[Parameters: Topic='ATMOSPHERE', Term='PRECIPITATION', Variable_Level_1='RAIN']
TRMM Precipitation Radar (PR) Level 2 Rain Characteristics Product (TRMM Product 2A23)Entry ID: GES_DISC_TRMM_2A23_V7
Abstract: The Tropical Rainfall Measuring Mission (TRMM) is a joint U.S.-Japan satellite mission to monitor tropical and subtropical precipitation and to estimate its associated latent heating. TRMM was successfully launched on November 27, at 4:27 PM (EST) from the Tanegashima Space Center in Japan.
The TRMM Precipitation Radar (PR), the first of its kind in space, is an electronically scanning radar, ... operating at 13.8 GHz that measures the 3-D rainfall distribution over both land and ocean, and defines the layer depth of the precipitation.
PR 2A23 produces a rain/no-rain flag. Its main objectives are (1) to detect bright band (BB), (2) to classify rain type, and (3) to detect warm rain.
2A23 uses two different methods for classifying rain type: (1) vertical profile method (V-method) and (2) horizontal pattern method (H-method). Both methods classify rain into three categories: stratiform, convective, and other. To make the results user-friendly, 2A23 outputs a unified rain type. Further information about 2A23 can be found in Awaka et al. (1998).
The V-method starts with the detection of BB. This detection is made by a spatial filter, based on the second derivative of Z with respect to range, and by imposing several conditions on BB (e.g., Z above BB peak should decrease appreciably, the height of BB should appear almost at the same height). One of the major conditions imposed on BB is that the height of BB must be located in a BB window, whose range is from freezH - 1.5 km to freezH + 1.5 km, where freezH is the height of freezing level estimated from a climatological surface temperature at sea level, Tsurface, by the following formula:
freezH = Tsurface / Tlapse,
and where Tlapse is the lapse rate of temperature (2A23 assumes that Tlapse = 6.0 (deg/km)).
After the BB detection, the V-method goes on to classify rain type. The outline of rain type classification by the V-method is as follows:
1. When BB exists, rain is basically classified as stratiform.
2. When BB is not detected, and maximum value of Z at a given angle bin exceeds a convective threshold, rain type for this angle bin is classified as convective.
3. Other type is defined as not-stratiform and not-convective.
It should be noted that other type of rain by the V-method is defined as not-convective and not-stratiform, i.e., (1) there exists appreciable radar echo, but it is not strong enough to be convective and (2) BB is not detected.
The H-method also classifies rain into 3 categories: stratiform, convective, and other. However, their definitions are different from those of the V-method.
The H-method is based on the University of Washington convective/stratiform separation method (Steiner et al., 1995), which examines the horizontal pattern of Z at a given height, where Z has a 2 km horizontal resolution. In 2A23, the following modifications are made:
1. Instead of examining a horizontal pattern of Z at a given height, a horizontal pattern of Zmax is examined; here, Zmax is the maximum of Z along the range for each antenna scan angle below freezH (minus 1 km margin).
2. Parameters are changed so that they may be suitable for the TRMM data with 4.3 km horizontal resolution. The parameters were chosen before the launch of TRMM, using test Ground Validation (GV) data in such a way that the 4.3 km resolution data produce almost the same result as that with a 2 km resolution data.
3. Other type of rain is introduced to handle noise.
In the H-method, detection of convective rain is made first. If one of the following conditions is satisfied for a pixel, which corresponds to the angle bin data being considered, it is judged that the pixel is a convective center: (1) Zmax exceeds a convective threshold or (2) Zmax stands out against the background area.
Rain type for a convective center is convective, and rain type for the pixels nearest to the convective center is also convective.
In the H-method, if rain type is not convective and if the rain echo is certain to exist, then rain type is stratiform.
In the H-method, rain type is "other" if the radar echo below freezH (minus 1 km margin) at a given angle bin is possibly noise. This means that the "other" type by the H-method includes the cases of (1) noise and (2) cloud.
2A23 outputs a unified rain type. The unified rain type is expressed by two digits: the first digit indicates the rain type (1: stratiform, 2: convective, 3: other) and the second digit indicates a level of confidence, which decreases as the number increases. Unification of rain type is made in such a way that the rain types by V-method and H-method can be reconstructed from the unified rain type by using a suitable table.
Detection of warm rain is also made in 2A23. When the following two conditions are satisfied, it is concluded that there exists warm rain:
1. Storm top of warm rain is much lower than the height of freezing level. (This condition implies that when BB is detected, it is not warm rain, because the existence of BB means that the storm top is higher than the height of freezing level).
2. Warm rain must be isolated from the other rain-certain areas.
1. Bright band (BB) detection has angle bin dependence because of smearing of BB near the antenna scan edges. Sample test indicates the rate of BB detection is about 80% near nadir directions (at antenna scan angle ranging from about -7 to about +7 degrees), but the rate decreases to as low as about 20% at the antenna scan edge.
2. When a large number of orbit data are processed, statistics of rain type show angle bin dependence, because the rain/no-rain judgment in 1B21 also seems to have angle bin dependence.
3. Bright band detection and rain type classification are carried out for rain-certain cases only. For rain possible, rain type is automatically classified as other type.
4. When rain is "certain" and the unified rain type is "other," it most probably indicates that there exists cloud only.
5. Side lobe clutter may still affect the detection of BB and, hence, affect rain type classification as well, although a side lobe clutter rejection is tried in the algorithm.
Spatial coverage is between 38 degrees North and 38 degrees South, owing to the 35 degree inclination of the TRMM satellite. This orbit provides extensive coverage in the tropics and allows each location to be covered at a different local time each day, enabling the analysis of the diurnal cycle of precipitation. There are, in general, 9150 scans along the orbit, with each scan consisting of 49 rays. The scan width is about 220 km.
The data are stored in the Hierarchical Data Format (HDF), which includes both core and product specific metadata applicable to the PR measurements. A file, containing a single orbit of data, has a size of about 10 MB (uncompressed). The HDF-EOS "swath" structure is used to accommodate the actual geophysical data arrays. There are 16 files of PR 2A23 data produced per day.
Data Set Citation
Dataset Originator/Creator: Tropical Rainfall Measuring Mission (TRMM)
Dataset Title: TRMM Precipitation Radar (PR) Level 2 Rain Characteristics Product (TRMM Product 2A23)
Dataset Publisher: Goddard Space Flight Center Distributed Active Archive Center (GSFC DAAC)
Version: 7Online Resource: http://disc.gsfc.nasa.gov/datacollection/GES_DISC_TRMM_2A23_V7.shtml
Start Date: 1997-12-07
Latitude Resolution: 4 km
Longitude Resolution: 4 km
Horizontal Resolution Range: 1 km - < 10 km or approximately .01 degree - < .09 degree
Temporal Resolution: 16 orbits per day
ISO Topic Category
Quality Scan Status is provided containing scan-by-scan information on quality, platform and instrument control data, and fractional orbit number. Status Flag is also provided indicating whether the data are obtained over sea or land and the degree of data reliability.
Access Constraints None
Data Set Progress
Distribution Media: FTP
Distribution Size: 15-16 files/day; 6600-6700 KB/file
Distribution Format: HDF
Email: awaka at de.htokai.ac.jp
Hokkaido Tokai University Department of Electronics and Information Engineering Minami-ku, Minami-sawa, 5-1-1-1
Province or State: Hokkaido
Postal Code: 005-8601
Role: TECHNICAL CONTACT
Email: gsfc-help-disc at lists.nasa.gov
Goddard Earth Sciences Data and Information Services Center Code 610.2 NASA Goddard Space Flight Center
Province or State: MD
Postal Code: 20771
Role: DIF AUTHOR
Email: Zhong.Liu-1 at nasa.gov
NASA Goddard Space Flight Center Mailstop 610.2
Province or State: MD
Postal Code: 20771
2A23 V7 User's Guide: http://pps.gsfc.nasa.gov/Documents/2A23_Users_Guide
File Specifications for TRMM Products: ftp://pps.gsfc.nasa.gov/pub/v7filespec/filespec.TRMM.V7.pdf
Metadata for TRMM Products: ftp://pps.gsfc.nasa.gov/pub/v7filespec/filespecMeta.TRMM.V7.pdf
TRMM V6 vs. V7 Major Data Format Changes: http://pps.gsfc.nasa.gov/Documents/formatChangesV7.pdf
Awaka, J., T. Iguchi, and K. Okamoto, 1998: "Early results on rain type classification by the Tropical Rainfall Measuring Mission (TRMM) precipitation radar," Proc. 8th URSI Commission F Open Symp., Aveiro, Portugal, pp. 143-146.
Steiner, M., R.A. Houze, Jr., and S.E. Yuter, 1995: "Climatological characterization of three-dimensional storm structure from operational radar and rain gauge data," J. Appl. Meteor., 34, pp. 1978-2007.
Extended Metadata Properties
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Creation and Review Dates
DIF Creation Date: 2011-07-15
Last DIF Revision Date: 2014-05-16