1. A Study of Variability in Tropical Tropospheric Water Vapor Robert L. Herman 1, Robert F. Troy 2, Holger Voemel 3, Henry B. Selkirk 4, Susan S. Kulawik 1, Gregory J. Flesch 1, Kevin W. Bowman 1, Annmarie Eldering 1, Ming Luo 1, Helen M. Worden 1, and Reinhard Beer 1 1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 2. Dept. of Earth and Space Sciences, UCLA, Los Angeles, CA 3. CIRES, Univ. of Colorado, Boulder, CO 4. Bay Area Environmental Research Institute, Sonoma, CA http://tes.jpl.nasa.gov A51A-0054 ABSTRACT Water vapor plays a significant role in climate forcing, but is challenging to characterize due to variability on short spatial and temporal scales. The recent Costa Rica Aura Validation Experiment (CR-AVE) and Ticosonde/CR-AVE 2006 Project characterized water vapor, temperature, and ozone (among other species) in the tropical troposphere and lower stratosphere over Costa Rica and the Eastern Pacific Ocean in the January-February timeframe. We will analyze the variability of water vapor in the tropical troposphere from these new observations, as well as A-Train satellite water retrievals. Measurements are provided by the Tropospheric Emission Spectrometer (TES) on the EOS Aura platform, cryogenic frostpoint hygrometers launched during Ticosonde, and aircraft in-situ instruments on the NASA WB-57F high-altitude research platform. ACKNOWLEDGMENTS The TES science team is supported by the NASA Aura Program. JLH thanks the NASA Upper Atmosphere Research Program and Radiation Sciences Program for support. Robert Troy is supported by a NASA Earth Science Graduate Fellowship. Thanks to Eric Jensen and Paul Newman, CR-AVE Project Scientists, and the field measurement team. Part of this research was carried out by JPL, Caltech, under a contract with NASA. BACKGROUND Comparisons of TES v002 retrievals with in-situ data have been carried out to validate the TES H 2 O retrievals in the troposphere. In the upper troposphere, sondes typically have biases due to calibration error and solar radiation error in the daytime [L. Miloshevich, pers. comm.]. For this reason, we use data from the Cryogenic Frostpoint Hygrometer (CFH) and in-situ aircraft instruments, which are thought to have higher accuracy than sondes in the upper troposphere, In order to evaluate the effectiveness of the validation, it is important to know whether differences between satellite and in-situ data are due to instrument bias or atmospheric variability. TES sensitivity to water vapor TES has sensitivity to H 2 O in the troposphere, as indicated by the TES H 2 O averaging kernels (left panels, Fig. 1 and 2). In the tropics, TES measures water up to ~150 hPa. At higher altitudes, TES v002 retrievals revert to a priori values spatially interpolated from GMAO GEOS-4. In this analysis, averaging kernels have been applied to the in-situ data for comparison with TES. Only TES retrievals that satisfy the H 2 O data quality criteria are shown. VALIDATION: Is the level of agreement between TES and in-situ H 2 O limited by spatial variability of H 2 O? Fig. 1. 22 Jan 2006 comparison between TES and WB-57F in-situ water from take-off, 10ºN, 84ºW (60-km, 1-hour proximity) shows large differences at 500-700 hPa. Fig. 2. 25 Jan 2006 comparison between TES and CFH in-situ water near San Cristobal, Galapagos Is., 0.9ºS, 89.6ºW (59-km, 1-hour proximity), shows better agreement at most pressure levels. Aqua MODIS Vis. Image courtesy of David Tobin. TROPICAL VARIABILITY: TES footprints (predicted) Scanning-HIS 900 cm-1 Tb SUMMARY TES v002 transect special observations have been compared with aircraft and balloon H 2 O profiles in the troposphere. TES v002 H 2 O retrievals are in better agreement with in-situ data provided the measurements are close in space and time, and the H 2 O field is spatially uniform (25 Jan 2006). On days with relatively uniform H 2 O, differences in H 2 O profiles are seen even on a 12-km scale. H 2 O validation efforts are limited by the spatial variability of H 2 O. A statistical approach is recommended. Fig. 3. Left: TES transect on 25 Jan 2006 shows relatively little H 2 O variability. Center: differences between CFH and TES are within TES error and within 1 st. dev. of TES retrieval variability except at 500-750 hPa. Right: fractional difference for 17 TES retrievals within 200 km of CFH. Differences up to 30% are seen between two TES retrievals 12 km apart and closest to CFH (red and green lines). Fig. 4. H 2 O curtain plots from two TES transects (22 Jan and 1 Feb 2006) show more spatial variability than 25 Jan 2006, particularly in the middle troposphere.