Cloud Variations under Subtropical High Conditions |
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Authors: | FENG Sh LIU Qi and FU Yun-Fei |
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Institution: | Laboratory of Atmospheric Observation and Climatological Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026,Laboratory of Atmospheric Observation and Climatological Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026,Laboratory of Atmospheric Observation and Climatological Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026 |
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Abstract: | The cloud variations under subtropical high (STH) conditions during summers
over a ten-year period are studied using combined data from the
International Satellite Cloud Climatology Project and the National Centers
for Environmental Prediction. The results reveal that clouds mainly
experience an isolated evolution in the STHs, which is designated in this
study by the 1540 gpm geopotential lines at 850 hPa. In the STH domain
throughout the Northern Hemisphere, the average amount of total clouds
exceeds 30%. Low clouds dominate in the STH domain, contributing over
60% of total cloud amount within the Pacific subtropical high and over
40% within the Atlantic subtropical high. The prevalence of low clouds in
above regions is determined by the circulation pattern around
150o--180oE and 850 hPa, which suppresses both the upward
development of the cloud tops and the water vapor divergences near the
surface. Furthermore, clouds present great geographical incoherence within
the STH domain. In the eastern STHs, the amount of middle and low clouds
increases to peak in the early morning and decreases to a trough in the
afternoon, while the amount of high clouds remains stable throughout the
day. Conversely, in the western STHs, the diurnal amplitude of low and
middle clouds is less than three, while high clouds dramatically reach the
maximum in the afternoon and drop to the minimum in the evening. Among the
nine cloud categories, stratocumulus clouds with greater optical thickness
account for the most under STH conditions, no matter their occurrence or
amount, causing more shortwave cloud radiative forcing to cool the local
atmosphere and surface as a consequence. |
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Keywords: | subtropical high clouds cloud variation cloud radiative forcing |
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