A Case Study in Modeling Low-Lying Inversions and Stratocumulus Cloud Cover in the Bay of Biscay

Abstract

any regional forecasting models struggle to simulate low-lying strong temperature inversions. To understand this apparent deficit for forecast improvements, a case study of a strong inversion occurring in the Bay of Biscay on 27 January 2003 is conducted. The event was characterized by extensive stratocumulus cloud cover beneath an extensive high pressure system in combination with a particularly strong inversion of 10?12 K at an altitude of 500?800 m. Simulations were performed at 2- and 12-km horizontal resolutions, with 60 vertical levels (13 levels within the first 1000 m), and with lead times of 12?72 h. The simulations were validated using in situ radiosonde and satellite data. Besides large-scale subsidence, turbulent vertical mixing is a key dynamical process for the formation of nocturnal inversions. Sensitivities to parameters for vertical mixing (the minimum threshold for eddy diffusivity and the turbulence length scale) are investigated. Results presented herein show the planetary boundary layer (PBL) profiles to be very sensitive to the minimum threshold applied for eddy diffusivity, whereas little sensitivity with respect to the turbulence length-scale parameter was found. PBL moisture and potential temperature ? profiles for hindcasts between 24- and 72-h lead times at both resolutions were adequately simulated. In simulations with an adequate representation of the vertical turbulent exchange, realistic cloud cover was simulated, while too high values of the aforementioned threshold produced a strong underestimation of the cloud cover. These results indicate that a realistic simulation of strong inversions and their associated cloud cover is feasible, provided the vertical turbulent exchange is adequately represented.