Daniel Kirshbaum - Interesting Weather

Radar image When moisture-laden airflow meets a mountain barrier, interesting weather often results. As the air is forced to rise up and over the barrier, it may saturate and produce clouds and precipitation. Moreover, the flow may also undergo a transition in stability (from stable to unstable) as it saturates, which can strongly enhance the mountain-induced precipitation. In stable flow, vertically displaced air parcels tend to return to their original position, which limits the amount of upward motion that can occur. In unstable flow, however, displaced air parcels continue to move in the direction that they were displaced and ascend or descend freely through the atmosphere. This transition in stability is caused by the latent heating of water as it condenses from vapor into liquid or vice versa.

I am studying the behavior of moist orographic (mountain-related) flows as they saturate and destabilize over relatively low and narrow mountain barriers. This type of flow regime occurs in numerous areas over the globe, including the French Alps, Japan, New Zealand, and the Pacific Northwest United States, and is notable because it may produce quasi-stationary rainbands that generate large precipitation accumulations over localized areas. My specific focus has been on banded convective precipitation events over the Oregon Coastal Range.

The accompanying figure shows radar imagery of quasi-stationary rainbands over the Oregon Coastal Range that produced over 69 mm at Mt.~Hebo but only 8 mm at Rye Mountain, two locations separated by only 20 km.

What causes the rainbands over the Coastal Range to remain stationary and deposit their rainfall over fixed locations? Our research has determined that each band is associated with a specific small-scale obstacle on the terrain. In this context, "large-scale" refers to the basic mountain ridge, and "small-scale" is all of the little bumps and valleys present on the ridge. Not all small-scale terrain obstacles, however, generate bands. Prior to entering the orographic "cap" cloud that sits over the mountain ridge, the flow is stable, and its response to a small-scale obstacle is an oscillating gravity wave. As this wave propagates downstream, it reaches the upstream edge of the unstable orographic cloud and perturbs the cloud. Only small-scale obstacles that produce gravity waves that perturb the cloud at a specific phase in their oscillations produce a band.

Dan in the fieldAnother related topic is the spacing between adjacent orographic rainbands, which varies between 5 and 20 km in the figure. This is a complex topic that we have only recently started to unravel. This spacing is governed by both the gravity-wave responses to the small-scale terrain on the Coastal Range and the rate of unstable growth of the perturbation inside the cloud, both of which depend on the horizontal scale of the embedded terrain obstacle. Larger-scale features on the terrain generate deeper waves that perturb the cloud more effectively, but smaller-scale perturbations grow faster inside the cloud. So the spacing between the bands is an intermediate scale that maximizes the net effect of these two factors.

ASP Spotlight December 2006
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