Updated: May 1, 2019
Normally pilots look forward to improving weather conditions after the passage of a cold front. The cold, dense air behind the front becomes negatively buoyant and sinks which tends to dry out the air. Moderate northwesterly winds will often prevail on the cold side of the front making for moderate mechanical turbulence sometimes extending up to 8,000 feet AGL. Other than some turbulent air, we don’t typically encounter much in the way of adverse weather behind such a cold front with few clouds, no precipitation and unlimited visibility.
During the spring, how many pilots think about the airframe icing threat that can occur in an overcast stratocumulus deck after the passage of cold front? Even a thin stratocumulus cloud deck like the one shown above can contain a liquid water contents approaching 0.5 g/m3 - especially near the tops. When the temperature is just right, these harmless-looking clouds can surprise a pilot with some moderate or even severe icing while climbing or descending through them. This is especially concerning to those pilots flying aircraft without certified ice protection systems (IPSs).
Stratocumulus decks have very distinct characteristics from other clouds. Although not completely smooth on top like a stratus deck, they have rather even tops with a quilted-like or lumpy appearance when viewed from above. While a stratocumulus deck can be broken or even scattered it is quite common for these cloud decks to be overcast when they occur after the passage of a cold front. They are rooted in the boundary layer near the surface similar to other cumuliform clouds, but an overcast stratocumulus deck can extend for hundreds of miles making them difficult to avoid.
Let's examine a case in the middle of April near Atlanta, Georgia where many pilots were reporting moderate icing in one of these stratocumulus decks.
This story starts out in the prior day. A strong cold front moved through the Atlanta area during the late morning, bringing with it severe thunderstorms with tornadoes and microbursts in the afternoon from northern Virginia down to the peninsula of Florida as shown above. Temperatures ahead of the front climbed into the low 80s. As the cold front (shown below) moved off the southeastern Atlantic coast the following day, this set the stage for a cold northwesterly wind to push a cold air mass over a fairly warm and wet ground in the Atlanta metro area. A warm and moist surface with cold air moving in aloft is the perfect recipe for the genesis of a juicy stratocumlus cloud deck.
Skies were generally overcast all morning throughout the Atlanta area as can be seen by the surface observations below. Ceilings were marginal VFR ranging from 2,100 feet to 2,600 feet. In an overcast cloud deck, there's no real way to know its depth from looking at the clouds from below. Depending on the sun angle, a darker cloud base is indicative of more condensate, but this was early in the morning where the bases will generally be darker. But in the Atlanta region, this was definitely an overcast stratocumulus deck like the one shown in the picture below.
KPDK 161453Z 30013G21KT 10SM OVC021 04/M01 A3000 RMK AO2 SLP167 T00391011 53014
KPDK 161353Z 27011G18KT 10SM OVC021 03/M01 A2998 RMK AO2 SLP160 T00331011
KPDK 161253Z 26013KT 10SM OVC022 03/M01 A2996 RMK AO2 SLP156 T00331011
KPDK 161153Z 28011KT 10SM BKN021 OVC028 04/M01 A2995 RMK AO2 SLP151 60000 70060
KPDK 161139Z 26010KT 10SM BKN026 OVC033 04/M01 A2996 RMK AO2 T00391006
Temperatures at the surface during the morning were a chilly +3°C to +4°C. But one characteristic of stratocumulus clouds is that there is very unstable air below. This means the temperature decreases at the dry adiabatic lapse rate of 3°C for every 1,000 feet gain in altitude. Except for right at the surface, this lapse rate is the most unstable that unsaturated air can be. This is best seen on a thermodynamic chart called a Skew-T log (p) diagram. The Skew-T analysis below for Dekalb-Peachtree Airport (KPDK) captured through the WeatherSpork Airports view demonstrates a textbook stratocumulus signature.
As can be seen above, the stratocumulus clouds extend from the bases at 2,500 feet MSL where the temperature and dewpoint converge with saturated conditions to the tops at roughly 5,500 feet MSL or where the temperature and dewpoint diverge. That's a depth of roughly 3,000 feet at 14Z (they likely varied in depth over the region). Notice the large lapse rate below the cloud bases with nearly moist absolute instability within the actual cloud deck itself. This is the most unstable lapse rate that can occur in saturated air. Remember that this area had received a fair amount of rain, so there was plenty of fuel being pulled into these clouds from below.
The tops are capped by a very strong temperature inversion which limits vertical cloud growth which is the classic signature to stratocumulus clouds. So you can think of this "system" as a lid on a pot of boiling water. The unstable air ascends, expands and cools to produce these clouds and given that the clouds are also unstable inside, the momentum in the capped updrafts gives those clouds the lumpy appearance when viewed from above. In fact, taking a closer look at this sounding analysis shown below with a parcel lapse rate added, there is a fair amount of convective available potential energy (CAPE) that allows for efficient transfer of water vapor into condensate (liquid drops). It's like squeezing the water out of a sponge.
While not specifically shown on the Skew-T diagram, the graph below is a trace of the liquid water content of an instrumented research aircraft that climbed through one of these cloud decks. The liquid water content is shown increasing to the right on the X-axis. Height is shown on the Y-axis. Because these clouds are rooted in the boundary layer, the have a median volumetric diameter drop size of less than 50 microns (small-drop icing scenario). A liquid water content value at the top of this cloud deck shown in the graph below of 0.5 g/m3 is huge. The liquid water content can be significant enough to overwhelm many aircraft when flying in the tops of these clouds.
Temperature is also important. The cloud top will typically exhibit the coldest temperatures in these clouds. In this case for Atlanta, the cloud top temperature is in the perfect range at -7°C if you want to see icing conditions. That guarantees an all-liquid process making for a juicy cloud containing no ice crystals. You can also see the cloud top temperature in the color-enhanced infrared satellite image below. Colors in the image are based on temperature in degrees Celsius. When clouds are present, this is the temperature of the cloud top, otherwise it's the temperature of the surface of the earth. The stratocumulus deck shows up nicely over northern Georgia as a yellow color implying a cloud top temperatures of -8°C which matches what is shown in the Skew-T diagram.
Some of this cloud cover in extreme northwest Georgia was covered under an icing G-AIRMET valid at 15Z for widespread moderate icing from the surface to 10,000 feet MSL (shown below).
Lastly, there were a number of pilot weather reports for icing in and around the Atlanta region that included an urgent pilot report from a Boeing 737 at 5,000 feet MSL south of Atlanta. The moderate ice reports during the morning tended to be around the tops of this cloud deck.
ATL UUA /OV ATL085020/TM 1245/FL050/TP B737/TA -3/IC MODERATE RIME
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist