Abstract
Quantification of long term changes in cloud distribution and properties is critical for the proper assessment of future climate. We show contrasting trends in cloud properties and cloud radiative effects over Northwest Indian Ocean (NWIO) in south Asian summer monsoon. Cloud top height (CTH) decreases in June (− 69 ± 3 myr−1) and July (− 44 ± 3 myr−1), whereas it increases in August (106 ± 2 myr−1) and September (37 ± 1 myr−1). These contrasting trends are investigated to be due to the changes in upper tropospheric winds and atmospheric circulation pattern. Strengthening of upper tropospheric easterlies and changes in vertical wind dampen the vertical development of clouds in June and July. In contrast, weakening of upper tropospheric winds over NWIO and strengthening of updraft favour the vertical growth of clouds in August. Further, changes in horizontal winds at 450–350 hPa and strengthening of Indian Ocean Walker cell favour the westward spread of high level clouds, contributing to the increase in CTH over NWIO in August. Decrease of cloud cover and altitude in June and July and increase of the same in subsequent months would affect the monsoon rainfall over the Indian region. Proper representation of these intra-seasonal contrasting trends of clouds in climate models is important for the better prediction of regional weather.
Highlights
Quantification of long term changes in cloud distribution and properties is critical for the proper assessment of future climate
Our study shows changes in atmospheric circulation pattern and their effects on cloud distribution over the south Asian summer monsoon region
This enhancement in westward movement of clouds leads to increase in amount of high level clouds and increase in mean Cloud top height (CTH) and cloud fraction (CF) over Northwest Indian Ocean (NWIO) in the second half of the season, whereas the hindrance to this westward movement of clouds causes decrease in amount of high level clouds and decrease in mean CTH and CF over NWIO in the first half
Summary
Quantification of long term changes in cloud distribution and properties is critical for the proper assessment of future climate. Cloud top height (CTH) decreases in June (− 69 ± 3 m yr−1) and July (− 44 ± 3 m yr−1), whereas it increases in August (106 ± 2 m yr−1) and September (37 ± 1 m yr−1) These contrasting trends are investigated to be due to the changes in upper tropospheric winds and atmospheric circulation pattern. Decrease of cloud cover and altitude in June and July and increase of the same in subsequent months would affect the monsoon rainfall over the Indian region. Proper representation of these intra-seasonal contrasting trends of clouds in climate models is important for the better prediction of regional weather. We show modifications in circulation patterns leading to contrasting trends in cloud properties, even within the same season, over the south Asian summer monsoon region
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