SPACEBORNE_MICROWAVE_OBSERVATIONS_OF_RAIN 1972-1997.ppt

• 1. SPACEBORNE MICROWAVE OBSERVATIONS OF RAIN 1972-1997
  • Thomas T. Wilheit

• Texas A&M Univ.

• Alfred T. C. Chang

• Formerly of NASA/GSFC

2. Period Runs from Launch of ESMR on Nimbus 5 to Launch of TRMM Al Chang (no longer with us) was a key player in all ofthis. Graphics a messy problem PowerPoint didnt exist in 1972 Color was rare. Organization of Talk IR-Based Rainfall Retrievals Passive Microwave over Ocean Passive Microwave over Land Profiling Algorithms Algorithm Workshops Radar 3. IR Measurements Estimates of Rainfall As far back as the 60s it was noted that cold clouds (IR) and bright clouds (VIS) were correlated with rainfall. Bright clouds seemed slightly better correlated but nighttime problem Thus, IR measurements attracted more interest. Geosynchronous satellites provided enough observations to be very useful. IR rain algorithms became a cottage industry. Precipitation Measurements from Space Workshop (1981) had presentations on 6 different techniques.I dont think any are in current use. 4. Phil Arkins Method One simple method seems to have survived. Phil Arkin used the ship radar data from the 1974 GARP Atlantic Tropical Experiment (GATE) and Geosync data to examine correlations between cloud top temperature and rainfall.6 hour accumulations had~80% correlations with cloud top temperatures colder than 235K.(slope ~ 3mm/hr) i.e. If the cloud top is colder than 235, its raining 3 mm/hr.Shows a useful degree of skill. Assorted refinements over the years The point is the desperation of the meteorological community for rainfall data. 5. Microwave Radiometry Comes to the Rescue Launch of Nimbus 5 December 1972 Electrically Scanned Microwave Radiometer:19.35 GHz,25km @nadir 50 cross-track scan (45 x 165 km resolution @ edges), H-pol 6. ESMR Calibration Accuracy ca. 5K (@Nadir)NE T ca. 2KSingle Frequency so Geophysical Noise was Worse. Initial Images Had Terrible Streaks.Traced to Cross Polarized Grating Lobes.Transformed so that All Beam Positions Statistically Like Nadir BP Coastlines and Ice Edge Blurred in Mosaics Predictive Ephemerides were Lousy. Deployment Mechanism Made the Antenna Rock 6 p-p After all this was cleaned up we could do some science. 7. Typical Quick Look Image from ESMR Land Features Obvious Features over the Ocean with TBs too high to be explained by SST, Wind, Non-Raining Cloud 8. 9. 10. 11. Quantitative Theory OK, we can see rain (of someunspecified intensity) over the ocean. Can we be more quantitative? Ed Rodgers and Merle Rao compared ESMR data with WSR-57 radar data from Miami.It looked reasonably good, but we needed a theory. Equations for radiative transfer in rain are messy but well-known. But how to solve them???To get the radiance in any one direction, we need the radiance scattered in from all other directions. Bob Curran had brought a program originally written by Ben Herman (U. Arizona) to GSFC.He gave Al Chang a copy and Al converted it for microwave. Now what do we put into the equation of radiative transfer? 12. 13. 14. 19.35 GHz Ground-Based 37 GHz Ground-Based ESMR vsMiami WSR-57 15. Early Applications Merle Rao, Bill Abbott and John Theon collaborated to generate an atlas of oceanic rainfall from ESMR Quality control problemsmostly from ephemerides. Freezing level problem First observation of the South Atlantic Convergence Zone? Bob Adler and Ed Rodgers looked at the energy balance of a hurricane Results were reasonable Beam Filling was ignored in all these applications 16. SSM/I First SSM/I was launched on DMSP F-8 in 198719.35 22.235, 37 & 85.5 GHz Dual Pol except @ 22.(85V failed earlyon F-8) 6 Subsequent Copies Additional Channels and Better CalibrationBetter Rainfall Retrievals. Rain Algorithm Developed for Global Precipitation Climatology Project 5 x 5 x 1 Month Boxes Freezing Levelfrom 19V /22V combination Rain from Histograms of 2*TB19V TB22VLinear Combination Mitigated Water Vapor Variability Fit Parameters of Mixed Log-Normal Rain PDF to TB histogram Chius Beam Filling Correction 17. 18. 19. SSM/I Derived Rainfall AmountAugust 1987 20. What about rain over land? High & Variable emissivity makes it difficult This is the late-70s view.Basis of 85.5 GHz channel on SSM/I 21. Nimbus-6 ESMR37 GHz, Conical Scan, Dual Pol Ed Rodgers and Honnappah Siddalingaiah looked at ESMR-6 over land 22. 1978 Tropical Storm Cora Flight Dont trythis kind of logic at home; Im a paid professional SSM/I land rain capability based on liquid hydrometeor scattering.Observed at 37 GHz/ Should be better at higher frequencies Ga. Tech had a 91.65 GHz radiometer suitable for flight on the NASA CV 990 CV-990 cannot fly over/through interesting land rain (too rough) It can fly through most oceanic precipitationAt these frequencies interesting rain (10s of mm/h) are opaque. Land surface emissivity doesnt matter. So we flew over ocean to test a land rain capability Expected to see Tbs of 240 to 250K with little polarization 23. 24. 25. 26. 27. 28. 29. Rain over land can beseenvia scattering by ice. Bergeron Rain Drop Formation Process Variability in size distribution/ layer thickness makes a quantitative relationship difficult. 30. Profiling Algorithms After Launch of SSM/ITwo Groups:Kummerow and Smith Interesting Problem Attracted Many New Researchers into Rain Two Obvious Pieces of Information in Oceanic Radiances Attenuation of Liquid LayerScattering by Frozen Layer Additional Degrees of Freedom More Subtle Kummerow Moved to Bayesian Approaches with AdditionalInformation from Database 31. Algorithm Intercomparison Projects NASA/WETNET PIP PIP-1 Aug-Nov 1987 Global PIP-21987-199317S to 60N(27 cases) PIP-3 1992Global + Jan. & Jul. 1991 &1993 Global Precipitation Climatology ProjectAlgorithm Intercomparison Project AIP-1 Summer 1987 Japan AIP-2Winter/Spring1991Europe AIP-3Austral Summer92-93 TOGA/COARE IR and Microwave Algorithms, Physical and Empirical Ground Truth Difficult to Impossible IR algorithms No Physics but Lots of Samples Microwave Scattering Weak Physicsand Very Poor Sampling Microwave Absorption Good Physics and Very Poor Sampling (Ocean Only) Performance Depends on How a Given Scenario Relates to Strengths& Weaknesses above 32. Why not fly a Radar? Suggested as early as the 50s (Harry Wexler) If you think of a Radar in isolation One you can afford is pretty much useless A useful one costs the gross national product. Any reasonable Radar will have a very narrow swath. No Sampling Think of a Radar as part of a rain measurement system. Radar is a physics probea calibrator. 33. Then TRMM was launched and everything changed.