Mechanisms of Along-Valley Winds: A Process-Based Model Evaluation of the ICON Model
Creators: name: Lezuo, Tobia
Publisher: name: ETH Zurich
Year: 2023
Subjects: Boundary layer flow
Valley wind system
Along-valley wind
Mountain meteorology
Model evaluation
ICON
info:eu-repo/classification/ddc/530
Physics

description: The valley wind system belongs to the most characteristic circulations of the at-
mospheric boundary layer in mountainous terrain. It dominates surface winds at
the valley bottom, contributes to the vertical air mass exchange with the free tropo-
sphere, and plays a crucial role in the horizontal air mass exchange with neighboring
plains and valleys. The accurate prediction of the valley wind system is therefore
crucial for forecasting the weather in complex terrain.

In this study, we assess the ability of the ICON model to resolve along-valley
winds in the Inn Valley, by comparing the simulation to measurements from a char-
acteristic valley wind day during the CROSSINN campaign (CROSSINN = Cross-
valley flow in the Inn Valley). The three-dimensional observations allow us to eval-
uate the simulation beyond classical statistical validation techniques and to further
investigate the underlying processes (process-based evaluation). Particularly, the
thermal forcing is investigated using radiosounding-derived valley-plain tempera-
ture and pressure gradients. Besides the model evaluation, we provide a compre-
hensive and detailed overview of the observed development of the valley boundary
layer during the selected day (13 September 2019, the 8th intensive observational
period (IOP8) of CROSSINN).

At 1 km horizontal grid spacing, the ICON model can simulate the daytime up-
valley flow well, including its strength, timing, and vertical structure (for the Inn
Valley with ridge-to-ridge distance of 10 km or 10 dx). However, during the night
the model is not able to correctly represent valley-plain temperature and pressure
contrasts, thus deviating from the observed nighttime down-valley flow. We identify
three main challenges for the simulation of IOP 8. The first challenge is the observed
westerly down-valley flow during the night and morning of IOP 8, which may in-
volve a scale interaction with the synoptic westerly flow. The second challenge is
the complex interplay between the up-valley wind, the slope circulation, and the
cross-valley vortex, which affects the detachment of the up-valley wind from the
surface and its transition to higher altitudes.The third challenge pertains to the sta-
ble boundary layer and valley cold air pool during the night after IOP 8. The model
limitations in representing these processes result in the failure to simulate thermal
forcing and the connected down-valley flow in the second night.

The main limitations of the model include the mis-representation of synoptic
flow (limited predictability), smaller-scale thermally-driven flows (limited resolu-
tion), and cold air pools (stable boundary layer issues). In summary, this study
contributes to a better understanding of along-valley winds under non-idealized
conditions, and provides ICON’s first process-based model evaluation on thermally
driven flows.