10 Island Effect Studies on Boundary Layer Modification and Cloud Street Evolution at Barbados using Large-Eddy-Simulations

Monday, 9 June 2014
Palm Court (Queens Hotel)
Michael Jähn, Leibniz-Institute for Tropospheric Research, Leipzig, Germany; and O. Knoth

Islands are modifying the maritime boundary layer (MBL), which is caused by two main mechanisms: diurnal heating and topographic effects. Since Barbados is the easternmost island in the Caribbean and steady trade winds are present, it is not affected by other surrounding islands. For that reason, Barbados is suitable for island effect studies.

The focus here lies on high-resolution numerical studies using the technique of Large-Eddy-Simulations (LES). The used model is the three-dimensional, hydrodynamic solver ASAM (All Scale Atmospheric Model). There, the inclusion of orography is realized by a cut cell approach with in a Cartesian model domain. Thus, the stratified atmosphere is better characterized than with terrain-following coordinates, which produce artificial forces. The model is initialized with observational profiles taken from the ongoing SALTRACE campaign (Saharan Aerosol Long-range TRansport and Aerosol-Cloud-interaction Experiment).

It will be carried out at which meteorological situation the thermal or the orographic effect is dominating, which is primarily dependent on the mean wind speed. Besides the island boundary layer structure and its spatiotemporal development, we are able to simulate narrow cloud streets, which can be observed by satellite images. They occur every 2-4 days and differ in length and direction. Trigger values of meteorological parameters for the development of these cloud streets will be derived from the model output.

Another focus lies on the distribution of Saharan dust layers, which reach the Caribbean via long-range transport over the Atlantic. With tracer studies it is possible to investigate effects like vertical mixing of dust into the boundary layer. These model results are compared with observations (ground-based + airborne in-situ and lidar measurements).

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