Inversions,and fog,stratus and cumulus formation in warm air over cooler water |
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| Authors: | J W Telford S K Chai |
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| Institution: | 1. Air Motions Laboratory, ASC, Desert Research Institute University of Nevada System, 89506, Reno, NV, USA
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| Abstract: | Two aspects of convection over oceans are discussed and the following conclusions are derived from theoretical considerations. - The air layer over the sea will usually convect even when the water surface is ten degrees or more colder than the initial air temperature.
- An inversion at stratus cloud tops is created by the stratus, and is not a necessary preexisting condition. Such inversions persist after subsidence evaporates the cloud.
- Radiation heat exchange does not play an essential role in stratus formation or maintenance, and can either heat or cool the cloud.
- Dry air convection does not erode inversions at the top of the convecting layer. Examples of soundings are discussed.
- Fogs are most likely to form at sea where the water is coolest, and need no radiation effects to initiate cooling, or a boost from patches of warmer water, to begin convection.
- Both stratus cloud growth, and the evaporation of clouds by cloud top entrainment, readjust the vertical structure of the air to leave a constant wet-bulb potential temperature with height.
These conclusions are supported by, firstly, a convective model which has been developed and which shows that vapor-driven convection over the ocean will proceed with zero or negative heat fluxes, at rates which saturate the lowest layer of the atmosphere in a few hours to altitudes of many tens of meters. Secondly, the availability of condensed moisture at the top of the surface layer cools the warmer entrained overlying dry air parcels so that when they descend they are no warmer than the sea surface temperature, and this induces downward moving plumes. This occurs if the wet-bulb potential temperature of the overlying air is less than the sea surface temperature, even if it is ten degrees C, or more, warmer in actual temperature. |
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