INLAND AND OFFSHORE PROPAGATION SPEEDS OF A SEA-BREEZE FROM SIMULATIONS AND MEASUREMENTS

The inland and offshore propagation speeds of a sea breeze circulation cell are simulated using a three-dimensional hydrostatic model within a terrain-following coordinate system. The model includes a third-ord er semi-Lagrangian advection scheme, which compares well in a one-dime nsional stand-alone test with the mole complex Bott and Smolarkiewicz advection schemes. Two turbulence schemes are available: a local schem e by Louis (1979) and a modified non-local scheme based on Zhang and A nthes (1982). Both compare well with higher-order closure schemes usin g the Wangara data set for Day 33-34 (Clark et al., 1971). Two-dimensi onal cross-sections derived from airborne sea breeze measurements (Fin kele et al. 1995) constitute the basis for comparison with two-dimensi onal numerical model results. The offshore sea breeze propagation spee d is defined as the speed at which the seaward extent of the sea breez e grows offshore. On a study day, the offshore sea breeze propagation speed, from both measurements and model, is -3.4 m s(-1). The measured inland propagation speed of the sea breeze decreased somewhat during the day. The model results show a fairly uniform inland propagation sp eed of 1.6 m s(-1) which corresponds to the average measured value. Th e offshore sea breeze propagation speed is about twice the inland prop agation speed for this particular case study, from both the model and measurements. The influence of the offshore geostrophic wind on the se a breeze evolution, offshore extent and inland penetration are investi gated. For moderate offshore geostrophic winds (-5.0 m s(-1)), the off short and inland propagation speeds are non-uniform. The offshore ext ent in moderate geostrophic wind conditions is similar to the offshore extent in light wind conditions (-2.5 m s(-1)). The inland extent is greater in light offshore geostrophic winds than in moderate ones. Thi s suggests that the offshore extent of the sea breeze is less sensitiv e to the offshore geostrophic wind than its inland extent. However, th ese results hold only if it is possible to define an inland propagatio n speed. For stronger offshore geostrophic winds (-7.5 m s(-1)), the s ea breeze is completely offshore and the inland propagation speed is i ll-defined.