Predictability of windstorms and gravity waves forced by complex terrain: Perspectives from T-REX

It is well know that the prediction of topographically-forced phenomena is very sensitive to the properties of the upstream flow, namely the cross-barrier wind speed and static stability. Mountain waves and downslope windstorms are examples of threshold phenomena that may occur when the basic properties of the upstream flow change through relatively small perturbations induced by the synoptic-scale or mesoscale flow. During the recent Terrain-Induced Rotor Experiment (T-REX), which took place in March-April 2006, an unprecedented suite of observations were collected in the Sierra Nevada Range including upstream radiosondes and numerous surface-based platforms on the eastern slopes in the Owens Valley. The T-REX dataset is well suited to address basic mesoscale predictability issues.

In this study, the recently developed adjoint and tangent linear models for the atmospheric portion of the nonhydrostatic Coupled Atmosphere/Ocean Mesoscale Prediction System (COAMPS) are used to explore the mesoscale sensitivity of mountain waves and downslope wind forecasts to the initial state during T-REX. Preliminary results indicate that the 24-h forecast downslope winds and mountain wave response are very sensitive to the initial state and in particular to synoptic-scale and mesoscale characteristics of mid-latitude cyclones. The mountain waves and strong downslope winds are most sensitive to upstream features in the initial state that are present in the lower troposphere. We will examine the adjoint sensitivity of the 24-h forecast conditions in the region of the Sierra Range to the initial state for a number of the T-REX Intensive Observation Periods. The results will be interpreted in the context of real-time high-resolution COAMPS forecasts performed during T-REX using a 2-km horizontal resolution.

A series of idealized predictability studies have also been conducted using the COAMPS adjoint modeling system. For flow over relatively small ridges in the hydrostatic wave regime, the sensitivity patterns exhibit a dual lobed structure that is a manifestation of a superposition of upward and downward propagating waves. Flow over higher obstacles near the gravity wave breaking threshold exhibits complex sensitivity patterns characterized by a wave-like packet of maxima and minima upstream of the middle- and upper-tropospheric region of wave breaking. In general, as the mountain height is increased, the tangent linear approximation becomes less accurate. Surprisingly, the strongest nonlinearity occurs for flows very near the wave breaking threshold, rather than fully within the wave breaking regime forced by higher terrain.