Updated: May 1, 2019
Supercooled LARGE drop (SLD) icing is a hazard for all aircraft. But is all SLD created equally? Not necessarily. It also helps to understand what's triggering the SLD event in the first place. A little sleuthing and some basic education goes a long way to understand the analyses and forecasts we use every day.
Below is the Current Icing Product (CIP) severity analysis for 7,000 feet MSL. CIP is an analysis of the icing environment that is always valid in the recent past...so it isn't a forecast - it's an analysis.
The blue-shaded regions correspond to icing severity categories of trace, light, moderate and heavy icing (shown below) that are expected at the top of the previous hour, in this case 1700 UTC. The SLD threat is overlaid on the severity analysis as a red-hatched region.
Notice that around Watertown, NY, there's a semi-circular region (pointed to by the red arrow) indicating a pretty widespread area of SLD. But what's triggering this? It's actually quite easy to deduce by noticing a few things. First, notice that the SLD threat has a circular appearance. Remember, this is not a forecast, so CIP is using surface observations as one component to build the icing severity and SLD analysis. That typically means that something is occurring at an airport right in the center of that circle (this is very common in Canada where observations are sparse). So let's take a look at the surface observations in this area around 1700 UTC.
At the center of that circle is Wheeler-Sack Army Airfield (KGTB) in upstate New York. At 1655 UTC, light drizzle (-DZ) was being reported at the surface. In fact, light drizzle and light rain were reported over the last several hours at this station. Assuming these automated reports of drizzle are correct (see this AvWxWorkshops member workshop for a discussion about why they may be incorrect), CIP SLD is using this observation to confirm the possibility of a drizzle-sized drops aloft. Drizzle drops are 200 to 500 microns in size, well above the median volumetric diameter of 50 microns that is used to define an SLD environment. So CIP is picking up on this single observation and that's triggering the SLD you see in this analysis.
Typically in a drizzle environment, the depth of the clouds is perhaps 4,000 to 12,000 feet. This can be seen on the Skew-T log (p) diagram. CIP uses the temperature and moisture data from the Rapid Refresh model shown below. The saturated atmosphere (as indicated by the red arrow) has a base of 2,500 feet MSL and tops of around 8,000 feet. Also notice that the cloud top temperature is roughly -10°C which is the perfect cloud top temperature to produce an all liquid cloud.
There are other clues as well. CIP also produces a "scenario" product that is available in the Imagery view. This CIP Scenario tries to describe why CIP is producing icing. In this case, it's due to a warm precipitation process (WMPCP) as can be seen by the red circle in the same location in upstate New York. Warm precipitation describes a scenario when non-snow precipitation is observed at surface with a cloud top temperature greater than -12°C.
The last thing to notice is the IR satellite image shown below. This depicts the cloud top temperatures as well. Around the same location (red circle below) the satellite image shows warm subfreezing tops as a yellow or pale green color which equate to about -12°C (pointed to by the blue arrow).
IR satellite image is also used by CIP to determine the SLD threat. All in all, the CIP SLD algorithm is showing a blotch of red-hatched region on the CIP Severity analysis shown above based on much of the data shown here. However, this CIP Severity analysis will show a red-hatched region even if there is as little as a 5 percent potential. When, in fact, there's actually less than a 50 percent potential of SLD in most of this circular region as can be seen on the CIP SLD potential analysis shown below.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist