[Personnel: Last_Name='HYDROLOGY DATA SUPPORT TEAM']
TRMM Precipitation Radar (PR) Level 2 Surface Cross-Section Product (TRMM Product 2A21)Entry ID: GES_DISC_TRMM_2A21_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.
The primary objective of the 2A21 is to compute the path integrated attenuation (PIA), using the surface reference technique (SRT). The SRT relies on the assumption ... that the difference between the measurements of the normalized surface cross section within and outside the rain provides a measure of the PIA.
Two types of non-rain surface cross section (sigma-zero) reference estimates are used: spatial and temporal. In the spatial surface reference data set, the mean and standard deviation of the surface cross sections are calculated over a running window of Ns fields of view before rain is encountered. These operations are performed separately for each of the 49+2 incidence angles of TRMM (corresponding to the cross-track scan from -17 degrees to + 17 degrees with respect to nadir). The two additional angle bins (making the total 51 rather than 49) are to account for non-zero pitch/roll angles that can shift the incidence angle with respect to nadir outside the normal range.
For the temporal surface reference data set, the running mean and standard deviation are computed over a 1 degree x 1 degree (latitude, longitude) grid. Within each 1 degree x 1 degree grid cell, the data are further categorized into incidence angle categories (26). The number of observations in each category, Nt, are also recorded. Note that, for the temporal reference data set, no distinction is made between the port and starboard incidence angles. So, instead of 49 incidence angles, there are only 25 + 1, where the additional bin corresponds to angles greater than the normal range.
When rain is encountered, the mean and standard deviations of the reference sigma-zero values are retrieved from the spatial and temporal surface reference data sets. To determine which reference measurement is to be used, the algorithm checks whether Nt >= Ntmin and Ns >= Nsmin, where Ntmin and Nsmin are the minimum number of samples that are needed to be considered a valid reference estimate for the temporal and spatial reference data sets, respectively. (Presently, Ntmin = 50 and Nsmin = 8). If neither condition is satisfied, no estimate of the PIA is made and the flags are set accordingly. If only one condition is met, then the surface reference data which corresponds to this is used. If both conditions are satisfied, the surface reference data is taken from that set which has the smaller standard deviation.
If a valid surface reference data set exists (i.e., either Nt >= Ntmin or Ns >= Nsmin or both) then the 2-way path attenuation (PIA) is estimated from the equation:
where sigma-zero(in rain) is the value of the surface cross section over the rain volume of interest and
To obtain information as to the reliability of this PIA estimate we consider the difference between the PIA, as derived in the above equation, and the standard deviation as calculated from the no-rain sigma-zero values and stored in the reference data set. Labeling this as std dev(reference value), then the reliability factor of the PIA estimate is obtained from:
reliabFactor = PIA - std dev(reference value)
When this quantity is large, the reliability is considered high and conversely. This is the basic idea. Specific definitions of the reliability flag and reliability factors are given in the definitions of the output variables.
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 contains a single orbit of data with a file size of about 9 MB (uncompressed). The HDF-EOS "swath" structure is used to accommodate the actual geophysical data arrays. There are 16 files of PR 2A21 data produced per day.
Data Set Citation
Dataset Originator/Creator: NASA,JAXA, Tropical Rainfall Measuring Mission (TRMM)
Dataset Title: TRMM Precipitation Radar (PR) Level 2 Surface Cross-Section Product (TRMM Product 2A21)
Dataset Publisher: Goddard Space Flight Center Distributed Active Archive Center (GSFC DAAC)
Version: 7Online Resource: http://disc.gsfc.nasa.gov/datacollection/GES_DISC_TRMM_2A21_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
Vertical Resolution: Surface
Temporal Resolution: 16 orbits per day
Temporal Resolution Range: Daily - < Weekly
ISO Topic Category
Quality A Table structure (called Scan Status) is included in the HDF file,
containing entries on a scan-by-scan basis. These entries include a
geolocation flag and a data quality flag, indicating the percentage of
pixels along each scan with values within a pre-defined range. There are
also various reliability flags and factor.
Access Constraints None
Data Set Progress
Distribution Media: FTP
Distribution Size: 15-16 files/day; file size: 6200-6300 kb/file
Distribution Format: HDF
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
File Specifications for TRMM Products: http://pps.gsfc.nasa.gov/Documents/filespec.TRMM.V7.pdf
Metadata for TRMM Products: http://pps.gsfc.nasa.gov/Documents/filespecMeta.TRMM.V7.pdf
TRMM V6 vs. V7 Major Data Format Changes: http://pps.gsfc.nasa.gov/Documents/formatChangesV7.pdf
Caylor I.J., G.M. Heymsfield, R. Meneghini, and L.S. Miller, 1997:
... Correction of sampling errors in ocean surface cross-sectional estimates
from nadir-looking weather radar. J. Atmos. Oceanic Technol., 14, 203-210.
Iguchi, T. and R. Meneghini, 1994: Intercomparisons of single-frequency
methods for retrieving a vertical rain profile from airborne or spaceborne
radar data. J. Atmos. Oceanic Technol., 11, 1507-1516.
Kozu, T., 1995: A generalized surface echo radar equation for down-looking
pencil beam radar. IEICE Trans. Commun., E78-B, 1245-1248.
Marzoug, M. and P. Amayenc, 1994: A class of single- and dual-frequency
algorithms for rain rate profiling from a spaceborne radar. Part I:
Principle and tests from numerical simulations. J. Atmos. Oceanic
Technol., 11, 1480-1506.
Meneghini, R. and K. Nakamura, 1990: Range profiling of the rain rate by
an airborne weather radar. Remote Sens. Environ., 31, 193-209.
Interface Control Specification Between the Tropical Rainfall Measuring
Mission Science Data and Information System (TSDIS) and the TSDIS Science
User (TSU). Volume 4: File Specifications for TSDIS Products-Level 2 and
Level 3. NASA Goddard Space Flight Center, March 5, 1999.
Extended Metadata Properties
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Creation and Review Dates
DIF Creation Date: 2011-07-15
Last DIF Revision Date: 2014-09-11