WRF simulations of north flow across the Alaska Peninsula: general characteristics of wakes, jets and mountain waves

Satellite imagery frequently reveals complex air flow patterns to the lee of the Alaska Peninsula during episodes of north winds as evidenced by the presence of wave clouds and vortices. The terrain on the southern half of the Alaska Peninsula and Unimak Islands which is the focus of this study consists of a number of strato-volcanoes of varying heights (1000-2800 m), either in isolation (Frosty Peak) or connected by a ridge line (Mt. Shishaldin-Mt. Isanotski-Mt. Roundtop). This provides a unique opportunity to study the impact that adjacent terrain of varying height and aspect ratio have on the lower tropospheric flows. Northerly wind episodes are characterized by strong low-level inversions at or near the top of the terrain with upstream wind speeds on the order of 10-15 ms-1. Positive or negative wind shear in or near these inversions adds a level of complexity to the response of the air moving over the terrain.

Using a series of WRF simulations the nature of air movement over the terrain is investigated within the nondimensional mountain height parameter space of 0.7-3.0. In general, the results of this study are in agreement with the numerous studies of a similar nature but which have used idealized terrain as well as homogenous upstream wind and temperature profiles. The main results indicate the importance of the distance between the base of the inversion and the top of the terrain to the nature of the flow downstream. Significant differences in flow properties are generated downstream of the various terrain features due to subtle changes in the height and strength of the inversion as well as in wind shear.

Specifically, the model indicates that wakes form in association with either strong or weak vertical vorticity (curl). Wave breaking typically generates wakes which contain stagnation and reverse flow; however, wakes of significant size also form in cases where mountain waves do not break. In addition, wake intensity and spatial extent can vary significantly in the vertical. If the base of an inversion descends below the crest of a mountain the flow impinging the upper mountain is diverted laterally (flow-splitting) with the result that the wake maybe weaker than when the base of the inversion was higher. As a rule-of-thumb the length of a wake extends 5-8 mountain half-widths downstream while the minima speed within a wake typically occurs 500-1000 m AGL but may vary at times. Jets emanating from sea-level gaps typically range from 150-200% of the upstream wind speed but values in excess of 250% can occur at times. In general, lee-side wind speeds evolve during the lifetime of a particular event and are difficult to categorize by one or two flow parameters.