Fine-Resolution WRF Simulation of Stably Stratified Flows in Shallow Pre-Alpine Valleys: A Case Study of the KASCADE-2017 Campaign

Michiel De Bode,Pierre Roubin,Pierre Durand,Thierry Hedde

doi:10.3390/atmos12081063

Abstract

In an overall approach aiming at the development and qualification of various tools designed to diagnose and/or forecast the flows at the local scale in complex terrain, we qualified a numerical model based on the WRF platform and operated in a two-way nested domain mode, down to a horizontal resolution of 111 m for the smallest domain. The area in question is the Cadarache valley (CV), in southeast France, which is surrounded by hills and valleys of various sizes. The CV dimensions (1 km wide and 100 m deep) favor the development of local flows greatly influenced by the diurnal cycle and are prone to thermal stratification, especially during stable conditions. This cycle was well documented due to permanent observations and dedicated field campaigns. These observations were used to evaluate the performance of the model on a specific day among the intensive observation periods carried out during the KASCADE-2017 campaign. The model reproduced the wind flow and its diurnal cycle well, notably at the local CV scale, which constitutes considerable progress with respect to the performances of previous WRF simulations conducted in this area with kilometric resolution, be it operational weather forecasts or dedicated studies conducted on specific days. The diurnal temperature range is underestimated however, together with the stratification intensity of the cold pool observed at night. Consequently, the slope drainage flows along the CV sidewalls are higher in the simulation than in the observations, and the resulting scalar fields (such as specific humidity) are less heterogeneous in the model than in the observations.

Highlights

The flows over complex terrain represent a challenge for both their observation and numerical simulation
With respect to the previous WRF simulations conducted in this area with a kilometric resolution, be it operational simulations as analyzed by [25], or case studies such as in [31,36], we demonstrated that a simulation with a hectometric resolution is clearly able to reproduce, with some accuracy, the observed winds and their diurnal cycle in the smallscale valley, which was unreachable with coarser resolutions
The study presented in this paper is a contribution to an overall approach aiming at the development and qualification of various tools intended to diagnose and/or forecast the flows at the very local scale in complex terrain areas

Summary

Introduction

The flows over complex terrain represent a challenge for both their observation and numerical simulation. The major mountain chains constrain the flows in the entire troposphere, and their effect has to be adequately taken into account for an accurate weather forecast. One may consider this question as having been satisfactorily addressed, some improvements are still required [1]. The topography has a direct impact on the lower layers of the atmosphere (the atmospheric boundary layer; ABL), with a marked diurnal cycle. While at a large scale, the mechanical effects of the topography are dominant, at the local scale, the flow is driven by a combination of both mechanical and buoyancy effects.

Results

Discussion

Conclusion