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GOES WATER VAPOR TRANSPORT CLIMATE DATAREADME DOCUMENTATIONGLOBAL HYDROLOGY AND CLIMATE CENTER |
This file contains text documentation to support the 1987/88 GOES Water
Vapor Transport Climate Dataset. This file describes the dataset, how
it was made, and its limitations.
Table of Contents:
- Directory Structure
- Directory Contents/Data Inventory
- Summary of Retrieval Technique
- Point Data Description
- Grid Data Description
- Contact Information
- References
1. Directory Structure
The data is contained in the following self-explanatory directory structure:
software/
pnt_dat/
daily/
bin/
mcidas/
grid_dat/
daily/
bin/
mcidas/
smpl_gif/
monthly/
bin/
mcidas/
smpl_gif/
2. Directory Contents/Data Inventory
The root directory contains the introductory HTML file- index.htm, the main HTML documentation- (this file) readme.htm, a text version of the documentation- Readme.txt, and other ancillary files.
readme.htm is the suggested documentation, since it contains links to supporting documentation, sample GIF images, as well as contact point information (URLs).
The /software directory contains subdirectories /unix and /dos which contain sample software for those systems.
The /pnt_dat directory contains a subdirectory called /daily which contains subdirectories /mcidas and /binary which contain data in those formats. The point data files in these directories are of different lengths depending on how many wind vectors were successfully retrieved. Unlike the gridded data, many of these files contain data which extends poleward of 45N and 30S.
The /grid_dat directory is separated into both /daily and /monthly subdirectories that are both further divided according to the data format (/mcidas and /binary). Both /daily and /monthly directories also have a /smpl_gif subdirectory to assist you in verifying the data is being read correctly. The gridded data contain 1 X 1 degree data fields for the 19 month GOES Pathfinder period with the exception of the days listed in Table 2.1. These days were missing due to either: errors in satellite navigation, lack of three consecutive hourly images to produce reliable winds, or large missing sectors in scan.
Table 2.1: Days where water vapor wind retrievals were not made due to scheduling, missing sections of scan, or navigation errors.
Beginning of dataset May 5, 1987
End of dataset Nov 31, 1988
Month Day of month Day of year
==================================
1987
Jun 1 152
3-4 154-155
28 179
Jul 1 182
4 185
8 189
17-18 198-199
20 201
23 204
25 206
Aug 2 214
14 226
23 235
Sep 1 244
Oct 31 304
Dec 3 337
1988
Jan 1-4 001-004
Feb 5 036
15 046
Mar 2 062
Apr 24 115
May 8 129
30 151
Jun 3 155
22 174
==================================
1987 total missing: 18
1988 total missing: 12
Total number of missing days: 30
Total possible days: 576 (5% missing)
3. Summary of Retrieval Technique
WINDSThe water vapor winds were derived using the Marshall Automated Winds (MAW) tracking algorithm (Atkinson 1984; Jedlovec et al. 1999). MAW uses a minimum-difference template matching scheme for feature identification. Three water vapor images (1100, 1200, and 1300 UTC) were used to determine a pair of wind vectors for each template of the image. Due to scheduling changes or missing data, three alternate hourly images were used on several days during the 19 month period (see Table 3.1). A template size of 49 X 49 pixels (pixel size: 8 X 8 km) was used to track the movement of features from image to image. The quality and control procedures used to create all gridded data fields included a vector pair discrepancy of 15 m/s in speed and 30 degrees in direction. Using these quality and control procedures reduces random errors with the GOES-VAS data to less than 4 m/s (Jedlovec and Atkinson 1996).
Table 3.1: List of alternate times used in making wind retrievals. For days not listed, default retrieval image times are 1100, 1200, and 1300 UTC.
Date Days of Year Times used (UTC) ================================================= 1987 May 5 - May 31 125-151 1300,1400,1500 Jun 4 - Jun 30 155-181 1400,1500,1600 Jul 2 183 1300,1400,1500 Jul 3 - Jul 18 184-199 1400,1500,1600 Jul 27 208 1400,1500,1600 Aug 3 215 1400,1500,1600 Nov 16 - Nov 17 320-321 1000,1100,1200 Nov 22 - Nov 23 326-327 1000,1100,1200 Dec 1 335 1400,1500,1600 Dec 15 349 1200,1300,1400 Dec 17 351 1400,1500,1600 1988 Mar 5 065 1200,1300,1400 Apr 13 104 1300,1400,1500 Apr 16 107 1300,1400,1500 May 9 130 1000,1100,1200 May 24 145 1300,1400,1500 May 26 147 1000,1100,1200 Jun 7 159 1400,1500,1600 Jun 15 167 1400,1500,1600 Jul 8 - Jul 9 190-191 1400,1500,1600 Jul 11 193 1400,1500,1600 Sep 19 - Sep 21 263-265 1000,1100,1200 Sep 29 273 1000,1100,1200 Oct 29 303 1300,1400,1500 Nov 5 310 1000,1100,1200 Nov 15 - Nov 16 320-321 1000,1100,1200 Nov 20 325 1000,1100,1200 Nov 24 329 1300,1400,1500 Nov 26 331 1000,1100,1200 =================================================
HUMIDITY
Relative humidity is calculated using a modified version of the Soden
and Bretherton (1996) brightness temperature to relative humidity conversion
technique. More details about the modifications are posted at
http://wwwghcc.msfc.nasa.gov/irgrp/wvti.html.
Relative humidity (RH) is converted to specific humidity (q) by
using the simple formula:
q ~= qs x RH / 100where the saturated specific humidity qs, is given by:
qs = (621.97 x esi) / p.p is the wind pressure height. The saturated vapor pressure is calculated with respect to ice; i.e.
esi = 6.11 X 10 **([9.5 * Tb] / [Tb+ 265.5]) (3.1)The template averaged brightness temperature is denoted by T b in Equation 3.1 above.
HEIGHT ASSIGNMENT
Pressure heights are assigned to each wind vector using the simple IR
window technique first proposed by Fritz and Winston (1962). This simply
involves matching the template averaged 6.7 µm brightness temperature
to a nearby model temperature profile. For this dataset, the NCEP/NCAR
reanalysis temperature profile at 5 X 5 degree resolution
at 1200 UTC daily was used.
4. Point Data Description
4.1 McIDAS Format (MD files)
The McIDAS MD files are named:
MDXyydddwhere yy is the year and ddd is the day of year (eg. 88239)
The McIDAS MD file structure places irregularly spaced data points into individual records within the file structure. Each record contains the variables outlined in an MD file "SCHEMA" which is located in the file /pnt_dat/daily/mcidas/DCGWVT.TXT. A sample record of a single retrieval point is shown in Table 4.1.
Table 4.1: Example of MD file record
--RECORD AT (ROW,COL) = ( 1, 164) |DAY = 88239 SYD | TIME = 120100 HMS | LAT = 22.2063 DEG |LON = 83.7576 DEG | U = -1.86 MPS | V = -10.24 MPS |P = 296 MB | T = 241 K | RH = 46 PCT |Q = 0.288 GPKG | FLAG = 2 | SDEV = 8 MPS |DDEV = 1 DEG |
FLAG corresponds to the error code for that particular record. Table 4.2 lists the possible error codes associated with each retrieval. The number for the error code may represent the sum of two or more errors.
SDEV is the speed deviation among two vectors used to produce the final wind vector. This is one of the quality and control constraints in producing the grid files.
DDEV is the directional deviation among the two vectors. A DDEV of 30° or greater was used in flagging vectors as bad for producing grids.
Table 4.2: Quality and control error codes for McIDAS MDFILE point data FLAG parameter. Actual error code may represent sum of different QC error codes.
-----------------------------------------------------------------
---
--- ERROR CODES:
--- -4 - MANUAL CHECK FAIL
--- 0 - NO ERROR
--- 1 - U DEPARTURE FROM GUESS
--- 2 - V DEPARTURE FROM GUESS
--- 3 - U & V DEPARTURE FROM GUESS
--- 10 - U ACCELERATION
--- 20 - V ACCELERATION
--- 30 - U & V ACCELERATION
-----------------------------------------------------------------
-4 : Manual check fails correspond to individual retrievals that
were flagged as bad vectors by subjective analysis.
1-3 : Departure from guess error codes should be disregarded since
no guess wind speed and direction were used to create the
dataset.
10,20,30 : Acceleration error code flags correspond to the 2 vectors
used to generate the final wind retrieval vector (i.e. V1 from
the 1100 and 1200 imagery; and V2 from 1200 and 1300 imagery).
If the second of 2 vectors (used to determine the final vector)
underwent acceleration greater than 5 m/s, then it is flagged.
-----------------------------------------------------------------
The binary version of the McIDAS MD files are named
MDXyyddd.bin
where yy is year (e.g., 88) and ddd is the day of year (e.g., 239).
The binary point data files contain several hundred sets of 11 values, corresponding to the parameters listed in Table 4.3. The number of sets varies with file. In each set, the first two values are 4-byte words and the remaining nine are 2-byte words (see figure below). Each set is therefore 26 bytes.
Set 1 Set 2
Lat Lon U V P T RH Q fl sd dd Lat Lon U V P T RH Q fl sd dd ...
|----|----|--|--|--|--|--|--|--|--|--|----|----|--|--|--|--|--|--|--|--|--| ...
The scaling factors needed to properly decode the values are found in Table 4.3 below. Values in binary file need to be divided by the appropriate scale factor to find the proper geophysical values.
Table 4.3: Scaling factors for decoding values in binary point data files.
Parameter Units Scale
factor
______________________________
Latitude deg 10000
Longitude deg 10000
U m/s 100
V m/s 100
Pressure hPa 1
Temperature K 1
RH % 1
Q g/kg 1000
flag N/A 1
speed dev m/s 1
dir dev deg 1
4.2.1 Sample Read Software
Sample read (C) software for point data is located in /software/read_mds.c.
The (standard) output of this program is an ASCII tabular listing of all
points in the file (with a 2 line header). Modify the source code
to meet your output format requirements. Compile the code, then execute
it as follows:
read_mds binary_MD_file
e.g., read_mds MDX88126.bin
The program was designed for either Unix or PC/VAX use. For Unix machines, no options/changes are required. For PCs or VAXs, either define the variable PCVAX in the source code or simply compile with the -D (Define) option like so:
cc -o read_mds read_mds.c -DPCVAXThe latter will define the variable PCVAX with a value of 1.
* To avoid potential processor differences, it is helpful to compile on the machine where the code will be executed.
5. Grid Data Description
5.1 McIDAS Format (GRID Files)
The McIDAS GRID files are named:
GRIyyddd
where yy is year (e.g., 88) and ddd is the day of year (e.g., 239).
The McIDAS grid file domain extends from 30S 120W in the lower left corner to 45N 30W in the upper right corner producing a total of 76 rows and 91 columns. Each grid file contains 10 grids (arrays) corresponding to 10 variables. A McIDAS listing example is given in Table 5.1. The grids were produced from the McIDAS point-data (MD) files using a Barnes style objective analysis (Barnes 1964) to generate the 1 x 1 degree resolution grids. The quality control constraints include a speed and directional deviation check. A pair of vectors are derived from three consecutive satellite images. Speed deviations greater than 15 m/s and directional deviations greater than 30° among the vectors were deemed bad and excluded from the gridding process. There was a final manual (subjective) check of outstanding wind vectors to screen out obvious errors.
Table 5.1: Example of McIDAS GRID file listing:
Grid file: 88239 **GOES WVT GRIDS 88239 **
# YYDDD HHMMSS NAME LEVEL SRC VT NR NC LLNW ROWINC COLINC
----- ----- ------ ---- ------- ---- -- -- -- -------- ------ ------
1 88239 120100 U TRO MDX 0 76 91 45/ 120 1.0000 1.0000
2 88239 120100 V TRO MDX 0 76 91 45/ 120 1.0000 1.0000
3 88239 120100 T TRO MDX 0 76 91 45/ 120 1.0000 1.0000
4 88239 120100 P TRO MDX 0 76 91 45/ 120 1.0000 1.0000
5 88239 120100 RH TRO MDX 0 76 91 45/ 120 1.0000 1.0000
6 88239 120100 Q TRO MDX 0 76 91 45/ 120 1.0000 1.0000
7 88239 120100 SPD TRO MDX 0 76 91 45/ 120 1.0000 1.0000
8 88239 120100 qv TRO MDX 0 76 91 45/ 120 1.0000 1.0000
9 88239 120100 qu TRO MDX 0 76 91 45/ 120 1.0000 1.0000
10 88239 120100 WVTI TRO MDX 0 76 91 45/ 120 1.0000 1.0000
---End of Listing
-------------------------------------------------------------------------
U is the East-West component of the wind (positive Westerlies)
V is the North-South component of the wind (positive Southerlies)
T is the template-averaged brightness temperature
P is the GOES wind pressure height
RH is the relative humidity
Q is the specific humidity
SPD is the total wind speed
qu is the zonal transport of specific humidity (q)
qv is the meridional transport of specific humidity (q)
WVTI is the Water Vapor Transport Index (q * SPD)
5.2 Binary Format
The binary version of the McIDAS GRID files are named
GRIyyddd.bin
where yy is year (e.g., 88) and ddd is the day of year (e.g., 239).
Each binary gridded data file contains 10 grids, each with 76 rows and
91 columns. There are no headers or separators. All values are 2-bytes.
Each array is therefore 6916 2-byte words long or 13832 bytes. With 10
grids, each file is therefore 138320 bytes. The grids are in the following
order:
Parameter Units Scale
factor
_______________________________
U m/s 100
V m/s 100
Temperature K 1
Pressure hPa 1
RH % 1
Q g/kg 1000
Wind Speed m/s 100
QV g/kg m/s 100
QU g/kg m/s 100
WVTI g/kg m/s 100
5.2.1 Sample Read Software
Sample read (C) software for daily and monthly grid data is located in
/software/read_grids.c.
The (standard) output of this code is an ASCII listing of all 10 grids
in the file (each with a 1 line header/title). Each line (except the title)
of the file has 91 values of varying field widths (depending on parameter).
Modify the source code to meet your output format requirements. Compile
the code, then execute it as follows:
read_grids binary_grid_file
e.g., read_grids GRI88126.bin
The program was designed for either Unix or PC/VAX use. For Unix machines, no options/changes are required. For PCs or VAXs, either define the variable PCVAX in the source code or simply compile with the -D (Define) option like so:
cc -o read_grids read_grids.c -DPCVAXThe latter will define the variable PCVAX with a value of 1.
* To avoid potential processor differences, it is helpful to compile on the machine where the code will be executed.
5.3 Sample Images
Samples of daily and monthly grids are available. These GIFs were created
using McIDAS. Any GIF viewer or browser will work.
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6. Contact Information
This dataset is produced at the Global Hydrology and Climate Center (GHCC) / NASA Marshall Space Flight Center. The dataset production is carried out by the Infrared Measurements Group of the Earth Sciences Division. Distribution and archiving is done by the Global Hydrology and Resource Center (GHRC). Technical assistance is available through the GHRC by phone at (256) 922-5932 or by email: ghrc@microwave.msfc.nasa.gov. Further technical inquiries may be directed towards Gary Jedlovec (gary.jedlovec@msfc.nasa.gov).
7. References
Atkinson, R. J., 1984: Automated mesoscale winds determined from satellite imagery. Interim Report on NAS8-34596, General Electric Company, Huntsville, AL, 51 pp. Barnes, S. L., 1964: A technique for maximizing details in numerical weather map analysis. J. Appl. Meteor., 3, 396-409. Fritz, S. and J.S. Winston, 1962: Synoptic use of radiation measurements from satellite TIROS-II. Mon. Wea. Rev., 90, 1-9. Jedlovec, G. J., and R. J. Atkinson, 1996: Quality and control of water vapor winds. Proceedings of the Eighth Conf. on Satellite Meteorology and Oceanography, AMS, Boston, 5-9. Jedlovec, G. J., J. A. Lerner, and R. J. Atkinson, 1999: A satellite derived upper-tropospheric water vapor transport index for climate studies. J. Appl. Meteor., (accepted) Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J. Woollen, Y. Zhu, M. Chelliah, W. Ebisuzaki, W. Higgins, J. Janowiak, K. C. Mo, C. Ropelewski, J. Wang, A. Leetmaa, R. Reynolds, R. Jenne, and D. Joseph, 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437-471. Soden, B.J. and F.P. Bretherton, 1996: Interpretation of TOVS water vapor radiances in terms of layer-average relative humidities: Method and climatology for the upper, middle, and lower troposphere. J. Geophys. Res. 101, 9333-9343.