Synoptic Scale Wind Field Properties from the SEASAT SASS
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NASA Contractor Report 3810 NASA-CR-381019840023662 l Synoptic Scale Wind Field Properties From the SEASAT SASS Willard J. Pierson, Jr., Winfield B. Sylvester, and Robert E. Salfi I GRANTNAGW-266 '" " . - - : __::7i!. ! JULY 1984 ' " ' 1 I NASA Contractor Report 3810 Synoptic Scale Wind Field Properties From the SEASAT SASS Willard J. Pierson, Jr., Winfield B. Sylvester, and Robert E. Salfi CUNY Institute of Marine and Atmospheric Sciences at the City College New York, New York Prepared for NASA Office of Space Science and Applications under Grant NAGW-266 \ National Ae[0nautics and SpaceAd\ministration ScientificandTechnical InformationBranch 1984 TABLE OF CONTENTS Introduction................................................. 1 - 3 Data......................................................... 4 - 7 Theoretical Considerations ................................... 8 - 12 Preliminary Data Processing .................................. 13 - 23 The Field of Horizontal Divergence ........................... 24 - 25 Boundary Layer and Wind Stress............................... 26 - 30 The Calculation of the Stress................................ 31 35 The Curl of the Wind Stress .................................. 36 - 37 Synoptic Scale Vertical Velocities ........................... 38 - 40 Independence and Covariances ................................ 41 43 Edges ........................................................ 44 Summary of Data Processing Methods ........................... 45 - 48 General Discussion........................................... 49 - 53 Programs, Data Tapes, Data Listings and Contoured Results.... 54 Result from the Preliminary Data Processing Procedures....... 55 - 74 Comparison Conventional Data and Their Analyses.............. 75 - 79 Revolution 1141 ............................................. 80 - 90 Revolution 1183 ............................................. 91 99 Revolution 1212 ............................................. I00 - 107 Revolution 1298 ............................................. 108 - 115 Wind Stress, Wind Stress Curl and Ocean Currents............. 116 - 141 Sea Surface Atmospheric Pressure Fields...................... 142 - 143 Conclusion ................................................... 144 - 145 Acknowledgements ............................................. 146 References ................................................... 147 - 152 Appendix A, B, C and D iii INTRODUCTION The ability to determine the winds in the planetary boundary layer over the oceans of the Earth by a radar scatterometer called the SASS on SEASAT has been demonstrated. The objectives of the SASS program were met, and perhaps exceeded, as described by Lame and Born (1982). A problem was identified when it was recognized that errors in the conventional data need to be better understood. The sources for the differences between a SASS-I winds, a conventionally measured wind, and a wind derived from a planetary boundary layer analysis using conventional data are many and varied. Yet to be produced are planetary boundary layer analyses of the winds based on the SEASAT SASS-I data only, augmented by a minimum amount of conventional data such as atmospheric pressures at the sea surface and air sea temperature differences. The purpose of this investigation is to prepare and analyse wind fields from the SASS data. The three most important applications of SEASAT-SASS-Iike data in the future will be (i) the correct description of the winds over the entire global ocean at an appropriate resolution, (2) the use of these data to produce vastly improved initial value updates for computer based synoptic scale numerical weather predictions and (3) as shown, by O'Brien, et al. (1982), the specification of the wind stress field and the curl of the wind stress at the sea surface for oceanographic applications. In this investigation, synoptic scale vector wind fields with a known error structure will be produced at a one degree resolution from the SEASAT SASS-I GOASEX data. These synoptic scale wind fields will be used to compute fields for the horizontal divergence of the winds in the planetary boundary, the vertical velocity at 200 meters, and to determine the error structure of the resulting fields. The vector wind stress fields can then be found from the vector wind fields, and the errors in these fields can be computed. Finally, the curl of the wind stress can be computed, with a specified error structure. Certain assumptions need to be made concerning the accuracy of the present SASS-I model function and the wind recovery algorithm. Also the drag coefficient that relates wind stress to the wind at ten meters is a matter of some uncertainty. If any of these assumptions need modification and updating for future systems,the main results of this study will.still be applicable. As a first effort, the atmospherewill be assumed to be neutrally stratified. An applicationof the Monin-Obukhovtheory for non-neutral stabilityas in Large and Pond (1981)would provide an improvedwind field at 19.5 (or i0) meters for the computationof divergence. The field for the air-sea temperaturedifferencewould be needed to an accuracy comparableto the neutral stabilitywind field. Technicalreports concernedwith the accuracyof the SASS winds have been written by Weller, et al. (1983)and Wentz and Thomas (1983). The former, if read too quickly, seems to suggestthat the meteorologically measuredwinds used to calibratethe SASS-I model functionfrom 4 to 16 m/s were 10% too high comparedto their true value. Closer study suggests perhaps 5 to 7 percent, and in our opinion,a closer analysis of Figs. 8 and 9 of their paper and of Tables 2 and 3 indicatepoor data analysis methods and suggest that the SASS-Imeteorologicalwinds were close to the mark. The SOS-SASS-Iwind recoveryalgorithmis questionable(Pierson 1983c)because it tends to give too high a wind for the 2 solutioncase and the "Y" base of the three solutioncase, which is the reason for the deletion of some of the data providedby the NASA Langley ResearchCenter (page 13). Wentz and Thomas (1983)derivewhat is claimed to be a model function superiorto SASS-I for which the wind speed "seems to be biassed about 1 m/s too high'_.The SASS-Imodel functioncan probablybe improvedoutside the range from 4 to 16 m/s. The SOS SASS-Irecoveryalgorithmis possibly the reason for part of these differences. Calibrationover a greater range of wind speeds and aspect angleswill be difficult,but much of the GOASEX data fall within the range from 4 to 16 m/s. A quotation from Guymer (1983a) summarizes the situation prior to Weller, et al. (1983). "The absoluteaccuracyof the measurementshas proved difficultto ascertain,and even subsequentcalibrationsand intercomparisons on land have failed to resolve the question (Weller personal communication)". Woiceshyn,et al. (1983)have provided the most convincingevidencethat there are difficultiesfor both V pol and H pol pairs for both very low and very high winds when these are comparedwith each other. The situationneeds further study along the lines outlinedby Pierson (1983c). The difficulties brought to light by this study show that the S.O.S. wind recovery algorithm, Jones, et al. (1982),is the major source of the discrepancy between co-located winds from V pol pairs and H pol pairs for low winds, especially for high incidence angles. The incorrect assumption that backscatter in either decibels (or bels) is normally distributed about its expected value as given by the model function in decibels, or bels, requires the determination of an equivalent variance that cannot be recovered if K is greater than one. SASS backscatter P values were discarded for three reasons; namely, (i) K > i, (2) received o P power negative, and (3) _ < -50 db. Since H pol is much weaker than V pol at high incidence angles, a large fraction of the H pol measurements were discarded for low winds and the winds that were recovered were biassed high. The analyses to follow may have winds that are too high in areas of light winds so that gradients in such areas will be increased. For winds above 8 or 9 m/s, the effects will not be too important. The methodology described in this paper can be re-applied to corrected wind fields by means of the use of MLE's as in Pierson and Salfi (1982). DATA The data used in this investigation were provided by the NASA Langley Research Center. They consist of the de-aliased SASS-I vector winds produced as the final product of the analysis of the SASS data for the SEASAT orbit segments chosen for intensive study during the Gulf of Alaska SEASAT Experiment (GOASEX). Preliminary results for this experiment are described by Jones, et al. (1979). Two workshop reports, Born, et al. (1979) and Barrick, et al. (1979), plus a summary of conventional data analyses, Woiceshyn (1979),give additional details. The SASS-I model function was based on the JASIN data with results summarized by Jones, et al. (1982), Schroeder, et al. (1982) Moore, et al. (1982) and Brown, et al. (1982). As described by Jones, et al. (1982), the GOASEX data were reprocessed by means of the SASS-I model function. De-aliasing was accomplished by selecting that wind direction closest to the planetary boundary layer wind fields obtained from conventional data. The model function recovers the effective neutral stability wind at 19.5 meters. The winds recovered from the SASS are very densely concentrated over the swath scanned by that instrument on SEASAT. Examples can be found for the raw data density in Wurtele, et al.