CAMEX-4 ER-2 Doppler Radar
Table of Contents
Introduction
The ER-2 Doppler radar (EDOP) is an X-band (9.6 GHz) Doppler radar mounted in the nose of ER-2. The instrument has two fixed antennas, one pointing at nadir and the second pointing approximately 33 degree ahead of nadir. The beam width of the antenna is 3 degrees in the vertical and horizontal directions which, for a 20 km altitude, yields a nadir footprint at the surface of 1 km. The ER-2 ground speed is nominally 210 m/s and the integration period used by the data system is 0.5 second. The transmit pulse is 0.5 second and the gate spacing is over sampled at 37.5 meter interval. Minimum detectable reflectivity is about -10 dBZ at an altitude of 15 km with a 0.375 meters range gate spacing. Additional details of the radar and processing are described by Heymsfield et al. (1996).
More detailed information about the ER-2 Doppler Radar may be found at the EDOP Home Page.
NOTE: Use of these files requires the commercial visualization program IDL available from ITT Visual Information Solutions.
File Naming Convention
EDOP files are 'tarred' together into daily (mission) files that appear as follows:
c4eedop_YYYY.JJJ_daily.tar
where YYYY.JJJ is the year and day of the year
When 'untarred' the tar file yields numerous swath data files named as follows:
yymmdd_ssss-eeee.uf
where yymmdd is 2 digit year (99) mm is month, dd is day
ssss and eeee is the start and end time of the swath in UTC.
Data Format
EDOP data is in Universal Format (UF) as documented in the UF-IDL.guide which is included with each dataset. IDL software is provided for reading the data.
EDOP currently collects data at a rate of 2 Hz from 4 Doppler channels with 738 gates. See paragraph 4 below for more information
Nadir rain (dynamic range optimized for rain signal -10-70 dBZ)
Nadir surface (dynamic range optimized for surface 20-100 dBZ)
Forward Co-polarization
Forward Cross-polarization
Each of these channels provides reflectivity, Doppler velocity, and spectral width estimated from the time series (autocovariance) approach.
Four levels of UF format are implemented for EDOP data:
Level 1: Conversion of EDOP uncalibated raw data format to UF
(uncalibrated power, Doppler velocity).
Level 2: Conversion of uncalibrated power to calibrated dBZ
Level 3: Doppler unfolding and aircraft motion removal.
Level 4: Attenuation correction of reflectivity.
The current distribution (Version 1) is in Level 2 format, and will have a ".uf.1" or ".caluf.1" appended to the file name. The ".1" is the version number; updated data sets will have higher numbers. All data files without a number appended are Version 0 and should be replaced with higher versions of the data. The parameter names for the Level 2 files are as follows:
PN Nadir_VV power
PS Nadir_VV surface power
DN Nadir_VV raw Doppler velocity [m/s]
DS Nadir_VV raw surface Doppler velocity [m/s]
ZN Nadir_VV reflectivity (dBZ)
ZS Nadir_VV surface reflectivity (dBZ)
EDOP CALIBRATION
The calibration of EDOP is described in Caylor et al. (1995), although this has been modified somewhat due to the change to the new digital-IF receiver and processing system in 1998. After the calibration is obtained, EDOP surface measurements (sigma_0) from both the nadir and forward beams are compared with previously published values as well as with statistics from TRMM estimated values. In addition, the reflectivity measurements in rain are compared with simultaneous measurements from the TRMM Precipitation Radar and several ground-based radars. However, only the EDOP internal and external calibrations are used, and NO adjustments are made to the EDOP reflectivities (unless otherwise noted) based on these comparisons. EDOP has a linear receiver so all calculations are performed linearly. Thus, if any error in calibration exists, the bias will be constant over the full dynamic range of the measurements. The final calibration of EDOP should be accurate to approximately 1 dBZ.
Software Provided
In addition to the file UF-IDL.guide mentioned in paragraph 3 above, two other documentation files are provided for your use. The first is the EDOP_README which was written by the dataset producer, Dr. Heymsfield. In addition is a file named EDOP-UF.txt that provides a text version of the Universal format for EDOP data (level 1). These three files are included as separate files in your dataset, but each are tarred into the dataset itself. Each of these (and the tarred software file) may also be found on our anonymous FTP site.
There is a file named software.tar that will, when expanded, provide you with numerous program files to be used with IDL that will extract specific pieces of information from the EDOP data files. One such file, qview_demo.pro is shown below:
NOTE: This is only one example of the numerous IDL programs that will be found once the software.tar file is 'untarred'.
;
;==========================================================================
;
; FILE: qview_demo.pro
;
; USAGE: qview_demo, filename
; ARGUMENTS filename Name of UF disk file
;
; ABSTRACT: Demonstration program to read a relative power field and
; display
;
; AUTHOR: I. Jeff Caylor
; Science Systems and Applications, Inc.
; 5900 Princess Garden Parkway, Suite 300
; Lanham, MD 20706
;
; at: NASA Goddard Space Flight Center
; Code 912
; Greenbelt, MD 20771
; Voice: (301) 286-3767
; Fax: (301) 286-1762
; Email: jeff.caylor@gsfc.nasa.gov
;
; CREATED: 10 April 1995
;
; MODIFIED: 11 April 1995 - IJC
; Changed rotate code from 2 to 7
; 13 July 1998 -LT
; Change count field to relative power field (no cuont field)
;
;==========================================================================
;
PRO qview_demo, filename
PRINT, 'Reading file: ', filename
OPENR, infile, filename, /GET_LUN
;
counts = fltarr(1500, 729)
;
i = 0
WHILE (EOF(infile) NE 1) DO BEGIN
UF_read_dwell, infile, ufbuf, /RDSS
;
; Extract nadir count data from the UF buffer
;help, i
UFnameddata, ufbuf, 'pn', nc, ngates, ERROR = status
counts(i, 0 : ngates - 1) = nc
print, UFstrtime(ufbuf), UFdwell(ufbuf), UFedop_ins_altitude(ufbuf), $
UFedop_ins_latitude(ufbuf), UFedop_ins_longitude(ufbuf), ngates
i = i + 1
ENDWHILE
CLOSE, infile
FREE_lun, infile
;
; Trim array and rotate
;
counts = counts(0 : i - 1, 0 : ngates - 1)
; counts = rotate(counts, 7)
;
; Display
;
image_cont, counts
END
References
Caylor I.J., G.M. Heymsfield, S. Bidwell, and S. Ameen, 1994: NASA-ER-2 Doppler radar reflectivity calibration for the CAMEX project. NASA Tech Memo 104611, 15 pp.
Heymsfield, G. M., S. Bidwell, I. J. Caylor, S. Ameen, S. Nicholson, W. Boncyk, L. Miller, D. Vandemark, P. E. Racette, and L. R. Dod, 1996: The EDOP radar system on the high-altitude NASA ER-2 aircraft. J. Atmos. Oceanic Tech., 13, 795-809.
Contact Information
To order these data or for further information, please contact:
Global Hydrology Resource Center
User Services
320 Sparkman Drive
Huntsville, AL 35805
Phone: 256-961-7932
E-mail: support-ghrc@earthdata.nasa.gov
Web: http://ghrc.nsstc.nasa.gov/
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