The influence of a down-valley wind on the development of a cold air pool in an Alpine valley

The stable atmospheric boundary layer which forms in mountain regions in winter is one of the challenges of current atmospheric modelling: under light synoptic forcing and clear sky, the evolution of the boundary layer is dominated by local processes which act at scales which are too small to be explicitely resolved by large-scale models (see f.i. Whiteman 2000, Zardi & Whiteman 2013). The social issues related to this topic are broad, ranging from air quality in urban area to the impact of the mountain atmosphere on climate changes. A deep understanding of these processes is needed before any parameterization can be derived for large-scale models.

The evolution of the stable boundary layer which develops inside a valley is characterized by slope and valley winds. Slope winds develop because of the horizontal thermal imbalance between the boundary layer close to the slope, and the layer of air at the same altitude far from the slope. During the night, due to the radiative cooling of the ground, the slope wind blows down-slope, towards the bottom of the valley, inducing a compensatory air motion upwards. On the other hand, valley winds are triggered by the thermal imbalance in the along-valley direction, for instance between the valley and a plain nearby. The boundary layer which develops in this environment is a complex interaction between radiative cooling, valley winds, and turbulent diffusion (Vosper et al. 2013).

It is well-known that in weak synoptic and wintertime conditions, an extreme cooling rate occurs at the ground (with values as low as -8 K/h), which leads to low temperature minima on the valley floor with a strong inversion layer above it. The relative role of the mechanisms leading to the formation of this Cold Air Pool (CAP) inside a valley has been investigated by Burns & Chemel (2013a, 2013b) for Alpine (i.e. deep and narrow) valleys. Their work focus on a two-dimensional idealized valley of finite extent along the valley axis, therefore no along-valley wind develops.

The present work aims to investigate the interaction between the along-valley wind, which develops because of the CAP, and the evolution of the CAP itself, in Alpine valleys. Our work is based on the numerical simulation of a three-dimensional idealized Alpine valley. The Advanced Weather Research and Forecasting model (WRF) in LES configuration is used for this purpose. For steep topography, Burns & Chemel (2013a) have shown that a very high resolution is needed in order to resolve the down-slope flow (2 m along the vertical, 30 m along the horizontal, depending upon the slope of the topography). Furthermore, they showed that a good representation of these winds are of fundamental importance in the evolution of the CAP inside the valley. In order to take into account the along-valley wind contribution on the CAP development and evolution, we relax the hypothesis of homogeneity along the valley axis. To minimize the influence of the lateral boundaries on our flow, a large computational domain is needed. In order to satisfy both the constraints of high resolution and large domain, we attack the problem using a LES nested in a LES, modifying the WRF-code as proposed by Moeng et al. (2007).

We first considered a simple idealized configuration, following Schmidli et al. (2010), which permits to use periodic boundary conditions in the horizontal directions. A comparison between our results and the results of Schmidli et al. (2010) allowed to validate qualitatively our simulation. This study also allowed to investigate the influence on the CAP evolution of purely thermally driven along-valley wind. We next moved towards a more realistic configuration, typical of Alpine landscape, of a valley exposed on a plain, and closed by a sill or a pass. Such a configuration is encountered at several locations in the Arve valley, which encompasses the Chamonix valley. In this more complicated framework, the along-valley wind is forced not only by thermal difference between the plain and the valley, but also by the down-slope wind from the pass which closes the valley on one side. Possible detrainment of this down-slope flow above the CAP and associated mixing processes will be analyzed carefully.

References

Burns P. and Chemel C. Evolution of cold air pooling processes in complex terrain. Boundary-Layer Meteorology, in press, 2013a.

Burns P. and Chemel C. Interaction between a down-slope flow and developing cold-air pool. Boundary-Layer Meteorology, submitted, 2013b.

Moeng C.-H., J. Dudhia J., Klemp J. and Sullivan P.P. Examining Two-Way Grid Nesting for Large Eddy Simulation of the PBL Using the WRF model. Mon. Wea. Rev., 135:2295--2311, June 2007.

Schmidli J., Billings B., Chow F.K., De Wekker M. et al. Intercomparison of mesoscale model simulations of the daytime valley wind system. Montly Weather Report, 139:1389--1409, 2010.

Vosper S.B., Hughes J., Lock A.P., Sheridan P.F., Ross A.N., Jemmett-Smith B., and Brown A.R. Cold-pool formation in a narrow valley. Quarterly Journal of the Royal Meteorology Society, DOI: 10.1002/asl2.439,2013.

Whiteman D. Mountain Meteorology. Fundamentals and applications. Oxford University Press, 2000.

Zardi D. and Whiteman D. Diurnal mountain wind systems. In: Mountain weather research and forecasting: Recent progress and current challenges. Chow, De Wekker and Snyder eds., Springer Atmospheric Sciences, Springer, New York, NY, USA, 2013.