The growth of the daytime convective atmospheric boundary layer, which plays a pivotal role in the vertical mixing and dispersal of water vapour and pollutants, is modulated by cloud radiative effects. Assessment of this cloud effect is sparse in any geographical region and non-existent over tropical coastal regions. We investigate the effect of clouds on the diurnal evolution of boundary-layer height over tropical coastal location Thumba (8.5°N, 77°E) during onshore and offshore flow using multi-year (2010–2016) microwave radiometer profiler observations. The boundary-layer height during both cloudy and clear-sky periods increases rapidly from 0800 LT (local time = UTC + 5.1 h) to attain a daytime peak around noon (400–1500 m). The seasonal mean noontime boundary layer height during cloudy periods is lower than that during clear-sky periods by > 900 m (> 400 m) when offshore (onshore) flow prevails during winter and pre-monsoon seasons. The forenoon growth rate of the boundary-layer height during clear-sky offshore flow is rapid (> 380 m h −1 ) compared to that during cloudy offshore (160 m h −1 ), clear-sky onshore (160–250 m h −1 ), and cloudy onshore (> 100 m h −1 ) flow. Effects of shortwave cloud radiative forcing and soil temperature on the noontime boundary-layer height and their interdependencies are presented. These observations reveal the contrasting and significant effect of clouds on the growth of the daytime boundary layer during onshore and offshore flow, and the coupled effects of cloud radiative forcing and soil temperature on boundary-layer height over tropical coastal regions, which provide essential constraints for evaluating model simulations.

中文翻译：

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云对热带海岸站大气边界层高度日变化的影响

白天对流大气边界层的增长在水汽和污染物的垂直混合和扩散中起着关键作用，它受到云辐射效应的调节。对这种云效应的评估在任何地理区域都很稀少，在热带沿海地区也不存在。我们使用多年（2010-2016 年）微波辐射计剖面仪观测，研究了在陆上和海上流动期间，云对热带沿海地区 Thumba（8.5°N，77°E）边界层高度日变化的影响。多云和晴空期间的边界层高度从 0800 LT（当地时间 = UTC + 5.1 h）迅速增加，在中午（400-1500 m）附近达到白天峰值。冬季和季风前的离岸（陆上）气流盛行时，多云期间季节性平均中午边界层高度比晴空期间低> 900 m（> 400 m）。与海上晴天（160 mh -1 ）和陆上晴天（160-250 mh -1 ）相比，海上晴空流动过程中边界层高度的午前增长速度较快（> 380 mh -1 ）和多云的陆上 (> 100 mh -1 ) 流。介绍了短波云辐射强迫和土壤温度对中午边界层高度的影响及其相互关系。这些观测结果揭示了在陆上和海上流动期间，云对白天边界层增长的对比和显着影响，