Sounding 1 Discussion

Discussion on Sounding 1 - Ice Pellets

The wind structure: Upon review of the wind structure, it appears the sounding environment is post frontal with broad northwesterly flow. However, the lowest wind gate is from the southwest-west which indicates that geostrophic warm advection is occuring in that layer. This is approximately from 0-4 kft in the lowest levels.

The temperature/moisture structure: From the wind profile discussion above, the warm layer in the sounding profile is located at about 910 mb. The temperature here is about -2C which falls in the -1C to -3C range. Stewart and King (1987) showed that typically this range produces a mixture of partially melted and completely melted snowflakes. [Assuming snowflakes enter that layer] Then, the precipitation particles will refreeze and typically yield either pure ice pellets, or a wintery mix of snow, sleet, freezing rain, and/or rain. Typically, ice pellets are observed with other precipitation types. Above 3C, freezing rain develops are particles totally melt to liquid and do not have the time to refreeze .

Question: Do we have ice particles, or nucleii, entering the 910 mb warm layer? Answer: We must. At first glance the sounding looks to maybe not be producing any precipitation. However, if ice pellets were observed at this time, some frozen precipitation must be introduced at some point. The key here: the 500-700 mb layer where it is saturated and temperatures range from -12C to -20C. This temperatures support ice growth via supercooled water drops.

What about evaporation of the particles in the sub-cloud layer near 500 mb?

Interestingly, it is near the 5000 foot threshold mentioned in the Smith (1995) paper. The particles fall as snow, just making it into the warm layer, begin to melt, and then refreeze in a more saturated boundary layer in the lowest 80 mb before reaching the surface.

Confusing?

- I would have expected that snow would come from this sounding for at least some time period based on the dry layer below the 500 mb cloud base. Evaporative cooling would occur and eliminate the warm layer for a time as Tw is < 0C, before the warm advection (if persistent), would recover the warm layer to once again have a temperature above 0C.

-Due to the dry nature to the sounding, I would have expected virga from this environment.


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