Relating Mean Radiosounding Profiles to Surface Fluxes for the Very Stable Boundary Layer
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A dataset collected during a measurement campaign in the middle of the Po Valley, Italy, is used to investigate the boundary-layer structure in stable conditions. Empirical formulations for temperature and wind profiles derived from Monin–Obukhov similarity theory are used as regression curves to fit radiosounding profiles in the lower half of the boundary-layer. The best fitting parameters of the regression are then compared to the surface turbulent fluxes as measured by a co-located sonic anemometer. This comparison shows significant discrepancies and supports earlier results showing that surface fluxes, in the limit of high stability, are not adequate scalings for mean profiles. The most evident differences are found for cases for which the bulk Richardson number turns out to be quite large. One of the practical consequences is that boundary-layer height diagnostic formulations that mainly rely on surface fluxes are in disagreement with those obtained by inspecting the thermodynamic profiles recorded during the radiosounding ascent. Moreover the incorrect scaling of similarity profiles in stable conditions leads to the erroneous diagnosis of 2-m air temperatures used in numerical weather prediction validation.
KeywordsBoundary-layer height Similarity theory Stable boundary-layer
We are in debt to Andrea Pitacco, Simone Righi and Sandro Nanni for logistic and technical support during the experimental phase of BASE:ALFA. Giovanni Bonafé is acknowledged for providing the surface fluxes used in this study and giving advice on their use. Many thanks to Anton Beljaars, Irina Sandu and Adrian Tompkins for clarifying some of the aspects related to the practical consequences in Numerical Weather Predictions of our work. The BASE:ALFA project was in part funded by the Emilia-Romagna Region (Bologna, Italy).
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