A study of vertical cloud structure of the Indian summer monsoon using CloudSat data
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Precise specification of the vertical distribution of cloud optical properties is important to reduce the uncertainty in quantifying the radiative impacts of clouds. The new global observations of vertical profiles of clouds from the CloudSat mission provide opportunities to describe cloud structures and to improve parameterization of clouds in the weather and climate prediction models. In this study, four years (2007–2010) of observations of vertical structure of clouds from the CloudSat cloud profiling radar have been used to document the mean vertical structure of clouds associated with the Indian summer monsoon (ISM) and its intra-seasonal variability. Active and break monsoon spells associated with the intra-seasonal variability of ISM have been identified by an objective criterion. For the present analysis, we considered CloudSat derived column integrated cloud liquid and ice water, and vertically profiles of cloud liquid and ice water content. Over the South Asian monsoon region, deep convective clouds with large vertical extent (up to 14 km) and large values of cloud water and ice content are observed over the north Bay of Bengal. Deep clouds with large ice water content are also observed over north Arabian Sea and adjoining northwest India, along the west coast of India and the south equatorial Indian Ocean. The active monsoon spells are characterized by enhanced deep convection over the Bay of Bengal, west coast of India and northeast Arabian Sea and suppressed convection over the equatorial Indian Ocean. Over the Bay of Bengal, cloud liquid water content and ice water content is enhanced by ~90 and ~200 % respectively during the active spells. An interesting feature associated with the active spell is the vertical tilting structure of positive CLWC and CIWC anomalies over the Arabian Sea and the Bay of Bengal, which suggests a pre-conditioning process for the northward propagation of the boreal summer intra-seasonal variability. It is also observed that during the break spells, clouds are not completely suppressed over central India. Instead, clouds with smaller vertical extent (3–5 km) are observed due to the presence of a heat low type of circulation. The present results will be useful for validating the vertical structure of clouds in weather and climate prediction models.
KeywordsClouds Indian monsoon Intra-seasonal Oscillation Active-break cycle
We are thankful to the NASA CloudSat project and CloudSat data processing centre for providing us the CloudSat data used in this study. We are grateful to Dr. Karen Milberger and Dr. Donald L. Reinke, CloudSat data processing centre, Cooperative Institute for Research in the Atmosphere (CIRA), Colorado State University in their kind help in sending us the CloudSat data in tape drives. We also thank the Global Modeling and Assimilation Office (GMAO) and the GES DISC for dissemination of MERRA. We are also thankful to three anonymous reviewers for their constructive comments and suggestions, which helped us to improve the quality of the paper. We also thank Dr. X. Jiang for his useful suggestions on CloudSat data sampling.
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