Thermal mesoscale circulations on the Baltic coast: 1. Numerical case study

Abstract

A three‐dimensional mesoscale numerical model is utilized to investigate a well‐documented seabreeze event taken from experimental data. The simulations are not set up as a model validation. Instead, the model is driven by generalized and simplified initial and boundary conditions extracted from the field experiment data. The aim is to simulate the main characteristics, if not all the details, of the observed case. This generates a comprehensive set of fully three‐dimensional and time dependent data that can be used to obtain a more complete description of the case characteristics. Analyzing these data facilitates understanding of the flow mechanisms in more detail. Once such a control simulation is done, the case can be resimulated for slightly different forcing and the cause of observed features can be illuminated. Significant features in the simulation are the significant temperature difference between land and sea, moderate coastal terrain, and an almost 90° change in coastline orientation. Among the conclusions from the control simulation presented here is that the seabreeze stage is preceded by a coastal wind jet stage with a significant influence on the flow by the quite moderate topography. This is due to the ratio of the depth of the marine boundary layer to the terrain height; the terrain is acting on the flow in three ways. First, the stability impedes vertical motions and the terrain thus acts as a barrier inhibiting geostrophic balance. The flow is blocked and deflected down the synoptic‐scale pressure gradient. Second, it generates a perturbation in the temperature field causing local baroclinicity and a thermal wind. Finally, a slope flow is also generated that acts to enhance the temperature perturbation by advection of cold air to the coastal zone. A seabreeze evolves on both coastlines. The first forms on the offshore flow coast but remains stationary at the shoreline until the second, later, seabreeze on the almost perpendicular coastline, where the coastal jet dominates the flow well into the morning, moves inland and removes the offshore flow balancing the first seabreeze at the shoreline. After this obstruction is removed, the initial seabreeze rapidly propagates inland.