An Auxiliary Tool to Determine the Height of the Boundary Layer |
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Authors: | Cecilia Johansson Hans Bergström |
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Affiliation: | (1) Department of Earth Sciences, Air and Water Science, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden |
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Abstract: | Results from radiosoundings, performed both over land and over sea, show that the ascent rate of a radiosounding balloon, the vertical velocity of the balloon, can be used to determine the height of the boundary layer. In many cases the balloon has a higher ascent rate in the boundary layer and a lower, less variable, ascent rate above. The decrease in ascending velocity appears as a jump at the top of the boundary layer. Two examples of potential temperature profiles for unstable stratification and one profile for stable conditions are shown with the corresponding ascent rates. A comparison between the boundary-layer height determined from potential temperature profiles and from ascent rates is presented for a larger dataset. The different ascent rates of the balloon in the boundary layer and above can be explained by a decrease in drag on the balloon in combination with a lowering of the critical Reynolds number in the boundary layer caused by turbulence. Hence, by simply logging the time from release of a radiosonde, it is possible to obtain additional information that can be used to estimate the height of both the unstable and stable boundary layers. |
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Keywords: | Ascent rate Boundary-layer height Drag coefficient Radiosounding Reynolds number. |
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