A radar-based climatology of 91 unique summertime (May 2000-August 2009) thunderstorm cases was examined over the Indianapolis, Indiana, urban area. The study hypothesis is that urban regions alter the intensity and composition/structure of approaching thunderstorms because of land surface heterogeneity. Storm characteristics were studied over the Indianapolis region and four peripheral rural counties approximately 120 km away from the urban center. Using radar imagery, the time of event, changes in storm structure (splitting, initiation, intensification, and dissipation), synoptic setting, orientation, and motion were studied. It was found that more than 60% of storms changed structure over the Indianapolis area as compared with only 25% over the rural regions. Furthermore, daytime convection was most likely to be affected, with 71% of storms changing structure as compared with only 42% at night. Analysis of radar imagery indicated that storms split closer to the upwind urban region and merge again downwind. Thus, a larger portion of small storms (50-200 km 2 ) and large storms (.1500 km 2 ) were found downwind of the urban region, whereas midsized storms (200-1500 km) dominated the upwind region. A case study of a typical storm on 13 June 2005 was examined using available observations and the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5), version 3.7.2. Two simulations were performed with and without the urban land use/Indianapolis region in the fourth domain (1.33-km resolution). The storm of interest could not be simulated without the urban area. Results indicate that removing the Indianapolis urban region caused distinct differences in the regional convergence and convection as well as in simulated base reflectivity, surface energy balance (through sensible heat flux, latent heat flux, and virtual potential temperature changes), and boundary layer structure. Study results indicate that the urban area has a strong climatological influence on regional thunderstorms.

A simple drag partition theory is developed for the classical problem of boundary layer flows over regular arrays of two‐ or three‐dimensional roughness elements. The theoretical expression for the ratio of the form drag on these elements to the total drag is shown to be in good agreement with wind tunnel observations. It is used for determining the contribution of form drag on pressure ridges to the total wind stress on the arctic pack ice. The theory also leads to an expression for the large‐scale roughness parameter as a function of mean ridge height, ridging intensity, small‐scale or local roughness parameter, and an average form drag coefficient. It is required for determining the average wind stress over large areas of the Arctic on a routine basis, using the so‐called geostrophic drag method, as envisaged in the Aidjex program.

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Various methods of measuring air stress on the arctic ice surface are discussed; however, none of them could possibly take into account the form drag due to pressure ridges. An expression is derived for the form drag per unit area in terms of certain key parameters of ridge statistics and a suitable drag coefficient. By using the available field and laboratory measurements of these parameters an estimate is made of the ratio of the form drag to the frictional stress. It depends on the geographical location in the Arctic, the season of the year, and the meteorological conditions in the atmospheric surface layer. It is found, contrary to the common assumption, that the form drag on pressure ridges may be much larger than the frictional stress on the ice surface, especially under stably stratified conditions.