Abstract
Using the Weather Research and Forecasting (WRF) – Advanced Research WRF (ARW) mesoscale model (WRF–ARW), we investigate how two nocturnal offshore rainbands occurring in the Mediterranean basin are modified in a warmer sea surface temperature (SST). After sunset, the thermal difference between land and sea air increases. Driven by drainage winds or land breeze, the inland cold air interacts with the relatively warmer and moister air over the sea. Vertical movement of sea air over the boundary between the two air masses may induce cloud and rain bands offshore. When an increase of SST is prescribed in the WRF simulations, a change in the precipitation pattern is simulated. The numerical experiments show an increase both in the extension and location of the rainbands and in the precipitation rate. These changes, induced by the modified SST, are analyzed by estimating and comparing several parameters such as the location of level of free convection (LFC), Convective Available Potential Energy (CAPE), or the triggering, deceleration and blockage terms of simplified conceptual models.
Highlights
Offshore precipitation cells and bands along the coastline are commonly observed phenomena during the night and early morning in several areas of the tropics [1,2,3,4] and in the Mediterranean basin [5,6,7,8,9]
The variables used in the analysis are: the 10-h accumulated precipitation, wind field, depth of the cold air mass and the estimated parameters that account for the triggering, blockage and deceleration that the cold air mass offers to the prevailing maritime air [1,11,17]
The influence of an sea surface temperature (SST) increase on the precipitation associated to two cases of interactions of warm–cold air masses in the Mediterranean basin has been performed by using Weather Research and Forecasting (WRF) simulations
Summary
Offshore precipitation cells and bands along the coastline are commonly observed phenomena during the night and early morning in several areas of the tropics [1,2,3,4] and in the Mediterranean basin [5,6,7,8,9]. Inland air cools faster than the air located at the coastline and over the sea This cooler and denser air begins to flow towards the coast as it descends from mountain ranges and follows rivers and dry streams. When it interacts with the warmer air over the sea, the structure of this relatively cold air mass resembles a coastal front that may enhance vertical movements of the warm and moist sea air. The warmer and moister maritime air (at potential temperature θw ) is forced to move upwards over the cold air (having potential temperature θc ) When ascending, this warmer air condensates forming stratiform clouds if the air reaches its lifting condensation level (LCL). Vc is influenced by roughness length over land [15]
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