Updated: Dec 28, 2021
As a pilot you've likely been taught that supercooled LARGE drop (SLD) icing is something that you should avoid at all costs. But if the entire temperature profile is below freezing, can there still be any chance of airframe ice, much less the chance for SLD?
Freezing rain (FZRA) and freezing drizzle (FZDZ) are two common examples of SLD. Some instrument pilots were taught to climb when they encounter freezing rain. The story goes that if freezing rain is falling, there's a warm layer aloft where the temperature is above freezing. In other words, the warm layer melts the snow which in turn becomes just a very cold rain. However, this classical structure only occurs in 8% of the freezing rain situations. Therefore, a significant portion of the remaining 92% you may find that climbing is not always sound advice. Let's take a look at such a case for freezing drizzle.
Below is the forecast sounding for Wiarton Keppel International Airport (CYVV) in Ontario, Canada. Notice that the entire temperature profile is below freezing all the way down to the surface. This means that you won't find a layer aloft that is warmer than 0°C. In fact, if you decide to climb, you'd better hope that you pop out of the cloud tops that are likely around 7,500 feet MSL based on this sounding. Certainly if you descended into this subfreezing saturated layer and were accreting ice rapidly, a quick climb out is likely the best solution to stop the accretion. Even so, with the temperatures several degrees below freezing, ice that has accreted will not melt off anytime soon.
This temperature and dewpoint profile speaks volumes. First, if there is any precipitation falling to the surface, it is likely to be freezing drizzle (FZDZ). This is due to a few signatures. There's a saturated layer from 1,200 feet MSL to about 7,500 feet MSL. In order to get rain drops (drops with a size of 500 microns or larger...1000 microns is a millimeter), you need a much deeper saturated layer aloft. With a depth of roughly 6,500 feet, you'd expect to see drizzle-sized drops (200 to 500 microns) and not rain drops. In fact, as you can see below from the EZWxBrief EZMap, that's what the 0200Z observation is reporting at CYVV...light freezing drizzle (-FZDZ).
The next signature is the unstable lapse rate (moist absolute instability) right at the tops of the clouds (between 5,000 and 7,500 MSL) where you'd likely expect this part of the atmosphere to be building those drizzle-sized drops. It's likely that the Rapid Refresh (RAP) model here is overdoing this unstable layer a bit (moist absolute instability is pretty rare). However, if you opted to climb out of this, you'd be climbing into the most serious icing near the tops. Also, this is likely to be a very clean environment given the temperature inversion from 2,000 to 5,000 feet MSL. Clean environments with warm subfreezing cloud top temperatures create a cloud that is dominated by supercooled liquid water.
This is nicely captured on the EZAirport meteogram in EZWxBrief. This suggests there is a high probability of SLD at 0200Z (far left of the meteogram) from 3,000 feet through 7,500 feet MSL as also indicated in the forecast sounding above. This is expected to continue through 0500Z.
What makes this a "non-classical" SLD environment? Well, it's an all-liquid process. That is, given the warmer cloud top temperatures, the cloud (and precipitation) is dominated by liquid water (few ice crystals) that are supercooled. As a result, this is also known to meteorologists as the supercooled warm rain process. Once the cloud top temperature falls below -12°C, ice nuclei begin to activate and that provides a better opportunity for ice crystals to begin forming producing a mixed phase cloud consisting of both supercooled liquid water and ice crystals that could begin to produce snow or even snizzle when the depth is shallow like it is here.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist