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Comparison of the Highly Reflective Cloud and Outgoing Longwave Radiation Datasets for Use in Estimating Tropical Deep Convection

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  • 1 California Space Institute, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California
  • | 2 Climate Research Division, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California
  • | 3 Earth Space Research Group, University of California at Santa Barbara, Santa Barbara, California
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Abstract

Currently, there are two long-term satellite-derived datasets most are frequently used as indices for tropical deep convection. These are the Outgoing Longwave Radiation (OLR) and Highly Reflective Cloud (HRC) datasets. Although both of these datasets have demonstrated their value, no direct comparison of these datasets has been conducted, to determine how well they agree when used to estimate tropical convection, nor has there been much work toward comparing these long-record datasets with more recently developed convection datasets. This information is vital since the inhomogeneous sampling of the in situ rainfall record makes it inadequate for many studies concerning tropical convection and the more modern datasets have not achieved a climatologically useful record length for all studies. The goal of this paper is to compare these two datasets in order to quantify their strengths and weaknesses. This information will provide guidance in choosing the most appropriate dataset(s) for subsequent studies, interpreting the results from those studies, and extending more modern convection datasets backward.

Comparisons are done in terms of their climatological and frequency-dependent characteristics, their consistency in identifying deep tropical convection, and their relationships to local sea surface temperature (SST). Additionally, use is made of the more modern, shorter-terrain International Satellite Cloud Climatology Project stage C2 dataset as a means of further comparison and validation. The results of this study reveal some important differences between the HRC and OLR in terms of their temporal and spatial scales of variability, their relationships to other geophysical fields, and the logistics of their use. Further, they suggest that for many applications the HRC more accurately represents the characteristics of cloud cluster-scale tropical convection. This is especially true in cases where 1) characterization of the spatial scales or frequency-dependent variability of convection is important, 2) the relationships between deep convection and SST or water vapor are being considered, and 3) the domain of interest is large enough to contain spatial inhomogeneities, such as land-sea contrasts or inhomogeneous SST and moisture fields.

One new and important finding of this study is that both the OLR-SST and HRC-SST relationships show that SSTs in excess of about 29.5°C tend to occur only under conditions of diminished convection. Thus, the maximum convective activity does not occur over the warmest (>29.5°C) water; rather, the warmest water occurs under “clear,” less-convective skies. Further, our results empirically demonstrate that in a highly convective regime the maximum equilibrium SST that can be supported is about 29.5°C. These results are further evidence that convective-cloud complexes provide a systematic and climatologically important cooling effect on the surface temperature.

Abstract

Currently, there are two long-term satellite-derived datasets most are frequently used as indices for tropical deep convection. These are the Outgoing Longwave Radiation (OLR) and Highly Reflective Cloud (HRC) datasets. Although both of these datasets have demonstrated their value, no direct comparison of these datasets has been conducted, to determine how well they agree when used to estimate tropical convection, nor has there been much work toward comparing these long-record datasets with more recently developed convection datasets. This information is vital since the inhomogeneous sampling of the in situ rainfall record makes it inadequate for many studies concerning tropical convection and the more modern datasets have not achieved a climatologically useful record length for all studies. The goal of this paper is to compare these two datasets in order to quantify their strengths and weaknesses. This information will provide guidance in choosing the most appropriate dataset(s) for subsequent studies, interpreting the results from those studies, and extending more modern convection datasets backward.

Comparisons are done in terms of their climatological and frequency-dependent characteristics, their consistency in identifying deep tropical convection, and their relationships to local sea surface temperature (SST). Additionally, use is made of the more modern, shorter-terrain International Satellite Cloud Climatology Project stage C2 dataset as a means of further comparison and validation. The results of this study reveal some important differences between the HRC and OLR in terms of their temporal and spatial scales of variability, their relationships to other geophysical fields, and the logistics of their use. Further, they suggest that for many applications the HRC more accurately represents the characteristics of cloud cluster-scale tropical convection. This is especially true in cases where 1) characterization of the spatial scales or frequency-dependent variability of convection is important, 2) the relationships between deep convection and SST or water vapor are being considered, and 3) the domain of interest is large enough to contain spatial inhomogeneities, such as land-sea contrasts or inhomogeneous SST and moisture fields.

One new and important finding of this study is that both the OLR-SST and HRC-SST relationships show that SSTs in excess of about 29.5°C tend to occur only under conditions of diminished convection. Thus, the maximum convective activity does not occur over the warmest (>29.5°C) water; rather, the warmest water occurs under “clear,” less-convective skies. Further, our results empirically demonstrate that in a highly convective regime the maximum equilibrium SST that can be supported is about 29.5°C. These results are further evidence that convective-cloud complexes provide a systematic and climatologically important cooling effect on the surface temperature.

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