Sounding 2 Discussion

Discussion on Sounding 2 - Freezing Rain

The wind structure: similar to sounding 1, geostrophic warm advection (winds veering with height) is apparent through the 0-6 km layer.

The temperature/moisture structure: the environmental sounding shows a near isothermal layer extending from the surface up to near 700 mb or 9000 feet. This layer is also saturated and the temperature (which =Tw=Td here) is near -7C at the surface warming to about -4C at 920 mb. It then remains near -4C to the top which is near 700 mb. According to cloud physics research, "essentially no clouds with tops as cold as -20C are ice-free, (while) ice anywhere in a cloud is unlikely unless the top extends to -5C or colder (R.R. Rodgers and M.K. Yau)." . Even if you are certain the warmest IN or nucleii condensation particles were introduced, the minimum temperature ice would form is still -4C. However, a good operational range to remember: at least -12 to -15C temperatures in a given cloud layer must be accomplished to have ice present, otherwise expect liquid or freezing precipitation types. It is important to watch for the introduction of seeding ice from a cloud layer above which is less than -15C.

Above the cloud top of the lower-mid level saturated layer, or 700mb, no seeding is taking place as no cloud is present.

Thus, precipate and cloud water is obviously present with the relative humidity near 100% through the bottom portion of the sounding. However, since no ice has been introduced into the cloud layer, any vertically falling precipitation will be in the form of freezing rain. It will be liquid, and with the surface temperature less than 32F, it will freeze on contact. Depending on the vertical motion, it may be drizzle as it is governed by warm rain processes. This is one reason freezing precipitation may be observed more in a drizzle than a hard rain.

What if? What if you saw a layer near 600 mb that was saturated? Here, the 600 mb temperature is near -9C. This cloud layer may have ice however the lieklyhood is not too large. Thus, a cloud layer at 600mb would have a small chance at having ice particles to seed the layer below.


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