ASCAT sigma0/TRMM PR rain/ECMWF wind (ATE) collocation dataset

J. Kjar and D. Long, Brigham Young University, 14 Jan 2022

The ATE data set contains a set of collocations of ASCAT sigma0/TRMM
PR rain/ECMWF wind vectors during 2009. It is available separately in
either ASCII comma-separated (.csv) or ASCII space-separated (.dat)
forms. Each type is packaged in seperate zip archives.

Each file contains data from one intersection of ASCAT-A and TRMM PR
orbit swaths that occurs within an hour of each other.  See the
Methodology description below for how data was collocated.

Each file represents a single TRMM-PR pass over an ASCAT-A swath
containing temporally and spatially collocated measurements. Only high
quality measurements from both ASCAT and TRMM-PR measurements are
used. Near surface rains come from V7 TRMM-PR 2A25 files and have a
spatial resolution of approximately 5 km on a 247 km wide swath. ASCAT
L1B full resolution (slice) sigma-0 slice measurements are processed
into a fine 2.5 km spacing grid using the Scatterometer Image
Reconstruction (SIR) algorithm. This interplates the ASCAT-A full
resolution sigma-0 measurements separately for each beam to the grid
locations. Values of sigma-0 at these grid points are collocated with
the locations of the TRMM-PR 2A25 surface rain measurements within a
time window using nearest neighbor, with time differences and
locations recorded for each point.

How the data is formatted:

Each ASCII file contains 23 columns with contents described below.

    1. time difference between ASCAT-A and TRMM-PR measurements in seconds
  2-7. year/month/day/hour/minute/secon of TRMM-PR measurement
  8-9. latitude and longitude of TRMM-PR rain measurements in deg
10-11. latitude and longitude of SIR-processed 2.5 km ASCAT grid centers in deg
12-14. ASCAT-A sigma-0 values in dB interpolated (using SIR) to grid center for
        fore, middle, and aft beams
15-17. ASCAT-A azimuth angle relative to north in deg corresponding to ASCAT-A
        sigma-0 for fore, middle, and aft beams
18-20. ASCAT-A incidence angle in deg corresponding to ASCAT-A sigma-0
         measurement interpolated to grid center for fore, middle, and aft beams
   21. TRMM-PR 2A25-reported surface rain in mm/hr
22-23. ECMWF u and v components of the wind in m/s trilinearly interpolated
       to the TRMM-PR locations

Methodology

0. Only unflagged (highest quality) measurements from TRMM-PR 2A25
   (surface rain) and ASCAT L1B (slice sigma-0) files are used.

1. All TRMM-PR passes over ASCAT swaths that occured within 30 mins of
   each other were identified for the year 2009.

2. For each of these passes, ASCAT-A L1B full resolution ("slice")
   sigma-0 measurements were interpolated to a 2.5 km grid using the
   Scatterometer Image Reconstruction (SIR) algorithm.  For each
   TRMM-PR measurement location with a valid rain measurement, the
   closest valid ASCAT 2.5 km grid location was identified. To be
   retained, the two locations had to be within 0.1 deg latitude and
   longitude and within 1 hour.  The time difference of each grid
   point collocation was determined and is included in the data set so
   that closer time collocations can be selected. Similarly, the
   lat/lon of the centers of the TRMM-PR and gridded ASCAT sigma-0
   measurements are recorded.

3. ECMWF surface wind fields (supplied as U and V wind components) are
   tri-linearly (time, lat, lon) interpolated to the locations of the
   TRMM-PR measurements. The ECMWF fields are 6 hour forecasts on a 50
   km grid.

4. The triple collocated measurements meeting the collocation critera
   are collected and stored in a single file for each pass. There are
   3609 passes with each pass containing about 5600 points. The exact
   number varies from pass to pass.  Files are recorded in fixed
   format columns separated by either spaces (.dat files) or commas
   (.csv).