by Scott C. Dennstaedt

Convective SIGMETs are not stalking us at the moment and our memories of dodging thunderstorms are now turning into visions of AIRMETs for icing as the cold air from Canada plunges south. We’re now looking directly into the countenance of winter.

Not to worry, the hottest new icing product on the web, called CIP Severity, became operational on December 6, 2006.  As a former NWS meteorologist, FAA-certificated instrument flight instructor and IFR magazine contributing editor, I provided a look at this newest NWS product in the December 2006 issue of IFR magazine (see http://chesavtraining.com/Articles/Dennstaedt_CIP_Severity.pdf).  An analysis of icing intensity has been long overdue.  While AIRMETs and SIGMETs still provide the official icing forecasts, the Current Icing Product or more simply CIP (pronounced “sip”) provides a better spatial and temporal resolution than the traditional advisories produced by the Aviation Weather Center (AWC) in Kansas City.  It is refreshing to see an icing intensity product that attempts to highlight the regions of airspace that instrument pilots should be sure to avoid.

Late March of 2006, I had the pleasure to visit with meteorologist and icing expert, Ben Bernstein.  Now retired from the National Center for Atmospheric Research in Boulder, CO, Ben was one of several meteorologists who was responsible for the research and development of CIP.  “Icing is a complex problem.  There are so many factors that play a role in what makes or breaks an icing situation,” says Ben.  He further emphasized, “I am not totally fond of rules of thumb with respect to icing.  There are some that are descent, but there are so many exceptions to every rule of thumb that they can be dangerous.”  Like many icing forecasters, Ben prefaces many of his statements with “it depends.”

He further explains that icing comes down to a competition.  Once you get into subfreezing temperatures and there are clouds present, there’s a competition between the production of ice crystals and the production of supercooled liquid water.  He offers this simple model.  “If there is sufficient lift and a cloud is formed, there are two possible outcomes; it will either become a cloud containing mostly supercooled liquid water or will contain mostly ice crystals.  The latter is a glaciated cloud and does not represent a structural icing threat.”

Depending on the specific scenario, both ice crystals and liquid water can coexist in the same cloud.   As the cloud continues to develop there’s a competition between the existing ice crystals trying to deplete the liquid water present in the cloud.  In other words, the ice crystals may grow at the expense of supercooled liquid water droplets. This occurs because water vapor will preferentially deposit on the ice crystals and not on the liquid drops.  In this rather unstable situation, the liquid drops begin to evaporate to grow larger ice crystals.  Lower liquid water content in the cloud means a lower risk of icing in that cloud.  However, “The devil is in the details,” Ben acknowledges.

Most of the time precipitation falling from a cloud also tends to deplete the liquid water content in it.  Snow is one of the best examples.  Snow falling from a cloud or into a cloud deck from above tends to deplete liquid water in that cloud or in cloud deck below it.  Therefore, if snow if observed at the surface, it tends to diminish the icing potential in the clouds producing the snow, but does not eliminate the possibility according to Ben.  I asked him, “If it’s snowing at the surface, is it safe to launch into the clouds?”  Ben emphatically says, “No, that’s not a good choice.”       

Ice pellets (sleet) or snow grains falling at the surface tend to be a good indicator of icing aloft.  Both ice pellets and snow grains are produced by liquid water “riming” on the snowflake or ice pellet.   Ben says, “If there’s riming, there’s liquid water in the cloud.”  “Rain, on the other hand, is a mixed bag,” says Ben.  “On some days, rain at the surface means there’s nasty stuff around and other days, rain might be depleting the liquid water content in the cloud.”

What about a plain overcast sky with no precipitation whatsoever?  Ben shares, “Now you’ve eliminated the depletion mechanism, but you may have also eliminated production because those clouds are not getting enough kick to get much going.  Even the most harmless looking stratus layer may be loaded with water or it may be rather weak and very thin with not much going on…in other words, it depends.”

So the $64,000 question I posed to our expert is, “Are there any weather products a pilot can use to make this determination between a harmful cloud loaded with ice and a harmless cloud with little or no icing potential?”  Ben answers, “That’s what we try to do in CIP.  We first try to identify that a cloud exists and then we try to get a handle on what’s happening in that cloud.  What are the mechanisms that relate to -- ‘it depends.’  What temperature is it occurring at?  How deep is the cloud?  What’s the cloud top temperature?  Is there precipitation falling and what type is it?  How intense is it?  How strong is the lift in the cloud?”  All of these things add up to how likely it is for supercooled liquid water to be in the cloud and how much is likely to be there.  “That’s what CIP tries to do,” Ben continues.   “CIP tries to digest all of these things to give you an answer that is somewhat credible, ergo, there’s a 30 percent chance of icing as opposed to a zero percent chance.”     

Even though CIP provides some help, pilots need to understand that icing prediction is a complex problem.  Even rather benign-looking situations can turn out to be problematic.  Ben watches pilot reports every day during the fall, winter and spring and is drawn to severe pilot reports to monitor how well CIP is doing to capture the icing threat.  

I asked him about an icing accident in January 2006 near Birmingham, Alabama where a Cirrus SR22 pilot who held an ATP certificate activated the Cirrus Airframe Parachute System (CAPS) after he lost control of his airplane while trying to get on top of cloud layer.  Ben commented, “Even though they were on the edge of the icing AIRMET, this was a classic case of an encounter with stratocumulus clouds.”

For this particular accident, a strong cold front with thunderstorms moved through the Birmingham region about 18Z ushering in much colder air.  At 18Z, the freezing level was about 9,000 feet.  Just before his departure around 21Z, the freezing level had dropped to 4,000 feet and there was an AIRMET for icing from 3,000 to 8,000 feet.  The accident occurred inside the AIRMET, but right on the edge of the AIRMET boundary.

According to Ben, post-cold frontal icing can occur when there is a low level destabilization of the atmosphere that can kick up the water content in the swaths of stratocumulus clouds behind the front.  While only a few thousand feet thick, stratocumulus clouds have characteristics of both stratus clouds and cumuliform clouds.  They have a large horizontal extent like stratus, but are vertically developed like cumulus clouds.  They are usually found behind a strong cold front and can have copious amounts of liquid water, especially right near the cloud tops.

This is all assuming that the temperature is just right.  The temperature in the icing layer near Birmingham was about -4 degrees Celsius which is a perfect for icing, especially clear ice.  Ben then pointed out, “The instability helps to generate condensation or liquid water if the cloud has some depth.”  

Sometimes it call comes down to how well the pilot prepares before the departure.  We don’t know exactly how this pilot prepared prior to launching into known icing conditions, however, the NTSB did state that the probable cause in part was “the pilot's inadequate preflight planning and failure to obtain a current weather briefing.”  Experienced or not, knowing how to do a complete self briefing is paramount.  My article, “Pump Up Your Web Wx” that appeared in the January 2007 issue of IFR, provides some great self briefing tips for even the most seasoned instrument pilot (see http://chesavtraining.com/Articles/Dennstaedt_Internet_Wx_Brief.pdf).

Determining that icing exists in a cloud is quite complex; determining the amount of icing in the cloud is either incredibly difficult or fundamentally impossible in some cases.  Nevertheless, the sole purpose of the new CIP Severity product is to provide pilots with a way to assess the location and altitude of supercooled liquid water that represents a serious icing threat.  CIP Severity also offers a masked variant that combines the CIP probability field with the intensity so pilots can quickly assess those regions that have the highest probability of moderate or greater icing.  This product masks out those lower probability events to only show areas that have a high likelihood of containing supercooled liquid water.  Additionally, the icing intensity levels of trace, light, moderate and heavy are preserved.  The term “severe” is not used since it is reserved for how the aircraft reacts to the meteorological conditions, not the meteorological conditions themselves.

Icing severity is primarily a function of three elements.  Factors of severe icing include the liquid water content in the cloud, temperature in the cloud and the drop size.  While temperature is fairly easy to predict, the other two factors are not.  Ben says, “Small errors in timing, vertical velocity in the cloud, stability and moisture initiation could mean the difference between a benign or dangerous event.”  There are classical freezing rain events that remain easy to identify, but are less common than most other large supercooled water drop scenarios.  According to Ben, “These other non-classical freezing rain events are about 9 times more common and have a more complex structure, and therefore, are more difficult to identify.”

All of the CIP products, including the severity products, can be found on the Aviation Digital Data Service (ADDS) web site at http://adds.aviationweather.gov/icing/ or on their experimental site at http://weather.aero/icing/.  

What about the Forecast Icing Product or FIP?  Will there be a severity product available soon?  Ben says, “It is one thing to try to determine icing severity based on observations like we do in CIP.  It is a totally different problem trying to forecast the potential for severe icing based purely on model data.”  Ben mentioned that there is a plan in place to have a FIP Severity product operational by June 2009 with an experimental product that’s currently available on the experimental ADDS web site.
 

Scott’s Bio

As a former NWS meteorologist and FAA-certificated instrument flight instructor, Scott offers a unique set of qualifications to his instrument students.  While he still occasionally teaches his in-person two-day aviation weather workshops, Scott now occupies most of his free time teaching instrument pilots online using live online sessions via GoToMeeting.  Some of these sessions are offered free of charge on a first come, first served basis and others require a small fee.  He also offers one-on-one online weather training tailored to the pilot’s specific needs or mission.  Scott is now building a complete aviation weather curriculum that will easily outpace the FAA standards including his latest CD called “Ice Is NOT Nice.”  Ice Is NOT Nice was a collaborative effort between Scott Dennstaedt and you guessed it, Ben Bernstein.
 

In addition to his online training, in February 2009 Scott will be launching a brand new subscription-based website called Aviation Weather Workshops.com (http://AvWxWorkshops.com).  AvWxWorkshops.com is a growing library of bite-sized aviation weather workshops designed to build a pilot’s weather acuity.  Each one of these “point-and-click” workshops contains both audio and video and range in length from 5 to 30 minutes.  The website also encapsulates The Weather Report aviation weather discussion forums.  The Weather Report is a categorical group of forums dedicated to aviation weather training that features the Internet Weather Brief Roadmap, a step-by-step guide for pilots who want to get the most out of their Internet preflight briefing.  For more details see http://chesavtraining.com.  Scott can be reached via e-mail at scott@chesavtraining.com.