainfall measurements from space. *Meteorology and Oceanography Group, Space Applications Centre (ISRO), Ahmedabad-380 053, India Address correspondence to Satyendra M. Bhandari, Meteorology & OceanographyGroup, 4372, Remote Sensing Area, Space Applications Centre (ISRO) Ahmedabad 380 053, India.

Received 8 September 1995; revised 2 March 1996.

REMOTE SENS. ENVIRON. 58:13-20 (1996) @Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010

INTRODUCTION

The inherent potential of spacebased measurements for providing global measurements of rainfall has long been recognized (Barrett and Martin, 1981). However, due to a number of limitations, the development of techniques for quantitative estimation of rainfall using data from a variety of sensors responding either to the reflected solar radiation or to the emitted thermal (infrared or microwave) radiation from the raining clouds has met with limited success (Bhandari, 1993; Petty, 1995). All attempts so far have depended on measuring the radiation emanating from the earth-atmosphere system in a passive mode. Active microwave probing of rain using C- and S-band radars, on the other hand, has been the most successful remote sensing technique for reliable area extensive measurement of rain from ground. This has, however, not yet found its counterpart in space. There are plans to orbit a rain radar in space onboard U.S.Japanese TRMM satellite sometime in the 1997/1998 time frame (Simpson et al., 1988). A few other nations are also envisaging similar radar and |idar missions in low-earth orbits (e.g., BEST by the French) before the turn of the century (Meneghini and Kozu, 1990). Meanwhile, during the last 20 years or so, a number of oceanographic satellites, for example, GEOS-3, SEASAT, GEOSAT, and ERS-1, and 2 have carried onboard sophisticated pulsed radar altimeters (RA) operating in the Ku-band (freq. - 13.5 GHz). As is well known, the 13.5-GHz signal is significantly affected by the presence of rain in the atmosphere. While this results in substantial errors and biases in the measurements of oceanographic parameters of interest, there are distinct possibilities of utilizing these RA measurements to detect 0034-4257 / 96 / $15.00 PII S0034-4257(96)00050-8

14

Bhandari and Varma

and estimate rain, especially over the oceans. Some attempts towards the former have already been made (Srokosz and Guymer 1988; Guymer and Quartly, 1992; Varma et al., 1994). The RA measurements of radar backscatter coefficients (tr °) are primarily sensitive to the ocean surface roughness produced by wind generated waves (JGR Sp. Issue, 1994). Rain falling on the ocean surface tends to modify the roughness characteristics. Light rain helps to smooth the surface by damping the short wavelength waves. Heavy rain, on the other hand, begins to make the ocean surface splashy, increasing the roughness in ways that are not fully understood as yet. Single frequency altimetric measurements of tr ° cannot easily distinguish between the wind/rain induced roughness changes from those resulting from the rain in the intervening atmosphere. During 1992, NASA launched a sophisticated high precision radar altimeter mission called TOPEX / Poseidon which provides simultaneous altimetric measurements of tr ° at two widely separated frequencies of 5.3 GHz (C-band) and 13.6 GHz (Ku-band). The differential attenuation due to rain at these two frequencies provides us with a potential tool to examine the characteristics of rain over the oceanic areas, by eliminating the effects of the changes in tr ° due to surface roughness modifications. In this article, we use the differential attenuation technique to examine the effect of rain on TOPEX/ Poseidon RA signals at C- and Ku-bands for estimation of rainfall associated with the 1993 SW monsoon season over the Indian region. DATA AND ANALYSIS The TOPEX/Poseidon radar altimeter data in the form of normalized radar backscatter coefficients (tr °) at 5.3 GHz and 13.6 GHz, along with a variety of other derived products (e.g., wind speed, significant wave heights, sea surface height, etc. with a spatial resolution of - l0 km along the subsatellite track) were made available on CD-ROMs by the TOPEX Project (Benada, 1993). In addition to the RA, the TOPEX mission also carries onboard a three-frequency nadir looking TOPEX microwave radiometer (TMR) operating at 18 GHz, 21 GHz, and 37 GHz. The TMR data provide information about integrated water vapor, liquid water, and rainfall in the atmosphere with a resolution o f - 50 km. TOPEX is orbiting in a non-sun-synchronous near-polar orbit such that the subsatellite tracks are spaced 315 km apart at the equator and the entire global coverage pattern, within :1:66 ° of latitude, repeats every 10 days (Benada, 1993). From the above data, we have extracted parameters of our interest, that is, a ° at two frequencies, brightness temperatures at the three frequencies, significant wave

height, information about liquid water, rain flags, and the geographical location and the time of each measurement. All measurements over land and coastal areas were excluded from the analysis based on information derived from TMR measurements. For the present study, we have analyzed all relevant TOPEX measurements pertaining to the period 28 May to 30 September 1993 covering the SW monsoon season over India. The geographical region of interest was defined as between 40-100°E and 5°S-25°N. From earlier studies based on ERS-1 it is known that the Ku-band RA tr ° values over the oceans are significantly reduced when the RA signal traverses convective cloudy regions infested with rain as delineated by INSAT-VHRR thermal IR band cloud images (Varma et al., 1994). In case of TOPEX, the possible presence of rain can be inferred quantitatively from TMR data using the 18 GHz brightness temperature T(18) and the Chiu et al. (1990) algorithm R (mm h- x)_ 5.26 log[102 / (274 - T)] =0

for 172 < T < 274 K, otherwise.

These instantaneous rainfall rate estimates (R) with a spatial resolution o f - 45 km can then be correlated with simultaneous measurements of tr ° at the two frequencies. However, in order to eliminate the changes in tr ° due to surface roughness effects, we use the difference t~tr° between 5.3 GHz and 13.6 GHz tr ° values, rather than tr ° themselves for delineating raining events. The t~tr° values [i.e., tr ° (5.3 GHz) - tr ° (13.6 GHz)] so derived are primarily attributable to the differential attenuation between the 5.3 GHz and 13.6 GHz signals on their two-way journey through the rain column. This is the case since the tr ° of the ocean surface is very weakly dependent on frequency over the range of frequencies under consideration (Walsh et al., 1984). RESULTS AND DISCUSSION Analysis of individual cases of rain indicated by TMR data and the presence of rain flags present in TOPEX data revealed that only the Ku-band signal was significantly affected. The C-band tr ° remained largely unaffected. The Ku-band tr ° dropped by as much as 3-15 db compared to clear (rain-free) ocean measurements. The scatter plot of t~a ° vs. rainfall rate R [inferred from the Chiu et al. (1990) algorithm] for all raining events for the 1993 monsoon period is shown in Figure la. A large range of 6tr ° values ( f r o m - 3 db to 13 db) is observed, whereas the rain rate is spread over a relatively small range. This is mainly due to the fact that the TMR measurements are spatially averaged over a much larger footprint of - 45 km, compared to ~< 10 km footprint of RA measurements. Therefore, depending on

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the spatial gradients of rainfall, the TMR estimates would be significantly diluted due to spatial averaging. Since intense rainfall is confined to more compact cells with steeper gradients, the underestimation in TMRbased rainfall estimates would be larger for higher rainfall rate events (Wilheit et al., 1977; Chiu et al,, 1990). In order to establish the possible relationship between 5o"° and R, we have averaged all values in bins of Sir ° containing approximately equal number of points, so that they have equal statistical weights. W e present, in Figure lb, a plot of average 6tr ° vs. average R along with a least squares line. While the correlation coeffi-

cient is very high, the relationship between 5tr ° and R is not considered to reflect the true relationship in view of the systematic biases present in estimation of R from TMR data (Wilheit et al., 1977; Chiu et al., 1990). Correction of these biases is likely to lead to a much steeper relationship than is portrayed by the least squares line. In view of the above, rather than quantifying 5tr ° in terms of rainfall rates at this stage, we have chosen a threshold value of 5 db to represent the presence of significant rain. To verify the overall effectiveness of t~tr° approach, we have limited our analysis to a study of geographical distribution of these significant rain events. The coverage pattern of TOPEX RA measurements during every 10-day cycle over the region of study is shown in Figure 2 (Benada, 1993). It is seen that there are large gaps between subsatellite tracks. Furthermore, each track is sampled only once in 10 days. This implies that the above analysis would be expected to bring out the monsoon characteristics only in an average statistical sense. To study the pattern of occurrence of rain events, we prepared monthly rain event location maps for the monsoon season. These are shown in Figures 3a-d. All events with 5tr°~> 5 db are included in these figures. In all these maps, it is observed that most rain events are spread over the Arabian Sea, Bay of Bengal, and the Equatorial Indian Ocean regions. A paucity of rain events is seen over the northern and western regions of the Arabian Sea. Also the number of events fluctuate from one month to another, as monsoon progresses. We have also analyzed other attributes of the rain events picked up by the above 5o"° analysis. The longitudinal distribution of the number of rain events derived from HA data is presented in Figure 4a. As expected for rainfall associated with the SW monsoon, the rain events depict a bimodal distribution with peaks over the Arabian Sea and the Bay of Bengal regions. It also shows the longitudinal distribution of mean rain rates in the same manner. It is observed that relatively higher rain rates are found to occur over areas with lesser number of events, that is, over the Arabian Sea region. The latitudinal distribution of the number of rain events and the mean rain rate in 5 ° latitude strips is presented in Figure 4b. During the analysis period, that is, 28 May to 30 September 1993, approximately 450 significant rain events have been delineated in - 5 ° to 0% 0 ° to + 5 °, 10-15 °, and 15-20 ° latitude bands each. The lowest number (100) of events occur for 15-20 ° band followed by - 350 for the 5-10 ° band. However, keeping the different extent of oceanic areas within each of the latitude bands in mind, these differences are not significant. Similarly, the mean rain rate is also not significantly dependent on latitude. Figure 4c shows latitudinal and longitudinal distri-

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Over the Indian region, monthly average rainfall information in the form of quantitative precipitation estimate (QPEs) maps with a spatial resolution of 2.5 ° x 2.5 ° are produced on a regular basis since 1986 using INSAT-VHRR IR images collected eight times daily (Arkin et al., 1989). These maps are produced following the Arkin and Miesner (1987) technique, which involves counting cloudy pixels colder than a predetermined threshold of 235 K and assigning these pixels a constant rainfall rate. Therefore, barring the constant multiplication factors, the QPE maps effectively represent the frequency of occurrence of rain events picked up by cloud top temperature thresh-

Figure 3. Geographical location of TOPEX-RA detected rain events on a month-by-month basis for 1993 SW monsoon season: a) June; b) July; c) August; d) September• JUNE

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analysis faithfully brings out the nature of distribution of the rain events over the Arabian Sea, Bay of Bengal, and the Indian Ocean regions. This in spite of the fact that the space-time sampling of TOPEX data (approximately every 10 km along track and - 300 km across track, and once in 10 days) is much coarser compared to the contiguous 1NSAT-VHRR coverage over the entire region with a spatial resolution of - 10 km x 10 km and temporal resolution of 3 h. The possibility of oversampling involved in generation of the QPE product is also

18

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indicated by Bhandari and Varma (1995) in their analysis of INSAT-VHRR data for monthly scale rainfall estimation by incorporating the knowledge of expected diurnal behavior of monsoon rainfall over the Indian region. In order to further establish the reliability of the 6or° analysis in delineating the rainfall activity associated with the SW monsoon, we have made a similar analysis for one nonmonsoon month. The distribution of rain events delineated from the Oa ° analysis described above for January 1993 is shown in Figure 6. The absence of activity over the Arabian Sea and the Bay of Bengal is

obvious. Further, as expected, the rainfall activity in this month is confined to ITCZ regions with a slight shift southward of equator compared to monsoon months. The above discussion clearly demonstrates the promise of Oa ° analysis in reliably picking up raining events over oceanic areas. The differential C- vs. Kuband attenuation is also shown to be strongly correlated with intensity of rainfall. Establishment of quantitative Oa ° vs. rainfall rate algorithm requires extensive oceanor island-based rainfall measurements coincided with altimeter data.

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CONCLUSIONS We have attempted, for the first time, the use of simultaneous dual-frequency radar altimetric measurements from TOPEX to demonstrate their utility in possible quantitative estimation of rainfall over the oceans for the 1993 SW monsoon season. The present analysis is able to separate out the effects of cr° variations resulting from surface roughness changes from those due to the presence of rain in the atmosphere. While in our analysis it has been possible to pick out rain events satisfactorily, it has not been possible to arrive at workable algorithm to estimate rainfall rate quantitatively. This is partly due to nonavailability of simultaneous high accuracy rainfall information for comparison with RA measurements over the oceanic areas of interest. The TMR-based algorithm for rainfall rate estimation has limitations of its own as it has not been validated for tropical/monsoon conditions (Chiu et al., 1990). In the present analysis, it has only been employed because it enabled us to find rainfall estimates simultaneous and colocated with all the RA measurements in the same TOPEX data set. More accurate rainfall estimates could be possible to obtain from multifrequency dual-polarization wide-swath SSM/I operating at 19.3-GHz, 22.2-GHz, 37-GHz, and 85.5-GHz onboard DMSP satellites. Although simuhaneons and colocated TOPEX and DMSP measurements may be relatively scarce, an attempt is being made to obtain SSM/I data for development of a TOPEX RA c~a° based quantitative rainfall estimation algorithm. It is observed that the geographical distribution of rain events analyzed using TOPEX RA data portray the basic features of rainfall activity associated with the SW monsoon very well. The monthly significant rainfall event maps based on c~cr° analysis also compare satisfactorily with the operational QPE product generated by IMD over the Indian region. It must also be noted that TOPEX altimeter data provide much finer spatial resolution and sampling

]9

(about a factor of 3-4) compared to state-of-the-art spaceborne microwave radiometers, albeit along the subsatellite track only. This is a significant advantage over the microwave radiometers particularly in convective rain situations. Examination of effects of rain on Ku-band radar altimeter data is considered as a useful first step in developing the rainfall estimation algorithms based on Ku-band rain radar data from the forthcoming TRMM (Simpson et al., 1988). Delineation of rainfall events using information about c~tr° also provides a very useful input to all oceanographic parameter extraction schemes based on radar altimeter data in filtering out the rain-corrupted measurements. Multiyear statistics of 5or° obtained from TOPEX data would provide a very valuable input in the design of Ku-band satellite communication systems, similar to those planned for INSAT-2C and other communication satellites (Bhandari et al., 1995). The scenario of rainfall estimation from space is likely to take a major step forward with the launch of TRMM carrying visible, thermal IR, and passive microwave radiometers supported by a rain radar operating at 13.4 GHz. The present altimeter based 5or ° technique can then be exploited to full advantage in synergism with TRMM data. Currently, efforts are on to combine INSAT-VHRR based visible and thermal IR band measurements with comparable spatial averaging to improve rainfall estimation over the oceanic region. For this purpose, island-based rainfall measurements would be utilized. In the EOS era, improved multibeam scanning type radar altimeters with improved resolution and wider swath are envisaged (NASA, 1987). These features would make it possible to realize precise quantitative radar altimeter based measurements of rain with frequent global repetitive coverage. Authors are thankful to the NASA--Physical Oceanography Active Archive Center at JPL, Pasadena, California, for distributing the TOPEX data to us. Thanks are also due to the Associate Editor and the reviewers for the encouraging and valuable comments. REFERENCES

Arkin, P. A., and Meisner, B. N. (1987), Spatial and annual variation in the diurnal cycle of large scale tropical convective cloudiness and precipitation, Mon. Wea. Rev. 115: 1009-1032. Arkin, P. A., Krishna Rao, A. V. R., and Kelkar, R, R. (1989), Large scale precipitation and outgoing longwave radiation from INSAT-1B during the 1986 southwest monsoon season, ]. Clim. 2:619-628. Barrett, E. C., and Martin, D. W. (1981), Use of SateUite Data in Rainfall Monitoring, Academic, New York, 340 pp. Benada, R. (1993), User's Handbook (PO.DACC Merged GDR (TOPEX/Poseidon)), Ver. 1.0, No. D-11007, JPL, California Institute of Technology Publication, Pasadena.

20 Bhandari and Varma

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