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Vanda Grubišić and Brian J. Billings


This note presents a satellite-based climatology of the Sierra Nevada mountain-wave events. The data presented were obtained by detailed visual inspection of visible satellite imagery to detect mountain lee-wave clouds based on their location, shape, and texture. Consequently, this climatology includes only mountain-wave events during which sufficient moisture was present in the incoming airstream and whose amplitude was large enough to lead to cloud formation atop mountain-wave crests. The climatology is based on data from two mountain-wave seasons in the 1999–2001 period. Mountain-wave events are classified in two types according to cloud type as lee-wave trains and single wave clouds. The frequency of occurrence of these two wave types is examined as a function of the month of occurrence (October–May) and region of formation (north, middle, south, or the entire Sierra Nevada range). Results indicate that the maximum number of mountain-wave events in the lee of the Sierra Nevada occurs in the month of April. For several months, including January and May, frequency of wave events displays substantial interannual variability. Overall, trapped lee waves appear to be more common, in particular in the lee of the northern sierra. A single wave cloud on the lee side of the mountain range was found to be a more common wave form in the southern Sierra Nevada. The average wavelength of the Sierra Nevada lee waves was found to lie between 10 and 15 km, with a minimum at 4 km and a maximum at 32 km.

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Juerg Schmidli, Brian Billings, Fotini K. Chow, Stephan F. J. de Wekker, James Doyle, Vanda Grubišić, Teddy Holt, Qiangfang Jiang, Katherine A. Lundquist, Peter Sheridan, Simon Vosper, C. David Whiteman, Andrzej A. Wyszogrodzki, and Günther Zängl


Three-dimensional simulations of the daytime thermally induced valley wind system for an idealized valley–plain configuration, obtained from nine nonhydrostatic mesoscale models, are compared with special emphasis on the evolution of the along-valley wind. The models use the same initial and lateral boundary conditions, and standard parameterizations for turbulence, radiation, and land surface processes. The evolution of the mean along-valley wind (averaged over the valley cross section) is similar for all models, except for a time shift between individual models of up to 2 h and slight differences in the speed of the evolution. The analysis suggests that these differences are primarily due to differences in the simulated surface energy balance such as the dependence of the sensible heat flux on surface wind speed. Additional sensitivity experiments indicate that the evolution of the mean along-valley flow is largely independent of the choice of the dynamical core and of the turbulence parameterization scheme. The latter does, however, have a significant influence on the vertical structure of the boundary layer and of the along-valley wind. Thus, this ideal case may be useful for testing and evaluation of mesoscale numerical models with respect to land surface–atmosphere interactions and turbulence parameterizations.

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