Friday, February 2, 2018

Lessons in Cloud Microphysics this Weekend

If you are in the skiing in the Wasatch Range the next few days, you might get a pretty good lesson in cloud microphysics if you pay attention.

For much of the period, we're going to be in northwesterly flow downstream of a broad upper-level ridge, with the jet stream to our north.  Just enough moisture will be spilling over that ridge to enable the development of shallow, mid-level, layered clouds known as altostratus, as is the case this afternoon over the Salt Lake Valley.

I despise altostratus for a number of reasons, but perhaps the biggest is that they are the ultimate do-nothing cloud.  They make for flat light for skiing and they generally don't produce much in the way of precipitation.  Worse yet, they often produce conditions favorable for rime or freezing drizzle where they are colder than 0ºC but warmer than -10ºC and intersect the mountains.

The NAM time-height section shows north-westerly flow predominating at crest level through Sunday with shallow moisture for much of the period.  The depth of the moisture does vary, however, and this will likely be important for cloud microphysical processes.

Below is the NAM sounding for 2300 UTC (4 PM MST) Saturday.  Note how the temperature and dewpoint traces meet from about 750 to 650 mb.  The NAM is basically producing a cloud in that layer.  The entirety of that cloud layer is colder than 0ºC, but the coldest temperature at cloud top is only about -5ºC.
Cloud droplets have a very difficult time converting into ice at those temperatures.  As a result, the cloud consists of a great deal of supercooled liquid water that tends to freeze on contact.  If the droplets are large enough, they can coat chairlifts or goggles pretty quickly.  You can play games and see how fast your goggles ice up at different speeds.  The faster you go, the more droplets you sweep out, and the faster your goggles ice up.

On the other hand, there are times when the clouds a deeper and cloud top is colder.  The skew-t below is a forecast for late afternoon Monday.  Rime would be more limited in a cloud like that.
At times, you may experience rime or freezing drizzle when you are in cloud the next few days.    When this happens, temperatures at cloud top are probably warmer than -10ºC.  You might get a little snow at times.  If the clouds are deeper and snow rates can pick up, you might find the riming decreases and goggles don't glaze as quickly.  Periods with deeper clouds and colder cloud top temperatures will likely produce less riming.  The models don't nail these variations, so watch for them when your skiing in cloud.  

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