The response of the marine boundary layer to mesoscale variations in sea-surface temperature

Dynamics of Atmospheres and Oceans, 8 (1984) 267-281

267

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE RESPONSE

MESOSCALE

OF THE MARINE BOUNDARY LAYER TO VARIATIONS IN SEA-SURFACE TEMPERATURE

J.A. BUSINGER National Center for Atmospheric Research (NCAR) *, P.O. Box 3000, Boulder, CO 80307 (U.S.A.) W.J. S H A W

Department of Meteorology, Naval Postgraduate School, Monterey, CA (U.S.A.)

ABSTRACT Businger, J.A. and Shaw, W.J., 1984. The response of the marine boundary layer to mesoscale variations in sea-surface temperature. Dyn. Atmos, Oceans, 8: 267-281. The effects of variations in sea-surface temperature on the surface fluxes of the marine atmospheric boundary layer have been investigated. The boundary model developed by Brown and Brown and Liu has been used to estimate these effects for near neutral conditions. Data taken on September 1, 1978, during the JASIN experiment have been used to corroborate the results obtained with Brown's model. Some speculations on secondary effects of the sea-surface temperature are given.

1. INTRODUCTION O v e r m o s t o f the o c e a n s the m a r i n e b o u n d a r y layer is n e a r l y neutral. U s u a l l y the air in the m a r i n e b o u n d a r y layer has travelled long distances o v e r w a t e r a n d has c o m e close to e q u i l i b r i u m with the u n d e r l y i n g ~'urface. Because the fluxes at the i n t e r f a c e are m o s t sensitive to variations in stability in n e a r n e u t r a l c o n d i t i o n s , v a r i a t i o n s in sea surface t e m p e r a t u r e m a y h a v e a n o t i c e a b l e effect o n the m a r i n e b o u n d a r y l a y e r . It i s o u r i n t e n t to estimate the m a g n i t u d e o f the effect o n the fluxes b y variations in sea-surface t e m p e r a t u r e , using the b o u n d a r y layer m o d e l b y B r o w n (1981) a n d B r o w n a n d Liu (1982). T h e s e estimates h a v e b e e n c o m p a r e d with the v a r i a t i o n s in fluxes a n d sea-surface t e m p e r a t u r e o b s e r v e d d u r i n g the J A S I N e x p e r i m e n t o n S e p t e m b e r 1, 1978. * The National Center for Atmospheric Research is sponsored by the National Science Foundation. 0377-0265/84/$03.00

© 1984 Elsevier Science Publishers B.V.

268 2. T H E E F F E C T O F STABILITY

A good deal of effort has gone into the search for a simple parameterization of the surface stress, given the geostrophic wind and the stratification of the boundary layer. Probably the first effort to relate the geostrophic wind to the surface stress which included the effect of stratification was made by Rossby and Montgomery (1935). However, not until Lettau (1959) presented his careful analysis of existing data, did it become evident that variations in stability in near neutral conditions have a dramatic effect on the fluxes of sensible and latent heat as well as on stress. Present day models do account for stability more or less accurately. We have adopted the model by Brown (1981) to estimate the effects of variations in stability on the fluxes. This model which was originally designed for use over land has been modified to be suitable for use over water (Brown and Liu, 1982). It incorporates the average effects of secondary flows, the drag coefficients proposed by Kondo (1975) (Fig. 1), and the diabatic surface layer profiles given by Businger et al. (1971). Figure 2 has been constructed with the use of Brown's model. It displays the effect of stability by using the difference in temperature between air (10 m) and sea surface, on u,/G (the so-called geostrophic drag coefficient), where G is the magnitude of the geostrophic wind. It is clear that the X 10- 3 20 t.6

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269 TABLE I Effect of change in stability in near-neutral conditions on the fluxes as calculated from the model Quantity

(A) Neutral

(B) Unstable

(C) Stable

(D) Average

T . - T, = 0

T . - T~ = - 1

T . - T~= + 1

(B+C)/2

u , (m s -1) u 2, (m 2 s - 2 )

0.285 0.082

0.32 0.102

0.235 0.055

0.28 0.079

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0.82 0.82

1.02 1.37

0.55 0.55

0.79 0.96

0.0 0.0 2.7 - 0.77 -2.20 69 0.0

- 4.3 - 1.37 3.3 - 1.41 -4.5 141 17

2.35 0.55 2.1 - 0.49 -1.15 36 -7

- 0.97 -0.41 2.7 - 0.95 -2.83 89 5

B , × 102 ( o C) u , O , × 102 ( ° C m s - l ) Aq (g kg -1) q , × 104 (g k g - 1) u , q , × 1 0 s ( g k g -1 s -1) F~ ( W m - a ) F h (W m - 2 )

u . = friction velocity, C D = geostrophic drag coefficient ( u 2 . / G 2), Ce = bulk transfer coefficient for water vapor, C h = b u l k transfer coefficient for sensible heat, 0 . = - w ' O ' / u , ; q . = - w ' q ' / u . , q = specific humidity, Fc = latent heat flux, F h = sensible heat flux. SST varies + 1 o C with T = 12 o C, G = 10 m s - 1, and 70% relative humidity in the boundary layer.

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270

maximum sensitivity is around ( T ~ - Ts)= 0 (the neutral case) and that for small changes in Ta - T~ relatively large changes in u . / G occur. The curves in this graph are calculated for the boundary layer in dynamic equilibrium with the underlying surface. In the exercise that follows we have assumed that the boundary layer remains in this dynamic equilibrium when the air moves over a varying sea-surface temperature. In reality we might expect that when the air moves across a temperature front that it will take some time for the boundary layer to adjust and that the fluxes show larger deviations than would be obtained with the equilibrium assumption. We believe, therefore, that our assumption leads to conservative estimates of the variations in fluxes. In Table I the equilibrium fluxes are given for a geostrophic wind, G = 10 m s-1 and for Ta - ~ = 0, - 1 , and + 1, respectively. The average sea-surface temperature T~ = 12 ° C and the relative humidity in the boundary layer is 70%. It is clear from this table that the latent and sensible heat fluxes increase substantially above the neutral case. The net effect is that a boundary layer which is neutral over a uniform sea-surface temperature (SST) becomes slightly unstable over a variable SST with the same average temperature as the uniform SST case. On the other hand, the effect on the stress in the average is negligible. However, the large variations in stress which are induced by the SST have other consequences which will be discussed in section 4. In the following section we shall look at a data set with significant mesoscale variations obtained during JASIN. 3. T H E C A S E S T U D Y O F S E P T E M B E R 1, 1978 D U R I N G J A S I N

3.1. Description The Joint Air Sea Interaction (JASIN) project has been described by Pollard (1978), and a number of results have been presented at a Royal Society discussion meeting in June 1982 which appeared in Royal Society (1983). We will restrict the description of J A S I N therefore to the information relevant to the case study of September 1. This day was selected because both the Electra and U C C130 flew the entire J A S I N triangle in sequence, which allowed us to carefully examine the mesoscale structure of the marine boundary layer and the sea surface. On August 30 and 31 a well-developed cold front passed through the J A S I N meteorological triangle, leaving the J A S I N area in a relatively cool northwesterly flow of 8 - 10 m s -1 near the surface which had weakened somewhat by September 1 (Fig. 3). Two surface weather chart sources, the British Meteorological Office (BMO) and the U.S. National Weather Service (NWS) agree on the cold frontal passage, but diverge in the details of their

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interpretation for September 1. The BMO analysis advances a very weak warm front from the southwest to within 1 0 0 - 2 0 0 miles of the JASIN triangle by 1200Z. The NWS, on the other hand, does not show a front, but rather a broad surface high of 1023 mbars centered 6 0 0 - 8 0 0 miles southwest of the JASIN area. Both analyses show relatively constant pressures of 1018 mbars over JASIN with the gradient slowly weakening during the day. Electra sampling of the boundary layer began at - 0630 G M T with a profile descent of 2.5 m s-1 at the Meteor and ended - 1000 G M T with a profile ascent, also at the Meteor. Figure 4 shows temperature and dewpoint from these traces. The descent shows the height of the synoptic inversion to be well-defined at - 1 km. The inversion lowers and becomes less sharp by the time of the ascent. The boundary layer at the Meteor was capped by stratus and stratocumulus with bases at 500 m during both profiles. The mean ABL temperature remained constant over the 4 h between profiles. During this period the Electra flew the JASIN triangle twice in the following sequence: from Meteor to Hecla to Endurer to Meteor and then the reverse from Meteor to Endurer to Hecla to Meteor. Figure 5, the satellite photograph, shows the various types of cloud features observed during the flight of the triangle. In the southern and western portions of the triangle, the ABL was topped by occasionally broken stratus and stratocumulus, with some altostratus present. As the Electra

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273

proceeded northeast, clouds became predominantly cumulus, with large breaks of clear sky between. Near the end of the first leg, the Electra passed what appeared to be a relatively large cumulonimbus to the east. The westward leg from Hecla to Endurer was distinguished by the presence of 4 or 5 distinct cloud lines oriented at an acute angle to the flight path. Because of the angle of interception, it was not possible to measure the width and spacing of the lines. However, they were irregularly spaced and had clear sky in between. The final leg of the first triangle showed little evidence of these cloud lines. What appeared to be a cloud line was evident on the eastern horizon at the beginning of the leg, hut the remainder of the leg appeared to be in a stratus and stratocumulus-topped ABL.

Fig. 5. Satellite photograph including the JASIN area on September 1, 1978 coincident with the Electra's second circuit (clockwise) of the triangle. Included are estimates of the Obukhov length from the circuit flown by the C130 following the Electra.

274

The reverse triangle has approximately the same features as the first one. The satellite photograph corresponds to the time just after the Electra had left the Hecla on its final leg toward the Meteor. Shortly after beginning this leg, the Electra passed under a cumulus congestus line which appeared to terminate in a sizeable cumulonimbus cloud to the east. This feature and the others previously described are evident in the satellite photograph, which indicates a broad region of stratus and altostratus in the southern and western part of the triangle and cloud lines in the northern and eastern portion, which increase in wavelength and finally break up into cellular convection east of the triangle.

3.2. The sea-surface temperature (SST) Both the Electra and the C130 were equipped with a PRT5 radiometer which senses the SST remotely. The measurements with this instrument usually indicate a bias and a drift. In flying the triangle the aircraft passed over the c o m e r ships which measured the SST directly (i.e., the bucket temperature). The aircraft observations were corrected to read the bucket temperature over the c o m e r ships. This resulted in a slight overestimation of the SST because the interface temperature is usually cooler than the bulk temperature due to heat loss at the surface, mainly evaporation and outgoing long wave radiation. However, under the conditions of near neutrality the vapor flux was rather modest and the difference between bucket temperature and interface temperature was probably < 0.2 o C. Another correction is due to reflection of the sky because the sea surface is not completely black for the wavelength band the Barnes radiometer senses, but has a reflectance of - 0.014 (Saunders, 1970). This correction can be made by using an upward looking PRT6 to determine the sky tempera-

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275 ture (Liu and Katsaros, 1983). Figure 6 displays the SST data after correction. The sea-surface temperature features from the first triangle are largely repeated in the second triangle. This indicates that the PRT5 can be used to determine the SST with a relative accuracy or resolution of + 0.2 o C and also that the SST is a slowly varying quantity, its mesoscale pattern does not change much in a few hours. Unfortunately the PRT5 on the C130 was not working during the following flight. Therefore we could not check how similar the SST still was in the afternoon of the same day. The average corrected SST from the two Electra triangles and the information available from ships and buoys were used to construct a SST map for 1 September (Fig. 7). This figure takes into account some of the features presented in the analysis presented in Guymer et al. (1983) for the period from 27 August to 5 September.

3.3. The fluxes The fluxes of momentum, sensible and latent heat were determined from the aircraft using the eddy correlation technique. The Electra was equipped with a gust probe, a Rosemount temperature sensor, a K-probe temperature sensor developed at NCAR, and Lyman-ot humidiometer. Unfortunately the Lyman-a was not adjusted for the humidity range in the boundary layer and

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276

did not provide useful data. The C130 was also equipped with a gust probe and a Rosemount temperature sensor, but had a microwave refractometer as humidity sensor. More detailed information about the aircraft instrumentation can be found in Nicholls et al. (1983). As can be seen from the SST map, Fig. 7, the temperature variations were most pronounced along the northern leg of the triangle from the Endurer to the Hecla. The distance is - 220 km. This leg has been selected to display the various quantities that were observed by the aircraft, which has been done in Fig. 8a, b. Near the Endurer the SST is relatively high, in the middle of the leg the temperature is low. From the middle to Hecla the temperature goes through a maximum and then drops to a low value at the Hecla. The temperature variation was only - 1 ° C, so no dramatic flux variations were expected. The wind speed and direction show a significant change betweeen the first Electra crossing, E(2), and the second crossing, E(5). The crossing, C(2), of the C130 which was several hours later, indicates wind speed and direction quite similar to E(5). The air-sea temperature difference, the m o m e n t u m flux and the vapor flux all display a shape which is similar to the SST. The vapor flux especially seems to be sensitive to the SST. The amplitude of the variations is about half the amplitude given in Table I for I"= 20m s-I

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Fe which is the order of magnitude we would expect because the measured SST variation is also about half the variation assumed in the table. The average geostrophic windspeed corresponds to the one assumed in the table. From the information in Table I we expect that the actual values of the sensible heat flux are < 10 W m - 2 . This is indeed the case. The measured heat fluxes vary between _+6 W m - 2 and although the individual 1 min averages cannot be trusted to _+ 10 W m - 2 the scatter is considerably less than that. The average sensible heat flux for the entire Electra flight was - - 0 . 5 W m - 2 w h i c h is for all practical purposes negligible. Similarly t h e average heat flux as measured by the C130 for the entire triangle was - 1 W m - 2 . This suggests that the average (T~ - Ta) = AT should have been slightly negative. Instead ATe=tr a -- 0.75 K and ATc130 = 0.4 K which indicates that the air temperatures measured by the aircraft were somewhat too low, or the SST somewhat too high. The aircraft measurements were corrected adiabatically from the height of 50 m to the surface. The latent heat flux as measured by the C130 has been plotted against Aq = qs - q~ in Fig. 9. Although the data scatter is considerable, as is to be

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Fig. 9. The latent heat flux, F~, as determined by the C130 for the JASIN triangle versus Aq = qs -- q.- The points represent 1 min averages, the dashed line is the trend prescribed by Brown's (1981) model and the solid line is the regression line through the observed points. expected for I min averages and because there is some uncertainty in the Aq, the trend of the points agrees quite well with the prediction of Brown's model as presented in Table I. This has been plotted as a dashed line for comparison. However, F e = 0 does not correspond with Aq = 0 as one would expect but with Aq _ 1.7 g k g - ] . This suggests that the humidity measurements on the C130 were too low or the SST values too high. Nicholls (1983) reported on the intercomparison between the Electra and the C130 that the dewpoint on the C130 was - 0.7 K lower than on the Electra. F u r t h e r m o r e that the air temperature on the C130 was - 0.8 K higher than the Electra. If we assume that the dewpoint on the Electra and the temperature on the C130 were correct and that the SST is too high by 0.2 K then most of the discrepancies between the fluxes and AT and Aq can be explained. The slope of the measured trend varies more rapidly with the latent heat flux than the calculated one. This confirms the suggestion that the equilibrium assumption leads to conservative estimates of the variations in fluxes. 4. CONCLUDING REMARKS A careful analysis of the data taken during JA SIN on September 1, 1978 shows that the model by Brown (1981) as adopt ed for the marine b o u n d a r y layer gives a reasonable account of the variations in the surface fluxes due to variations in sea-surface temperature. T he SST variations over the JA SIN triangle although small have been clearly identified and these variations have

279

their imprint on the marine boundary layer. It is quite desirable to repeat this type of exercise over an a r e a of the ocean where the surface variations are significantly larger. The imprint of the sea surface on the marine boundary layer is systematic and this fact leads us to speculate how it may be detected through remote sensing. As a thought experiment let us assume that a steady west wind blows over a warm ocean eddy. Several interactions of this warm eddy with the atmospheric boundary layer can be visualized: (1) the boundary layer will be modified by enhanced impact of latent and sensible heat. This will tend to increase the height of the boundary layer. If there is a stratus stratocumulus deck present, this cloud layer will tend to become thicker unless the air above the boundary layer is dry and cool enough to allow entrainment instability to occur. In this case the SC-deck may break up into individual cumulus clouds; (2) the enhanced convective activity over the warm eddy will tend to cause a general inflow along the edges of the eddy and develop a modest "heat island effect"; (3) the drag coefficient over the warm water will be larger than over the cool surrounding water. Consequently there will be convergence of flow on the upwind edge of the eddy and divergent flow on the downwind edge as sketched in Fig. 10; and (4) similarly the curl of the stress, V x ~', will be positive along the north edge and negative along the south edge of the eddy. This has also been indicated in Fig. 10. It is not clear how all these interactions will combine. They may set up secondary flows with markedly different cloud patterns, etc. However, if we COOL

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280

naively assume that the various effects are additive we would expect to have maximum cloudiness in the N W region of the eddy and minimum cloudiness in the SE region. If this is so and if the boundary layer wind is more or less steady including modulation by its own eddies the effects o f the ocean eddy will have a systematic imprint on the cloudiness over this eddy. This might be observed from a satellite. Similar qualitative discussions can be given for the boundary layer interactions with a cold eddy or a meandering ocean front, etc. The challenge is to transform these speculations into reliable quantitative results. Further experimental and modeling work is needed in this area. ACKNOWLEDGMENTS

The authors of the aircraft was supported ATM77-14934

are grateful to Richard J. Lind for the careful data reduction data in relation to the sea-surface temperature. The research in part by the National Science Foundation through grants and ATM80-19753.

REFERENCES Brown, R.A., 1981. Modelling the geostrophic drag coefficient for AIDJEX: the model and the data. J. Geophys. Res., 86: 1989-1994. Brown, R.A. and Liu, W.T., 1982. An operational large scale marine PBL model. J. Appl. Meteorol., 21: 261-269. Businger, J.A., Wyngaard, J.C., Izumi, Y. and Bradley, E.F., 1971. Flux profile relationships in the atmospheric surface layer. J. Atmos. Sci., 28: 181-189. Garratt, J.R., 1977. Review of drag coefficients over oceans and continents. Mon. Weather Rev., 105: 915-929. Guymer, T.H., Businger, J.A., Katsaros, K.B., Shaw, W.J., Taylor, P.K., Large, W.G. and Payne, R.E., 1983. Transfer processes at the air-sea interface. Philos. Trans. R. Soc. London, Ser A: 308: 253-273. Kondo, J., 1975. Air-sea bulk transfer coefficients in diabatic conditions. Boundary-Layer Meteorol., 9: 91-112. Large, W.G., 1979. The turbulent fluxes of momentum and sensible heat over the open sea during moderate to strong winds. Ph.D. Thesis, Univ. British Columbia, 180 pp. Lettau, H.H., 1959. Wind profiles, surface stress and geostrophic drag coefficients in the atmospheric surface layer. Adv. Geophys., 6: 241-257. Liu, W.T. and Katsaros, K.B., 1984. Mesoscale variations of sea-surface temperature and air temperatures as measured on an airplane during JASIN. J. Geophys. Res. (submitted). Nicholls, S., 1983. An observational study of the mid-latitude, marine, atmospheric boundary layer. Ph.D. Thesis, Univ. Southampton, 188 pp. Nicholis, S., Bri~mmer, B., Fiedler, F., Grant, A., Hanf, T., Jenkins, G., Readings, C. and Shaw, W., 1983. The structure of the turbulent atmospheric boundary layer. Philos. Trans. R. Soc. London, Set A: 308: 291-309. Pollard, R.T., 1978. The Joint Air-Sea Interaction Experiment--JASIN 1978. Bull. Am. Meteorol. Soc., 59: 1310-1318.

281 Rossby, C.G. and Montgomery, R., 1935. The layers of frictional influence in wind and ocean currents. MIT paper 3: 3-101. Royal Society, 1983. Results of the Royal Society Joint Air-Sea Interaction Project (JASIN). The Royal Society, London, 229 pp. Saunders, P.M., 1970. Corrections for airborne radiation thermometry. J. Geophys. Res., 75: 7596-7601. Shaw, W.J., 1982. A study of coherent structures in the near-neutral marine atmospheric boundary layer. Ph.D. Dissertation, Univ. Washington, Seattle, 197 pp. Smith, S.D., 1980. Wind stress and heat flux over the ocean in gale force winds. J. Phys. Ocean., 10: 709-726. Smith, S.D. and Banke, E.G., 1975. Variations of the sea surface drag coefficient with wind speed. Q. J. R. Meteorol. Soc., 101: 665-673. Wu, J., 1969. Wind stress and surface roughness at air-sea interface. J. Geophys. Res., 74: 444-455. Wu, J., 1980. Wind-stress coefficients over sea surface near neutral conditions--a revisit. J. Phys. Ocean., 10: 727-740.