Saturation With Respect to Ice

[griteater]

During the Feb 12-13 winter storm, we had a situation in parts of central and western North Carolina where snow switched over to sleet.  At first glance, the thought was that a warm nose had moved in aloft.  However, the culprit instead appeared to be reduced moisture in the snow growth region leading to the absence of ice nucleation, and instead, the freezing of supercooled water droplets into sleet. 

 

My question is...what relative humidity percentage is considered saturated with respect to ice?  Or put another way, what RH is needed in the snow growth region to produce snowflakes (and when I say snow growth region, I'm going with the -10C to -20C zone, with -12C to -18C being optimum)

 

I have previously read that 80% RH is considered saturated with respect to ice, while 100% RH is considered saturated with respect to water.  Is that correct?

 

Here's the sounding text from Greensboro, NC, right close to the time when snow began mixing with sleet.  At the height where the temperature is -12.8C, the RH is 90%...and at the height where the temperature is -17.5C, the RH is 84%.  Those values would appear to be sufficient to produce snowflakes given my statements above...but I may be off with the 80% RH threshold...also, would a reduction in vertical lift play a role as well?  Any insight would be appreciated. 

 

[isohume]

During the Feb 12-13 winter storm, we had a situation in parts of central and western North Carolina where snow switched over to sleet.  At first glance, the thought was that a warm nose had moved in aloft.  However, the culprit instead appeared to be reduced moisture in the snow growth region leading to the absence of ice nucleation, and instead, the freezing of supercooled water droplets into sleet. 

 

 

Super cooled droplets will not form sleet. Sleet is the process of ice crystals melting then totally refreezing before reaching the ground. The supercooled water droplets wouldn't have an ice nuclei in order to freeze into sleet. They would fall as freezing drizzle or drizzle. The IP you experienced was due to ice crystals melting then refreezing. The warm nose didn't reach the GSO area at the time of the sounding...assuming you were in CLT

.

That GSO sounding is certainly moist enough aloft for saturation wrt ice. It depends on the environmental temperature/pressure as well. The lower the pressure, the less the saturated vapor pressure of ice needs to be to equal it. The same is true for temperature since P and T are directly related according to the ideal gas law.

Here's a chart that shows the saturation relationships btw ice and supercooled water wrt to temperature. For example, if the temp is -20, then the RH to reach saturation wrt ice is about 80%.

 

 

As far as vertical motion goes, yes a reduction aloft (increasing subsidence) would lower the possibility of temps reaching or remaining close to dewpoints and saturation levels.

[griteater]

isohume - thanks for the response...and for the clarification on super cooled water droplets.  That's a nice chart.

 

So, on that chart, it looks like if the temperatures are in the -10 to -15 range, the RH to reach saturation with respect to ice is around 90%.  Please correct me if wrong.

 

And if I'm understanding this correctly, I didn't realize you had to have ice nuclei in order to see sleet...but that makes sense...and given all of the above, it does look like a warm nose working west into central and western portions of NC was the culprit for the changeover to sleet (just looked at some re-analysis maps at 850mb).