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New Atlas Presents Global Analyses of Surface Marine Data
Earth System Monitor (Vol. 6, NO. 2, December 1995)
Arlindo M. Da Silva, Data Assimilation Office, Goddard Space Flight Center NASA
Sydney Levitus, National Oceanographic Data Center, NOAA/NESDIS
|A recent collaboration between researchers at the Department of Geosciences of the University of Wisconsin--Milwaukee (UWM) and the Ocean Climate Laboratory (OCL) at the National Oceanographic Data Center (NODC) has culminated in the production of objective analyses of observed and derived surface marine parameters. These global analyses are based on individual observations found in the Comprehensive Ocean--Atmosphere Data Set (COADS) Release 1 complemented by an interim release for the 1980's. Results of the analyses are presented in Atlas of Surface Marine Data 1994 , a five volume series depicting 45-year seasonal climatologies (1945--89), anomalies, and standard deviations of various quantities characterizing the surface marine climate (da Silva 1994 a,b,c,d,e). The analyses of monthly climatologies, anomalies and standard deviations are available in a set of 3 CD-ROMs or on exabyte tape. The raw statistics are also available on exabyte tape for those users desiring to perform their own objective analysis. The CD-ROMs are available from the NNDC Online Store.|
The analyses presented in Atlas of Surface Marine Data 1994 are derived from the individual observations found in Compressed Marine Reports--Product 5 (CMR-5) of COADS Release 1 (Slutz et al. 1985, Woodruff et al. 1987). Observations included in COADS/CMR-5 consist of reports of wind, air and sea surface temperature, sea level pressure, humidity, and present weather made primarily by ships of the Voluntary Observing Fleet. Observations included in this historical collection have also been taken by various military ships, ocean weather ships, light ships, research vessels, buoys, and bathythermographs. In addition to the observations, COADS/CMR-5 provides metadata such as quality control flags and measurement type indicators. Although COADS/CMR-5 includes observations as far back as 1854, the analyses in Atlas of Surface Marine Data 1994 cover only the years 1945 through 1989.
The joint project between UWM and OCL (known also as the UWM/COADS project) is an effort to improve and expand the standard COADS statistics. The first improvement is an attempt to reduce known biases in the ship reports in addition to the quality control provided in COADS/CMR-5. In the UWM/COADS project, bias corrections have been applied to individual observations of cloudiness, present weather and Beaufort-estimated winds.
In order to account for biases in wind speed estimates based on sea-state, a new Beaufort equivalent scale was developed. The other bias corrections are the interpretation of certain missing present weather observations as clear weather, trimming cloudiness observations taken in low light conditions, and interpreting certain sky obscured cloudiness observations as 100% cloud cover.
A second improvement is higher resolution. The COADS/MST are on a 2 degree x 2 degree grid while the UWM/COADS raw statistics have been calculated on a 1 degree x 1 degree grid identical to that used by Levitus (1982), Conkright et al. (1994) and Levitus et al. (1994a,b,c). In addition to the raw statistics, UWM/COADS includes objectively analyzed fields, where a successive correction analysis has been applied to interpolate values to missing boxes and remove small scale, noisy features. A similar objective analysis of COADS/MST has been performed at NOAA's Geophysical Fluid Dynamics Laboratory. This analysis is essentially the same as that used by Levitus (1982), Conkright et al. (1994) and Levitus et al. (1994a,b,c).
The UWM/COADS data set also extends the COADS/MST by providing a complete set of heat (sensible, latent, radiational), momentum, and fresh water (evaporation, precipitation, buoyancy) fluxes. These fluxes have been estimated using wind-speed dependent transfer coefficients and stability effects (Large and Pond 1981, 1982), unlike the pseudo fluxes found in the COADS/MST where transfer coefficients have been omitted. Precipitation estimates are based on Present Weather reports using the algorithm of Tucker (1961) with a variant of Dorman and Bourke's (1978) corrections.
By calculating a complete array of atmospheric forcing parameters, and by using the same grid and analysis scheme as in Conkright et al. (1994) and Levitus et al. (1994a,b,c), the UWM/COADS data set is intended to be used as a complement to the surface and subsurface marine analyses found in Conkright et al. (1994) and Levitus et al. (1994a,b,c).Organization
The multi-volume Atlas of Surface Marine Data 1994 (da Silva et al. 1994a,b,c,d,e) is organized into two main categories. The first category, in-depth documentation of the data set, is found in Volume 1. Besides describing all parameterizations, calculations, and analysis methods used in creating the data set, this volume discusses possible biases the COADS data may contain and explains the bias corrections employed. Other subjects discussed in Volume 1 are the constraining of heat and fresh water fluxes by oceanographic transport estimates, sampling and fair weather biases, surface layer formulation, equations for astronomical calculations (e.g., altitude of the sun), and NetCDF data access software.
The second main category is the graphical representation of the data set. Volumes 2 through 5 consist mainly of seasonal contour plots of climatology, standard deviation, and anomalies of the analyzed quantities. The quantities are organized according to parameter type. Volume 2 contains directly observed quantities, Volume 3 contains heat and momentum fluxes. Fresh water fluxes can be found in Volume 4, and Volume 5 has an assortment of miscellaneous derived quantities.
The data files associated with Atlas of Surface Marine Data 1994 are available in both their analyzed and unanalyzed forms on CD-ROM and exabyte tape. The majority of the analyzed quantities are contained in a 3 CD-ROM set, organized by parameter type. Disc 1 contains directly observed quantities (and number of observation files). Disc 2 contains heat, momentum, and fresh water fluxes. Disc 3 contains the remaining miscellaneous parameters.
On the CD-ROMs or magnetic tape, the 12 analyzed monthly climatologies (1945--1989) and the 540 analyzed monthly anomalies (12 months x 45 years) are stored in a single binary file for each quantity using Unidata's Network Common Data Format (NetCDF). NetCDF allows access to binary files from most computer platforms, using a single set of FORTRAN or C subroutines. Easy to use FORTRAN access software is provided for those unfamiliar with NetCDF. Number of observations per square are stored similarly. Standard deviation fields are stored in smaller files (12 monthly fields per file) also using NetCDF. Each anomaly or number of observations file is approximately 46 megabytes. The standard deviation files are a little more than 1 megabyte in size.
Unanalyzed data for each parameter is stored using NetCDF as well, with separate files for each year. An unanalyzed file contains the 12 monthly fields of mean, standard deviation, and number of observations for a single year. Information is stored for only those squares containing 1 or more observations during that month. While the size of these yearly files varies greatly, the total byte count for each parameter is around 150 megabytes. At this time, the unanalyzed data is available only on exabyte tape. A CD-ROM set is under consideration.Acknowledgments
This research has been supported by NSF grant ATM 9215811 (AMdS/CCY) and by NOAA's Climate and Global Change Program (SL/CCY). This work was concluded after A. da Silva joined the Data Assimilation Office of NASA's Goddard Space Flight Center; the support of R. Rood is acknowledged. The data sets and products represented by this atlas are for distribution internationally, without restriction.References
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