Tuesday, October 11, 2011

The Cloud Storm

Yesterday we had a classic example of what meteorologists jokingly refer to as a cloud storm.  Common in the Intermountain West and other arid/semi-arid regions, the cloud storm produces precipitation aloft, which evaporates or, in the case of snow, sublimates, before reaching the ground.

Yesterday's Salt Lake cloud storm at 6 PM MDT 10 Oct 2011.  Note the
ragged cloud base and how the high peaks of the Oquirrhs are obscurred.
A scary sky over the Garner Funeral Home.   A harbinger of things
to come for Halloween?
Critical for a good cloud storm is a high cloud base and a layer of dry air at low levels.  Both were evident in the 0000 UTC (6 PM MDT) sounding from the Salt Lake Airport.

Source: NOAA/SPC
Cloud base is at 650 mb, 2–2.5 km above the valley floor, where the dewpoint (green) and temperature (red) traces converge.  Below cloud base is a classic "inverted-V" sounding, with a 28F dewpoint depression at the surface.  Precipitation falling into such a dry layer will sublimate and evaporate before hitting the ground.  Indeed, I felt only a few drops of rain as I walked home from the U.

Later in the evening, rain began to fall across the valley.  This occurred as precipitation continued to cool and moisten the low levels.  Typically when this occurs, the temperature and dewpoint converge on something known as the wet-bulb temperature.  Evidence for this happening is apparent in the sounding as below the stable layer the temperature and dewpoint traces sit right along the moist-adiabat that roughly represents the wet-bulb temperature trace within the afternoon convective boundary layer (see thin blue line).  Indeed, note how at KSLC the temperature fell to 52F and the dewpoint rose to 45F during the evening, approaching the surface wet-bulb temperature in the sounding above.

Source: MesoWest
The importance of sub-cloud sublimation and evaporation is largely overlooked in the mountain meteorology literature as an important contributor to mountain–lowland precipitation contrasts.  Fortunately, it will be discussed in a chapter in the forthcoming book, Mountain Weather Research and Forecasting, which is being written and edited by a number of mountain meteorologists.

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