Low clouds play a key role in the Earth-atmosphere energy balance and influence agricultural production and solar-power generation. Smoke aloft has been found to enhance marine stratocu-mulus over the Southeast Atlantic in austral spring through aerosol-cloud interactions, but its role in regions with strong human activities and complex monsoon circulation remains unclear. Here we show that biomass burning aerosols aloft strongly increase the low-cloud coverage over both land and ocean in subtropical southeastern Asia. The degree of this enhancement and its spatial extent are comparable to that in the Southeast Atlantic, even though biomass burning emissions in Southeast Asia are only one-third of those in Southern Africa. Our results indicate that a cou-pling of aerosol-cloud-boundary-layer feedback with the monsoon is the main reason for the strong semi-direct effect and enhanced low-cloud formation in Asia.

A B S T R A C T Flow regimes of dry, stratified flow passing over an isolated two-dimensional (2-D) orography mainly concentrate at two stagnation points. One occurs on the upslope of the orography owing to flow blocking; another is related to gravity wave breaking (GWB) over the leeside. Smith (1979) put forward a hypothesis that the occurring of GWB is suppressed when the low-level vertical wind shear (VWS) exceeds some value. In the present study, a theoretical solution in a two-layer linear model of orographic flow with a VWS over a bellshaped 2-D orography is developed to investigate the effect of VWS on GWB's occurring over a range of surface Froude number Fr 0 0U 0 /Nh (U 0 is surface wind speed, h is orography height and N is stability parameter), over which the GWB occurs first and the upstream flow blocking is excluded. Based on previous simulations and experiments, the range of surface Froude number selected is 0.6 5 Fr 0 52.0. Based on this solution, the conditions of surface wind speed (U 0 ) and one-to-one matching critical VWS (Du c ) for GWB's occurring are discussed. Over the selected range of Fr 0 , GWB's occurring will be suppressed if the VWS (Du) is larger than Du c at given U 0 . Moreover, there is a maximum value of Du c over the selected range of Fr 0 , which is labelled as Du max , and its matching surface wind speed by U 0m . Once the Du is larger than Du max , the flow will pass over the orography without GWB's occurring. That means, over the selected range of Fr 0 , the flow regime of 2-D orographic flow related to GWB occurring primarily will be absent when Du ! Du max , regardless of the value for U 0 . In addition, the vertical profile of atmospheric stability and height of VWS could result in different features of mountain wave, which leads to different Du c and Du max for the GWB's occurring. The possible inaccuracy of estimated Du c in the present linear model is also discussed.

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