Satellite Retrieval of Atmospheric Water Budget Over Gulf of Mexico-Caribbean Sea Basin Pablo Santos 1 & Eric A. Smith 2 1 National Weather Service, Miami, FL [Florida State Univ. Dept. of Meteorology Graduate Student Program] 2 NASA/Goddard Space Flight Center, Greenbelt, MD Presented at 81st AMS Annual Meeting Conference on Global Change & Climate Variations [January 14-18, 2001; Albuquerque, NM]

Motivation and Objectives n interest in understanding all terms of atmospheric branch of hydrological cycle within enclosed ocean basin n interest in quantifying water budget components from satellite measurements only n interest in extending past research which has been restricted largely to requiring balance between divergence of WV transport & E - P n develop remote sensing-based & portable retrieval methodology, using measurements from SSM/I & GOES satellites for calculating atmospheric water budget, verifying divergence residue with radiosonde-based estimates n quantify mean & variance properties of Gulf of Mexico- Caribbean Sea Basin atmospheric water budget in time-space n understand drawbacks within convectively active region under assumption that local rate of change of PW and cloud LWP are negligible in context of regionally & seasonally based atmospheric water balance

Study Area

Data n Research utilizes data from three sources: DMSP (SSM/I) –used to retrieve precipitable water (W), cloud liquid water path (W c ), precipitation (P), surface air temperature (Ta), surface specific humidity (qa), & surface wind speed (U) GOES (Imager) –used to retrieve sea surface temperature (SST) & surface air temperatue (Ta) -- plus W, W c, P, & qa with adjustments by SSM/I Upper Air T-q-V profiles

Methodology n development of algorithms for water budget variables n calculation & validation of water budget n analysis of water budget atmospheric water budget equation for vertically integrated total water in atmosphere where: W = precipitable water; W c = cloud liquid water path; Q = vertically integrated water vapor transport; E = evaporation; & P = precipitation :

Surface Specific Humidity calculated from GOES as function of SST & adjusted by SSM/I qa via Schlüssel (1996) SSM/I ∑ NESDIS SSM/I alg (Ferraro et al., 1998) cross-validated against 4 other SSM/I algs ∑ SSM/I retrievals within expected uncertainty of ground truth (Smith et al., 1998) SSM/I Greenwald et al. (1995), Lojou (1994), Petty (1994), Wentz (1995) GOES Split Window Technique Chesters (1980), Crosson et al. (1993) Combined SSM/I-GOES GOES W estimate adjusted by SSM/I SSM/I Alishouse (1990), Greenwald et al. (1995), Lojou (1994), Weng and Grody (1994) Combined SSM/I-GOES ∑ GOES detects cloud area ∑ GOES LWP estimate equals closest SSM/I retrieved mean cloud LWP in time ∑ data from two SSM/Is are used to increase data sampling S ea Surface Temp (GOES) Legeckis (1997) Surface Wind Speed (SSM/I) Bates (1991), Clayson & Curry (1996), Schlüssel (1996) GOES E estimate uses closest in time SSM/I mean U retrieval from two SSM/Is Surface Air Temperature (SSM/I-GOES) Clayson and Curry (1996) SSM/I - cld classification GOES - cld top temp & SST Ta func of cld type & SST Combined SSM/I-GOES ∑ GOES retrievals based on probability matching (PM) between SSM/I retrieved rainrates & GOES EBBTs ∑ PM technique based on work of Turk et al. (2000) & Grose et al. (2000) Rain Screening Ferraro et al. (1998) Cloud detection based on NESDIS & NAVY operational screening techniques Legeckis (1997) & May (1998)

Water Budget Calculation Once precipitable water (W), cloud liquid water path (W c ), precipitation (P), & evaporation (E) are retrieved at each grid point, divergence of vertically integrated water vapor transport ( —∑ Q) for time t at grid coordinate (i,j) is calculated using centered difference form of water balance equation, as follows:

Validation of Water Budget n Divergence of water vapor transport obtained as residue in water balance equation. n Residue term compared to direct measure of divergence obtained from ring of operational radiosonde stations surrounding basins in study area. n Direct measure obtained from line-integral technique (application of Greens’ theorem) in conjunction with sounding ring:

Analysis n Algorithm sensitivity analyses and multiple-algorithm intercomparisons are being used to define retrieval system uncertainties. n Budget calculations are being performed on 2 x 4 km grid mesh with hourly mean composites generated on 0.25 x 0.25 degree grid mesh for 6 one-month periods (Oct 1997; Jan, Apr, Jul, Oct 1998; Jan 1999). n Analyses are aimed at quantifying regional water budget variability of Gulf of Mexico-Caribbean Sea Basin to enable pursuit of key scientific objectives. n Final conclusions are to be based on analyses of filtered daily time series of sub-regional averages & gridded monthly/diurnal-composites with respect to individual terms of water budget equation.

Algorithm Intercomparison Results - Jan 98

Precipitable Water (W) intercomparison results (Jan/Jul 98) SSM/I-GOES W algorithm (Santos Adj-GOES) used for final budget calculations

Cloud Liquid Water Path (W c ) intercomparison results (Jan 98) Alishouse SSM/I algorithm used as control for GOES-8 retrievals SSM/I-GOES W c algorithm used for final budget calculations

Precipitation (P) intercomparison results (Jan 98) NESDIS SSM/I algorithm used as control for developing probability matching lookup tables between SSM/I rainfall & GOES-8 EBBTs (these tables used for GOES-8 rain retrievals) SSM/I-GOES P algorithm used for final budget calculations

Sea Surface Temperature (SST) intercomparison results (Jan/Jul 98) SST is input parameter for Clayson & Curry (1996) Evaporation algorithm Legeckis GOES algorithm used for final SST retrievals

Surface Air Temperature (Ta) intercomparison results (Jan/Jul 98) Ta is input parameter for Clayson & Curry (1996) Evaporation algorithm SSM/I-GOES algorithm (modified Clayson & Curry where GOES used in deriving cloud type) used for final Ta retrievals

Surface Specific Humidity (qa) intercomparison results (Jan/Jul 98) qa is input parameter for Clayson & Curry (1996) Evaporation algorithm SSM/I-GOES algorithm used for final qa retrievals

Surface Wind Speed (U) intercomparison results (Jan/Jul 98) U is input parameter for Clayson & Curry (1996) Evaporation algorithm Schlüssel SSM/I algorithm used for final U retrievals

Ocean surface latent heat flux from Clayson & Curry (1996) Evaporation algorithm (based on surface renewal theory) Comparisons to vertical q gradient over surface layer [qs(SST) - qa] & to surface wind speed (U) [Jan 98]

Ocean surface latent heat flux from Clayson & Curry (1996) Evaporation algorithm (based on surface renewal theory) Comparisons to vertical q gradient over surface layer [qs(SST) - qa] & to surface wind speed (U) [Jul 98]

Jan 98 Hourly Composites of Rate of Change of Precipitable Water (W)

Jan 98 Hourly Composites of Rate of Change of Cloud Liquid Water Path (W c )

Jan 98 Hourly Composites of Input Parameters for Clayson (1996) Evaporation Algorithm: Sea Surface Temperature (SST), Surface Air Temperature (Ta), Surface Specific Humidity (qa)

Jan 98 Hourly Composites of Surface Evaporation (E)

Jan 98 Hourly Composites of Divergence of Water Vapor Transport ( —∑ Q)

Time Series Comparisons of Retrieved versus Sounding-Based —∑ Q

20-day time series of —∑ Q retrieved from satellite (green lines) compared to value calculated using line integral technique applied to sounding data (red lines) series extends from day for Jan 98 at 00Z with value for each day corresponding to 10- day average centered on respective day

20-day time series of —∑ Q retrieved from satellite (green lines) compared to value calculated using line integral technique applied to sounding data (red lines) series extends from day for Jan 98 at 12Z with value for each day corresponding to 10- day average centered on respective day

Summary n A portable satellite-based retrieval system for obtaining the atmospheric water budget has been developed for applications with combination of SSM/I & GOES data. n Preliminary single layer (troposphere) calculations have been generated for January-1998 over Gulf- Caribbean Basin (3-layer calculations will be made). A verification analysis is underway to determine the validity of retrieving the (E - P) and ∂(W+W c )/∂t terms of the atmospheric water budget while obtaining —∑ Q as a residue term, thus circumventing the need for measurements of the wind velocity profile. n Analysis is underway to determine the uncertainty imposed on a regional-seasonal atmospheric water budget in a highly convective tropical basin, by ignoring the W and W c storage terms.

Preliminary Conclusions n Preliminary results for January-1998 indicate that the algorithm system captures the main features of the regional budget on both daily and monthly/diurnal- composite time scales. Budgets for Oct-97, Apr-98, Jul- 98, Oct-98, and Jan-99 are also being calculated to understand how well the methodology represents seasonal variability over an annual cycle. Although validation checks are limited by the lack of continuous upper air data over the southern half of the Gulf of Mexico & parts of the Caribbean Sea basins and given that there are episodes of poor sounding data, preliminary —∑ Q intercomparisons are within sufficient agreement to justify pursuing technique development and the underlying budget estimates.