Sounding 3 Discussion

Discussion on Sounding 3 - Freezing Rain

The wind structure: this is a bit different from the first two cases in that the environment is located to the northwest of a surface cyclone center. The northeasterly flow in the lowest gate gives this away. Again, geostrophic warm air advection is occurring through the lowest 5000 feet but not above the top of the inversion

The temperature/moisture structure: The basic temperature structure reveals a classic precipitation-type problem ...a surface anticyclone moving to the east - with a depth from the surface to near 850 mb. The strong inversion exists through that layer with a topical air mass returning over the colder air dome. The warm layer is centered near 830 mb...on top of the cold air.

This sounding is bit tricky because of the close-to-saturated conditions above the warm layer. Clearly, the lower layers are saturated (RH=100%) through about 690 mb where the coldest layer temperature is only about -6C (at surface). Above 690mb, the dewpoint depression is > 5C. Recall that the layer must be saturated with respect to ice (a bit lower than 100% relative humidity) to have any ice growth if a nuclei is present.

This case produced freezing rain. Based on the guidelines that at least -12 to -15 C in the cloud layer must be reached to produce ice...it is straight forward. Operationally, the concern here would be ice seeding from above which may cause the change to frozen or IP.

What if? What if the cloud layer depicted in the sounding extended to near 500 mb or -20C? In this case, ice would be introduced into the cloud. At that point, you need to concentrate on the warm layer maximum temperature to assess the surface-based precipitation type expected.

Assuming the surface temperature is <32F:

Warm Layer Max Temperature

Precipitation Type
with ice introduced

Precipitation Type
without ice introduced

<1C

Snow Freezing Rain

1C to 3C

Sleet(1C) to Mix (3C) Freezing Rain

> 3C

Freezing Rain Freezing Rain

>Check out below!!

Getting back to the surface observed weather for this case at 00Z, it is easily seen how the precipitation type follows the above table. From A to B, the type starts at snow and then changes to rain as the warm layer aloft thickens and is warmer. Thinking about the warm air wrapping northwestward around the low, the warmest air is nearest to the warm sector. Conversely, the smallest wedge of warm air aloft is further northwest toward A or the snow line. At the snow line, the warm layer aloft is no warmer than 1C. However, immedately to the southeast, the warm layer temperature is 1-3C (PE), and then over southeastern MN, it has warmed to over 3C (FZRA).


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