Pathfinder Matchup Database - Overview
This document describes the compilation of a multi-year, multi-satellite database of approximately co-temproal, co-located in situ sea surface temperature and AVHRR measurements. These redords, called “matchups”, can be used to develop and test algorithms for the estimation of sea surface temperature using AVHRR data.
Introduction
The Pathfinder program was jointly developed by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) with the goal of providing long-term, consistently calibrated global change-related data sets to Earth scientists. In addition to providing data sets for global change research, the processing and end-to-end management involved in generating consistent Pathfinder products serve as testing grounds for handling the large volumes of data expected in the next few years from Earth observing systems (NOAA-NASA, 1994).
One of the data sets selected for the Pathfinder project was collected by the Advanced Very High Resolution Radiometer (AVHRR), a scanning radiometer flying onboard polar orbiters operated by NOAA. The AVHRR Pathfinder involves four separate elements: Atmosphere, Land, Oceans and Calibration. The objective of the AVHRR Oceans Pathfinder is to develop a long (>10 years) and consistent time series of global sea surface temperature (SST) fields for climate studies.
Algorithms to estimate AVHRR multichannel SSTs traditionally have been based on equations derived from simplifications of radiative transfer theory. Once a functional form is adopted, algorithm coefficients are typically estimated following two main methodologies. The first method involves the use of a radiative transfer model and a set of atmospheric temperature and humidity profiles to simulate brightness temperatures at the satellite sensor; the coefficients are estimated through regression between the surface temperature and the simulated brightness temperatures. This approach has been used, for instance, in the development of SST algorithms for the Along-Track Scanning Radiometer (ATSR) onboard the European satellite ERS-1 (Závody et al. 1995). The second method involves the regression of satellite-sensed brightness temperatures against in situ SST observations. The latter approach has been followed by NOAA for the derivation of AVHRR SST algorithms (Strong and McClain 1984). This method requires co-temporal, co-located AVHRR and in situ observations, called “matchups”.
One of the AVHRR Oceans Pathfinder highlights is that, for the first time, a large validation data set will be distributed to accompany the Pathfinder global SST fields. This document describes a multi-satellite, multi-year database of AVHRR and in situ SST matchups. The Pathfinder Matchup Data Base (PFMDB) involves several files (organized by satellite, year, and month) distributed separately; more details are given below. The purpose of the PFMDB is to allow interested investigators to develop and validate alternative formulations for the computation of AVHRR-derived SSTs. We have assembled a fairly large collection of high-quality in situ SSTs with a reasonably wide distribution in space and time. This will allow investigators to validate algorithm performance under a wide range of atmospheric and oceanic regimes.
In the following sections we will detail the steps involved in the compilation of the Pathfinder Matchup Database, describe the quantities included in the matchup records, and provide tables summarizing the composition of the PFMDB. At the end of the document, we provide appendices showing (a) currently available matchup files, and (b) maps of the geographic distribution of the available matchups by year.
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