Predictability of windstorms and mountain waves using an adjoint model: 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. Results indicate that the 36-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 also examine the adjoint sensitivity of the 12-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 indicate that predictability of mountain waves and windstorms are limited by rapid perturbation growth on multiple scales. On the synoptic-scale, rapid growth associated baroclinic waves occurs that impact the stability and cross barrier wind speed upstream of the Sierras. On the smaller scales, the mountain waves and lee side windstorm are predominantly influenced by the upstream stratification and crest-level wind shear. Implications for predictability of terrain-forced phenomena will be addressed.