When we think about how mountains affect the production of clouds, we often assume that the air rises, cools, and produces clouds on the windward side of a mountain range (i.e., the side facing the flow) and sinks, warms, and dissipates clouds on the leeward side (i.e., the side facing away from the flow.
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| Source: Whiteman (2000) |
This is often a good assumption at low levels if the flow is moist and sufficiently strong to surmount the mountain range. However, there are times when cloud generation occurs on the leeward side of mountain range.
This happened overnight downstream of the Deep Creek, Schell Creek, and Snake Ranges of Nevada. As shown in the photo below, mid- and high clouds formed or became enhanced downstream (east) of these ranges.
A schematic of this process is shown below. The flow moves over the mountain barrier at low levels, resulting in cloud generation at low levels over the windward slope and dissipation over the leeward slope (this isn't happening in the satellite image above because the low-level airmass is too dry). At upper levels, however, there is what meteorologists refer to as a vertically propagating gravity wave. Such waves are sometimes generated by flow over mountains and they lead to an upstream tilt in the wave troughs and crests and rising motion over the lee slopes at mid and upper levels.
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| Source: Durran and Klemp 1983/The Comet Program |
This can lead to cloud generation downstream of the mountains. In some situations, such clouds can extend well downstream of the mountain barrier.
Instead of clear skies, you might get overcast skies. Meteorologically you might say no biggie. However, such clouds strongly affect power generation by solar farms and thus are a consideration for power management.
Today's convection was much more interesting than I had anticipated. I think shear turned out to be a bit higher than advertised, as storms were able to get some rather impressive structure (at least as far as northern Utah goes). Multiple good-looking hail cores and even some weak rotation at times in many of the stronger cells.
ReplyDeleteI'm wondering if some of the leeward development you are seeing in Utah might also be due to increasing atmospheric moisture eastward. The current storm affecting southern Colorado is producing much more precip on the leeward side of the eastern mountain ranges that are adjacent to the Great Plains, even though the precip bands still generally track from west to east. The dew points are a lot higher to the east because Gulf of Mexico moisture is wrapping into the system via low-level upslope flow. I think this moisture has become quite deeper than usual and itis definitely spilling over the tops of at least some ranges. Could some of this Gulf of Mexico moisture be making it all the way to Utah now? It is a pretty strong system, but snow levels are high, like 10-11k ft.
ReplyDeleteAt the time I took that photo, probably not. For the subsequent thunderstorms the past couple of days, probably yes.
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