PASSIVE MICROWAVE TECHNIQUES: RAINFALL ESTIMATION

1. PASSIVE MICROWAVE TECHNIQUES: RAINFALL ESTIMATION

2. B(Tc) Tc ? ? Visible and Infrared Technique Principle: Rainfall at the surface is related to cloud properties observed from space IR cloud top temperature VIS reflected sunlight from cloud VIS/IR radiometer measures cloud-top properties instead of rain

3. IR/VIS Cloud Indexing Life history Bi-spectral Cloud Model Microwave Emission-based (low-frequency, ocean) Scattering-based (high-frequency, ocean + land) Combined emission and scattering Radiative transfer model - profiling Rainfall Retrieval Methods

4. Cloud indexing (GOES Precipitation Index, Arkin(1979)) RR = ( # pixels ≦ Tb / total # pixels ) × r r RR : Rainfall( rate, mm/hr ) # pixels ≦Tb : Number of convective cloud pixels Tb : Threshold temperature to identify convective clouds Total#pixels : Total number of pixels in the region rr : rain rate( mm/hr ) Tb = 235 (K) r r = 3.0 (mm/hr) for 0.5×0.5deg. boxes

5. (a) and (b) are 0.5 × 0.5 deg. boxes, and the numbers are temperature(K) of each pixel in the boxes, respectively. What is the rainfall(rate(mm/hr)) for each box ? RR = ( 4 / 16 ) ×3.0 = 0.75 (mm/hr) RR = ( 10 / 25 ) ×3.0 = 1.2 (mm/hr)

7. Visible-Infrared-Microwave

8. Pros & Cons for VIS/IR and Microwave Methods • A natural solution: Combing Microwave w/ VIS-IR

9. PASSIVE MW TECHNIQUES • At Vis-IR rain estimation is from observations at top of clouds • At MW frequencies MW radiation penetrates clouds • Precipitation size drops interact strongly with the radiation and detected by radiometers • The Dis advantage is that the MW radiometers have poor resolution

10. Waves • Electromagnetic Waves, Sound waves • … UV, Visible, infrared, microwave, radio wave … • Active or Passive • Lidar, Radar, Radiometer • Methods • Emission-based, scattering-based, extinction-based

11. Some Satellite Missions with Microwave Measurements • DMSP - SSM/I, SSM/T, SSM/T-2 • NOAA(17&18) – AMSU-A/AMSU-B • TRMM – TMI • AQUA – AMSR-E • IRS-P4/Oceansat-1 – MSMR • ADEOS-II – AMSR • NPOESS – CMIS • GPM Core – GMI

12. Common Frequencies • 6 GHz – soil moisture • 10 GHz – soil moisture, rain • 19 GHz – cloud liquid, rain, sea ice, wind • 22 GHz – water vapor • 37 GHz – cloud liquid, rain, sea ice, wind • 60 ± aGHz – temperature profile • 85 GHz – cloud liquid/ice, rain, snowfall • 150 GHz – cloud ice, rain, snowfall • 183 ± a GHz – water vapor profile

13. Different Ways of Remote Sensing Emission Extinction/ Scattering Active Scattering

14. Rainfall - emission & scattering • Emission - based • Low frequency (< 30 GHz) • Over ocean • Scattering – based • High frequency (> 80 GHz) • Over both ocean and land • Combined • Using both low and high frequencies

15. SCATTERING & ABSORPTION PROPERTIES OF RAIN Rain rate (mm/hr) Rain rate(mm/hr)

16. SCATTERING & ABSORPTION PROPERTIES OF RAIN • Spencer et al (1989) calculated the scattering and absorption properties of rain for 3 frequencies used for rain estimation • The following important properties can be noticed:- • Ice does not absorb MW radiation • Liquid drops both absorb and scatter but absorption dominates • Scattering and absorption increases with increasing precipitation

17. SCATTERING & ABSORPTION PROPERTIES OF RAIN • Two general conclusions can be drawn • Firstly, MW spectrum can be divided in to three parts • Below about 22 GHz, absorption is the primary mechanism affecting the transfer of MW radiation • Above 60 GHz, scattering dominates • Between 22 GHz and 60 GHz both absorption and scattering are important • Second, at different frequencies MW observe different parts of the rain structure

19. ABSORPTION AND SCATTERING • At 22 GHz any ice is nearly transparent, mw radiometers respond directly to the rain layer • Above 60GHz, scattering is the main process, it senses the ice and not the rain below • Hence rain estimates made at higher frequencies are more indirect than at lower frequencies • It is important to note that cloud droplets, water vapour and oxygen all absorb but not scatter and thus have the potential to confuse rain estimates

20. Over land, emissions from land at 37 GHz have about the same magnitude as emissions from precipitation, making it more difficult to detect precipitation over land. Over the ocean, energy leaving the cloud is greater than the surface-based energy entering the base of the cloud from below.

21. High frequency microwave scattering channels, such as 85 GHz, provide different information than lower-frequency microwave or infrared channels. Upwelling energy comes from the surface, cloud water, and raindrops below the freezing level. However, above the freezing level, the energy is attenuated due to scattering by precipitation-sized ice particles. Thus, the net effect of these large ice particles is to depress brightness temperatures seen by the satellite.

22. BRIGHTNESS TEMPERATURE vs RAIN RATE • If the model of precipitation and it’s atmospheric environment are assumed then the RTE can be used to calculate the MW brightness temperature as a function of rain rate • These calculations depend on the assumptions made (e.g. amount of cloud water and the structure of rain layer) but they show the general behaviour of MW BT in presence of rain • One set of calculations is shown in the Fig.

23. BRIGHTNESS TEMPERATURE vs RAIN RATE FOR THREE FREQUENCIES

24. SIMPLE RTE is the directionally weighted mean BT If we ignore scattering, then the Eqn becomes If we assume the temp is nearly constant in rain (TA) and note that volume absorption coefficient is nearly Zero except in rain. Integrating the simplified equation gives TB≈ TBS + TA (1- ), where  is the transmittance of rain layer

25. The transmittance of rain layer is approximately given by  ≈ exp(- aD), D is the depth of the rain layer These equations best represent low frequencies where ice above the rain layer is nearly transparent For  = 1 (no rain), TB = Ts For  = 0 ( rain), TB converges to TA For water surfaces TB increases dramatically with rain rate, thus rainy areas can easily be identified For land surface the change in TB with increasing rain Rate is small, thus rainy areas can not easily be identified

26. DIFICULTIES OF THE SCHEME • Useful only over ocean for less rain rate • Rain rate is proportional to a but the TB is proportional to D a • Cloud drops and water vapour and oxygen contribute an un-known amount to a • Increasing rain rate or MW frequency causes scattering to become more important

27. ABSORPTION SCHEMES • The launch of ESMR on board Nimbus-5 (1972) provided the first opportunity to estimate rain using MW data • Wilheit et al (1977) used these data to estimate rain rate over ocean • Used the RTE to calculate TB as a function of rain rate for 19.35GHz • Since it was an absorption scheme rain layer needs to be considered. Thicker rain layer leads to higher optical depth and hence higher TB • It was assumed that rain layer extended from Freezing level to surface • Compared with radar estimates of rain • In general the ESMR-5 estimates are with in a factor two of radar estimates

28. BRIGHTNESS TEMP AS A FUNCTION OF RAIN RATE

29. BRIGHTNESS TEMP AS A FUNCTION OF RAIN RATE : ESMR 5 vs WSR-57 RADAR Calculated BT for 4 Km FL Departure of 1mm/hr The dots are radar and crosses are inferred from ground measurements

30. Several changes from earlier data Conical scanning Measurements in dual polarisations Measured 37 GHz to estimate rain rate over land Polarisations measurements discriminate dry ,wet ground and rain It does not work for temp less than 15 deg celsius Does not retrieve rain rate NIMBUS-6 : ESMR-6 DATA Study by Rodgers et al (1979)

31. NIMBUS-7 : SMMR-6 DATA • Had 5 channels with dual polarisation • Spencer et al (1985) used the 10 BTs to estimate precipitation • Used Step wise regression approach to relate the 10 BTs to rain rate • The US Air Force follows this method to calculate rain rate using SSM/I data

32. ABSORPTION SCHEMES : PROBLEMS • Two problems with the absorption approach • Cloud water and rain water are difficult to separate using single wave length. SMMR data is useful as some of the channels contain independent info on liquid water • MW radiometers have large foot prints. The rain rate can not be same with in this foot print. The radiometer averages the BTs. As BT is highly non linear in rain rate, the average BT does not produce good estimate of mean rain rate with in the foot print • The rain rate estimate of radiometer is always an under estimate. But the multi wavelength approach may give better result as diff wavelength explores diff part of the cloud structure

33. SCATTERING TECNIQUE • Precipitation is the only atmospheric constituent that scatters MW radiation • If scattering radiation can be detected and quantified, precipitation estimates will be more accurate • If we consider conically scanning instrument (ESMR-6,SMMR, SSM/I) that views each scan spot with 50 deg then,

34. POLARISATION DIAGRAM • The solid line is the result of plotting THB andTVB for all values of  (0 to1) • If non-scattering material is added over non raining and oceanic surface then the observed BT moves along No-scattering line • If scattering particles (rain) are introduced, the observed Bt will be in between no polarisation and no-scattering line • The rain rate is proportional to the distance from the no-scattering line • Spencer studied 85 GHz data from SSM/I and calculated PCT which gives a measure of distance from the no-scattering line Study by Spencer et al (1989)

35. PCT = 1.818 – 0.818 • It was found that 255K corresponds well with out line of precipitating regions

36. Thank You