Abstract
Data from the Advanced Very High Resolution Radiometer (AVHRR) have been used to create several long-duration data records of geophysical variables describing the atmosphere and land and water surfaces. In the Climate Monitoring Satellite Application Facility (CM SAF) project, AVHRR data are used to derive the Cloud, Albedo, and Radiation (CLARA) climate data records of radiation components (i.a., surface albedo) and cloud properties (i.a., cloud cover). This work describes the methodology implemented for the additional estimation of the Outgoing Longwave Radiation (OLR), an important Earth radiation budget component, that is consistent with the other CLARA variables. A first step is the estimation of the instantaneous OLR from the AVHRR observations. This is done by regressions on a large database of collocated observations between AVHRR Channel 4 (10.8 µm) and 5 (12 µm) and the OLR from the Clouds and Earth’s Radiant Energy System (CERES) instruments. We investigate the applicability of this method to the first generation of AVHRR instrument (AVHRR/1) for which no Channel 5 observation is available. A second step concerns the estimation of daily and monthly OLR from the instantaneous AVHRR overpasses. This step is especially important given the changes in the local time of the observations due to the orbital drift of the NOAA satellites. We investigate the use of OLR in the ERA5 reanalysis to estimate the diurnal variation. The developed approach proves to be valuable to model the diurnal change in OLR due to day/night time warming/cooling over clear land. Finally, the resulting monthly mean AVHRR OLR product is intercompared with the CERES monthly mean product. For a typical configuration with one morning and one afternoon AVHRR observation, the Root Mean Square (RMS) difference with CERES monthly mean OLR is about 2 Wm−2 at 1° × 1° resolution. We quantify the degradation of the OLR product when only one AVHRR instrument is available (as is the case for some periods in the 1980s) and also the improvement when more instruments are available (e.g., using METOP-A, NOAA-15, NOAA-18, and NOAA-19 in 2012). The degradation of the OLR product from AVHRR/1 instruments is also quantified, which is done by “masking” the Channel 5 observations.
Highlights
The Climate Monitoring Satellite Application Facility (CM SAF) [1] has been established to provide an important contribution to the global climate monitoring
This RMSE was calculated by collecting all instantaneous Outgoing Longwave Radiation (OLR) observations (CERES Single Scanner Footprint (SSF)) and corresponding OLR estimations (AVHRR-derived) and computing the root mean square over all their differences
Higher error was associated with high level clouds, reflecting the combination of increased Advanced Very High Resolution Radiometer (AVHRR)-Clouds and Earth’s Radiant Energy System (CERES) matching error and, in the case of cirrus, increased anisotropy
Summary
The Climate Monitoring Satellite Application Facility (CM SAF) [1] has been established to provide an important contribution to the global climate monitoring. One of the main successes of this project is the generation of the Cloud, Albedo, and Radiation (CLARA) dataset based on the global observations from the Advanced Very High Resolution Radiometer (AVHRR) [2] onboard the NOAA and METOP satellites. A first edition of the CLARA dataset [3] was released in 2013, and a second one followed in 2017 [4]. CM SAF is currently developing a third edition, CLARA-A3, with a planned release at the end of 2021.
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