A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface

Nina Svensson,Erik Nilsson,Merete Badger,Hans Bergström,Anna Rutgersson,Erik Sahlée

doi:10.1155/2017/9015891

Nina Svensson, Erik Nilsson + Show 4 more

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https://doi.org/10.1155/2017/9015891

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Abstract

Streaky structures of narrow (8-9 km) high wind belts have been observed from SAR images above the Baltic Sea during stably stratified conditions with offshore winds from the southern parts of Sweden. Case studies using the WRF model and in situ aircraft observations indicate that the streaks originate from boundary layer rolls generated over the convective air above Swedish mainland, also supported by visual satellite images showing the typical signature cloud streets. The simulations indicate that the rolls are advected and maintained at least 30–80 km off the coast, in agreement with the streaks observed by the SAR images. During evening when the convective conditions over land diminish, the streaky structures over the sea are still seen in the horizontal wind field; however, the vertical component is close to zero. Thus advected feature from a land surface can affect the wind field considerably for long times and over large areas in coastal regions. Although boundary layer rolls are a well-studied feature, no previous study has presented results concerning their persistence during situations with advection to a strongly stratified boundary layer. Such conditions are commonly encountered during spring in coastal regions at high latitudes.

Highlights

Larger areas of coherent streaky structures are often seen in satellite images over sea surface
Case studies using the WRF model and in situ aircraft observations indicate that the streaks originate from boundary layer rolls generated over the convective air above Swedish mainland, supported by visual satellite images showing the typical signature cloud streets
Boundary layer rolls are elongated vertical circulations aligned approximately in the along-wind direction, consisting of counterrotating vortices, creating areas of alternating up- and downward motion. This vertical wind speed pattern translates into the horizontal wind field, where lower horizontal winds are seen in the upwind regions and higher in the downwind regions, which makes it possible to observe the rolls in satellite images where wind speed is inferred from backscatter from the roughness elements at the sea surface

Summary

Introduction

Larger areas of coherent streaky structures are often seen in satellite images over sea surface In many cases this is due to an instability pattern in an area with certain atmospheric conditions. Boundary layer rolls are elongated vertical circulations aligned approximately in the along-wind direction, consisting of counterrotating vortices, creating areas of alternating up- and downward motion This vertical wind speed pattern translates into the horizontal wind field, where lower horizontal winds are seen in the upwind regions and higher in the downwind regions, which makes it possible to observe the rolls in satellite images where wind speed is inferred from backscatter from the roughness elements at the sea surface. Most of the observational studies concerning boundary layer rolls have been performed in cold-air outbreaks over water [3], when air flows from a cold land or ice surface out over relatively warmer water This causes convective conditions over the sea surface, where boundary layer rolls are formed by thermal instability.

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Discussion and Conclusions