Cloud and precipitation formation is one of the key issues in the tropical precipitation study.For the first time TRMM provides such rich products that cover many components of cloud and precipitation formation.TRMM data include hydrometeors distribution, latent heat profiles derived from the Goddard Cloud Ensemble Model, separated convective and stratiform rainfall data, cloud electrification, and so on.Asthe time of writing this page, the reported applications to cloud andprecipitation include study of tropical rainfall diurnal cycles; study of the electrification ofcyclones and hurricanes; and the improvement in cloud and precipitation parameterization used in numerical general circulation models.
Diurnal variation of precipitation is systematically investigated by using the TRMM precipitation products retrieved from TRMM microwave imager (TMI), precipitation radar (PR) and TMI/PR combined algorithms for year 1998. Temporal variations of diurnal cycle of rainfall derived from TRMM 2A12, 2A25, and 2B31were averaged over ocean and land separately (35S-35N).
The figure below is the diurnal cycle over the tropical ocean derived from those three TRMM data products (top: 2B31, middle: 2A25, bottom: 2A12).The contour is the rainfall (mm/day) varying with months (horizontal axis) and local time(vertical axis).
- A rainfall peak in early morning (high value contours) exists consistently in the all three data products
- Seasonal variation on intensity of rainfall diurnal cycle (relatively large values during the late spring and early fall)
A similar rainfall diurnal cycle figure but for land is shown below.
| (above two figures are provided by Dr. S. Yang at GSFC, NASA)
A rainfall peak in early-mid afternoon (high value contours) exists consistently in the all three data products
Seasonal variation on intensity of rainfall diurnal cycle (relatively large values on the winter and fall)
Further detailed discussion about the spatial features of precipitation diurnal cycles is given by S. Yang et al. ( Reference: Yang S., Smith E.A., Randall D. and Lin X. 1999: An observational basis for diurnal variability of global tropical rainfall. Submitted to Journal of Climate TRMM Special Issue).
TRMM 2A12 algorithm explores the application of profiling techniques (Kummerow et al., 1996) to TRMM microwave imager (TMI) data.The profiling techniques use the Goddard Cumulus Ensemble Model, and generate vertical hydrometeror profiles on a pixel by pixel basis. For each pixel, cloud liquid water, precipitation water, cloud ice water, precipitation ice, and latent heating are given at 14 vertical layers based upon the nine channels of the TRMM microwave imager (TMI).The top of each layer is given at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0,3.5, 4.0, 5.0, 6.0, 8.0, 10.0,14.0, and 18.0 km above the surface. The surface rainfall and the associated confidence indicator is also calculated.Each data granule is one orbit plus 50 scan lines of pre-orbit overlap and 50 scan lines of post-orbit overlap.Each data granule consists of two parts: metadata and swath data.
Both version 4 and version 5 of TRMM 2A-12 data are available at GDAAC. But users need to know that there were two known deficiencies in the version 4 TRMM 2A-12 data.The rainfall rates were affected by the calibration problems in TMI level 1B data, and the latent heating field was not reliable.The version 5 of TRMM 2A-12 algorithm has fixed the calibration problem, but temporarily sets the latent heating profile zero. It is expectd that the version 6 of TRMM 2A-12 algorithm restore the latent heating profiling.
TRMM scientists are also working on the discrepancy in the rainfall climatology between TMI 2A12 and TRMM precipitation radar (PR) data. 2A12 data produces rainfall climatology about 20% higher than PR. The coming version 6 is expected to improve this.
TRMM 2A25, precipitation radar (PR) profile, produces an estimate of vertical rainfall rate profile for each radar beam.The rainfall rate estimate is given at each resolution cell of the PR radar.To compare with ground-based radar data, the attenuation corrected Z factor profile is also given.The average rainfall rate between the two pre-defined altitudes is calculated for each beam position.Other output data include parametersof Z-R relationships, integrated rain rate of each beam, range bin numbers of rain layer boundaries, and many intermediate parameters.
TRMM 2B31, TRMM combined, includes vertical hydrometeor profilesderived from the data of PR radar and 10 GHz channels of the TMI.It alsoincludes the correlation-corrected mass-weighted mean drop diameter, the correlation-corrected relative spread of mass-weighted mean drop diameter,the correction made to the inpput path-integrated attenuation estimate andtheir standard deviations. The granule size is one orbit and has a PR based geometry.
Data are in HDF format.The description of all these three data sets can be obtained from the Volume 4 - Levels 2 and 3 File Specifications provided by the TRMM Data and Information System (TSDIS).A Sample Fortran and C program to read this data set is available at Hydrology on Line Information.
Kummerow, C., Olson, W.S., and Giglio, L., 1996: A simplified Scheme for Obtaining Precipitation and Vertical Hydrometeor Profiles from Passive Microwave Sensors. IEEE Trans. on Geosci. and Remote Sensing , 34 , 1213-1232.
Three dimensional structure of rain:
TSDIS provides an Orbit_viewer Graphic Interface for plotting and analyzing 3-dimension features of some TRMM dataproducts ( 3-dimension Viewer ). Below are two snap-shoots of hurricane Floyd (Sept. 13, 1999) over the westernAtlantic Ocean (center at about 24oN, -71oE). Top figureis precipitation water derived from TMI (TRMM 2A12) data, and the bottom is rain rate derived from PR (TRMM 2A25). The precipitation structure around the hurricane center is clearly captured by the TRMM instruments.