QuikSCAT SeaWinds Data

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$Id: QuikSCAT.html 11305 2017-03-14 17:52:08Z hendric@ucar.edu $

DATA SOURCES / PROGRAMS / MODULES / NAMELIST / ERRORS / FUTURE PLANS / TERMS OF USE

Overview

NASA's QuikSCAT mission is described in http://winds.jpl.nasa.gov/missions/quikscat/. "QuikSCAT" refers to the satellite, "SeaWinds" refers to the instrument that provides near-surface wind speeds and directions over large bodies of water. QuikSCAT has an orbit of about 100 minutes, and the SeaWinds microwave radar covers a swath under the satellite. The swath is comprised of successive scans (or rows) and each scan has many wind-vector-cells (WVCs). For the purpose of this document, we will focus only the Level 2B product at 25km resolution. If you go to the official JPL data distribution site http://podaac.jpl.nasa.gov/DATA_CATALOG/quikscatinfo.html , we are using the product labelled L2B OWV 25km Swath. Each orbit consists of (potentially) 76 WVCs in each of 1624 rows or scans. The azimuthal diversity of the radar returns affects the error characteristics of the retrieved wind speeds and directions, as does rain, interference of land in the radar footprint, and very low wind speeds. Hence, not all wind retrievals are created equal.

The algorithm that converts the 'sigma naughts' (the measure of radar backscatter) into wind speeds and directions has multiple solutions. Each candidate solution is called an 'ambiguity', and there are several ways of choosing 'the best' ambiguity. Beauty is in the eye of the beholder. At present, the routine to convert the original L2B data files (one per orbit) in HDF format into the DART observation sequence file makes several assumptions:

  1. All retrievals are labelled with a 10m height, in accordance with the retrieval algorithm.
  2. Only the highest-ranked (by the MLE method) solution is desired.
  3. Only the WVCs with a wvc_quality_flag of zero are desired.
  4. The mission specification of a wind speed rms error of 2 ms (for winds less than 20 m/s) and 10% for windspeeds between 20 and 30 m/s can be extended to all winds with a qc flag of zero.
  5. The mission specification of an error in direction of 20 degrees rms is applicable to all retrieved directions.
  6. All retrievals with wind speeds less than 1.0 are not used.
  7. The above error characterstics can be simplified when deriving the horizontal wind components (i.e. U,V). Note : this may or may not be a good assumption, and efforts to assimilate the speed and direction directly are under way.

DATA SOURCES

The NASA Jet Propulsion Laboratory (JPL) data repository has a collection of animations and data sets from this instrument. In keeping with NASA tradition, these data are in HDF format (specifically, HDF4), so if you want to read these files directly, you will need to install the HDF4 libraries (which can be downloaded from http://www.hdfgroup.org/products/hdf4/)

If you go to the official JPL data distribution site http://podaac.jpl.nasa.gov/DATA_CATALOG/quikscatinfo.html, we are using the product labelled L2B OWV 25km Swath. They are organized in folders by day ... with each orbit (each revolution) in one compressed file. There are 14 revolutions per day. The conversion to DART observation sequence format is done on each revolution, multiple revolutions may be combined 'after the fact' by any obs_sequence_tool in the work directory of any model.


PROGRAMS

There are several programs that are distributed from the JPL www-site, ftp://podaac.jpl.nasa.gov/pub/ocean_wind/quikscat/L2B/sw/; we specifically started from the Fortran file read_qscat2b.f and modified it to be called as a subroutine to make it more similar to the rest of the DART framework. The original Makefile and read_qscat2b.f are included in the DART distribution in the DART/observations/quikscat directory. You will have to modify the Makefile to build the executable.

convert_L2b.f90

convert_L2b is the executable that reads the HDF files distributed by JPL. DART/observations/quikscat/work has the expected mkmf_convert_L2b and path_names_convert_L2b files and compiles the executable in the typical DART fashion - with one exception. The location of the HDF (and possible dependencies) installation must be conveyed to the mkmf build mechanism. Since this information is not required by the rest of DART, it made sense (to me) to isolate it in the mkmf_convert_L2b script. It will be necessary to modify the mkmf_convert_L2b script to be able to build convert_L2b. In particular, you will have to change the two lines specifying the location of the HDF (and probably the JPG) libraries. The rest of the script should require little, if any, modification.

set JPGDIR = /contrib/jpeg-6b_gnu-4.1.2-64
set HDFDIR = /contrib/hdf-4.2r4_gnu-4.1.2-64

There are a lot of observations in every QuikSCAT orbit. Consequently, the observation sequence files are pretty large - particularly if you use the ASCII format. Using the binary format (i.e. obs_sequence_nml:write_binary_obs_sequence = .true.) will result in observation sequence files that are about half the size of the ASCII format.

Since there are about 14 QuikSCAT orbits per day, it may be useful to convert individual orbits to an observation sequence file and then concatenate multiple observation sequence files into one file per day. This may be trivially accomplished with the obs_sequence_tool program in any model/xxxx/work directory. Be sure to include the '../../../obs_def/obs_def_QuikSCAT_mod.f90' string in input.nml&preprocess_nml:input_files when you run preprocess.

obs_to_table.f90, plot_wind_vectors.m

DART/diagnostics/threed_sphere/obs_to_table.f90 is a potentially useful tool. You can run the observation sequence files through this filter to come up with a 'XYZ'-like file that can be readily plotted with DART/diagnostics/matlab/plot_wind_vectors.m.


NAMELIST

This namelist is read from the file input.nml. We adhere to the F90 standard of starting a namelist with an ampersand '&' and terminating with a slash '/' for all our namelist input. Character strings that contain a '/' must be enclosed in quotes to prevent them from prematurely terminating the namelist. The following values are the defaults for these namelist items.

&convert_L2b_nml
   l2b_file = '',
   datadir = '.',
   outputdir = '.',
   lon1 = 0.0, 
   lon2 = 360.0, 
   lat1 = -90.0, 
   lat2 = 90.0,
   along_track_thin = 0,
   cross_track_thin = 0
 /

It is possible to restrict the output observation sequence to contain data from a region of interest throught the use of the namelist parameters. If you need a region that spans the Prime Meridian lon1 can be a larger number than lon2, for example, a region from 300 E to 40 E and 60 S to 30 S (some of the South Atlantic), would be lon1 = 300, lon2 = 40, lat1 = -60, lat2 = -30.

Contents Type Description
l2b_file character(len=128) name of the HDF file to read - NOT including the directory, e.g. QS_S2B44444.20080021548
datadir character(len=128) the directory containing the HDF files
outputdir character(len=128) the directory for the output observation sequence files.
lon1 real(r4) the West-most longitude of interest in degrees. [0.0, 360]
lon2 real(r4) the East-most longitude of interest in degrees. [0.0, 360]
lat1 real(r4) the South-most latitude of interest in degrees. [-90.0, 90.0]
lat2 real(r8) the North-most latitude of interest in degrees. [-90.0, 90.0]
along_track_thin integer provides ability to thin the data by keeping only every Nth row. e.g. 3 == keep every 3rd row.
cross_track_thin integer provides ability to thin the data by keeping only every Nth wind vector cell in a particular row. e.g. 5 == keep every 5th cell.


KNOWN BUGS

There are no known bugs at this time.


FUTURE PLANS

  1. There is one bit of error-checking that did not survive the conversion from F77 to F90. I need to restore the check that the HDF file being read is a 'Level 2B' product.
  2. There is a lot of error-checking that is not being done. I need to bulletproof the code more.
  3. We need namelist options to select something other than the highest-ranked ambiguity.
  4. We need namelist options to select more QC flags - not just the ones with the 'perfect' QC value of 0
  5. Add an option to leave the observations as speed and direction instead of converting them to U,V components. This is a natural implementation of the instrument error characteristics. However, it would require writing a specialized forward operator in order to assimilate obs of this type (speed, direction), and there is still a numerical problem with trying to do the statistics required during the assimilation of a cyclic direction value.

Terms of Use

DART software - Copyright UCAR. This open source software is provided by UCAR, "as is", without charge, subject to all terms of use at http://www.image.ucar.edu/DAReS/DART/DART_download

Contact: Tim Hoar
Revision: $Revision: 11305 $
Source: $URL: https://svn-dares-dart.cgd.ucar.edu/DART/releases/Manhattan/observations/obs_converters/quikscat/QuikSCAT.html $
Change Date: $Date: 2017-03-14 11:52:08 -0600 (Tue, 14 Mar 2017) $
Change history:  try "svn log" or "svn diff"