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

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.

A Method to Improve Land Use Representation for Weather Simulations Based on High‐Resolution Data Sets—Application to Corine Land Cover Data in the WRF Model

“…Most noteworthy is the sudden jump in humidity seen 20 February 2013 16:00 UTC, as it resembles a moist air flow system described in de Bode et al. (2021) with most winds coming from the south east, that looks like winds matching the CV orientation, while the 110‐m AGL GBA tower measures above the CV sidewall tops. In the morning, the humidity quantities match again.…”

“…de Bode et al. (2021) shows that this is due to a lack of proper stability development in the valley resulting in the moist winds from the south east reaching the surface whereas in the observations they remain above valley. However, no radiosonde profiles are available during the night to check the correctness of a moist air layer.…”

“…(2016) attribute the DTR to unresolved orography, but in the fine‐resolution simulation of de Bode et al. (2021) in spite of the added two domains that refined the horizontal resolution to 111 m, the expected improvement in DTR simulated values was not observed. To fix this problem de Bode et al.…”

“…Table 1 shows a selection of LC maps suited for meteorological use that are available for research without cost. For our WRF applications (de Bode et al., 2021), we require from a data set a spatial resolution of O (100 m), many classes for a detailed surface representation, and regular updates. USGS (Anderson et al., 1976) has 24 classes (WRF extends it to 27, but not over Europe), with a 1 km resolution MODIS shares the same characteristics but with a yearly update and 20 classes.…”

“…Furthermore, our goal is to evaluate the impact of LC on the overall atmospheric structure, and not to resolve very local circulations, as done by de Bode et al. (2021). To investigate the influence of land cover, we outlined the two experiments described below.…”