Updated: Nov 8, 2021
For those pilots that utilize the airframe icing analysis and forecasts found on aviationweather.gov, it's important to take a closer look at the guidance for supercooled LARGE drop (SLD) icing. The Current and Forecast Icing Products (CIP and FIP) provide three distinct forecasts. This includes an analysis and forecast for (1) icing probability, (2) icing severity and (3) supercooled LARGE drop (SLD) icing. First, let's define SLD.
What is SLD?
Clouds are made up of tiny droplets that are suspended in the air. Most of those droplets are 15 to 40 microns in size. For those who are micron-challenged, 1000 microns is equal to 1 millimeter. The naked eye can see objects as small as 40 microns. As a point of reference, the diameter of a human hair is approximately 100 microns, and a red blood cell is 8 microns. When the droplets grow in size to 50 microns or greater, they are considered a "large" drop (and drop the "let" at the end of droplet).
For icing certification purposes the drop sizes are actually based on a medium volumetric diameter (MVD). That is, when the median size of the volume of drops exceeds 50 or more microns, this becomes a large drop environment. That means when a drop becomes barely perceptible to the naked eye, it's considered a large drop for certification purposes. If the temperature is below 0°C this is called a supercooled LARGE drop environment. When the MVD is less than 50 microns, it's considered a small drop environment. Certified ice protection systems (IPS) are only certified into small drop environments.
Simply put, SLD is a "large drop" icing environment. If you hear terms like freezing rain (FZRA), freezing drizzle (FZDZ) or thunderstorms during your weather briefing, you are also likely dealing with the risk of SLD. Despite what some pilots will tell you, SLD does not stand for supercooled liquid drops; they are truly missing the point. Yes, they are liquid, but SLD stands for supercooled LARGE drops.
SLD can be divided into two primary categories, namely, convective and non-convective. Convective SLD occurs within deep, moist convection - or more specifically, vertically-developed cumuliform clouds (as shown above). This includes towering cumulus (TCU) and cumulonimbus (CB) clouds. Convective SLD is usually encountered during the warm season at much higher altitudes. The best way to avoid convective SLD is to remain outside of cumuliform cloud boundaries when flying at or above the melting level (0°C). This is usually easily accomplished since most convective SLD is produced by broken or scattered clouds.
Non-convective SLD includes freezing rain (MVD greater than 500 microns) and freezing drizzle (MVD between 100 and 500 microns) environments. This does not imply that the temperature at the surface has to be below 0°C. In fact, it doesn't imply that precipitation is reaching the surface either. Both of these forms of SLD can occur aloft and may remain aloft carried by upward mixing or may evaporate before reaching the surface.
Non-convective SLD can be split into two categories: classical and non-classical Most pilots were taught about classical SLD that produces freezing rain. It is usually produced in very deep saturated environments during the cold season where the clouds aloft are producing snow. Pilots were taught that this snow falls through the cold clouds to eventually fall into a layer of air that is warmer than 0°C (called a warm nose). In turn, this completely melts the snowflakes into raindrops. Lastly, the raindrops fall into a subfreezing layer (usually very close to the surface) to form freezing rain which is considered SLD. This classical structure can be seen on the image above.
Non-classical freezing rain is a bit more challenging to understand. But, 92% of all freezing rain events are non-classical. So it is important to understand. The biggest difference is that the cloud top is warmer (usually warmer than -12°C) and the clouds do not produce snow. There is often a dry layer above the tops. The temperature profile (shown above) can be very similar as the classical freezing rain structure including the presence of a warm nose. The difference, however, is that this non-classical structure results in an all-liquid process down to the surface producing freezing drizzle (or freezing rain).
However, the most significant difference is that the non-classical freezing rain structure may not have a layer above freezing (see above). In other words, the entire temperature profile may be colder than 0°C, but there still is a dry layer aloft and warm cloud tops. Most of the time, this kind of profile will produce freezing drizzle, not freezing rain. It is typically more shallow of an environment, but can produce a hazard over a larger range of altitudes because there's no warm nose that contains temperatures above freezing.
In addition to the forecast soundings shown above, SLD is also forecast over a geographical area. As shown below, you'll first notice that on the the icing analysis (CIP) and icing forecast (FIP) charts that the SLD "threat" is depicted as a red hatched area overlaid on the icing severity charts. This is not a calibrated probability like the CIP/FIP icing probability analysis and forecast. The SLD threat is called an icing "potential" forecast or more simply, the likelihood that an SLD threat exists at the altitude shown on the chart. Here's the important point. Essentially, any chance of SLD above a likelihood of 5% is shown on these charts.
This broad brush threat forecast for SLD makes it difficult to determine where SLD may exist, but the actually likelihood is low. The EZImagery provides the same charts that you'll find on the Aviation Weather Center website, but takes the SLD analysis a bit further. For example, the image below shows the SLD analysis that includes a large area of SLD threat at 7,000 feet MSL over the southern Appalachian Mountains.
However, if you compare this to the SLD Potential analysis from the EZImagery shown below, it is exactly the same analysis except that it contours the SLD at 10% intervals. That means the SLD threat shown above has a much less chance of occurring on the southern portion over northern Georgia. In fact, it's less than a 10% likelihood. On the other hand, the northern portion of this SLD threat over western Virginia is much more likely at 70-80%.
Even better, the EZRoute Profile shown below also takes advantage of the higher resolution for SLD potential that clearly shows a route from Danville, Virginia to Nashville, Tennessee at 7000 feet through this northern area in western Virginia would encounter a 67% SLD potential.
What is causing this area of SLD? To answer that, drilling down using a Skew-T diagram is a perfect way to further decipher the threat. Below is the 4-hour forecast sounding in this region from the Rapid Refresh (RAP) model. This is a non-classical SLD scenario. The cloud tops are about 7,500 feet MSL and are in the warmer subfreezing temperatures (-5°C). There is dry air aloft and the tops are quite unstable showing a moist absolute unstable layer (MAUL). In this case, the clouds are not deep enough to produce precipitation, however, there is likely some orographic lifting occurring in this area. The clouds are capped by an isothermal layer which limits their growth. The moist absolute instability and isothermal layer above can trap a significant amount of moisture just under the cap causing coalescence of smaller drops, thus, creating the SLD threat in the tops of these clouds.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist