Case Study 009"Severe Icing
Event"
6 March 1996
The following lab was presented as part of the AES Managers Mesoscale Course
Widespread In-Flight Aircraft Icing During a Winter Storm Event
M. Politovich, NCAR/RAP
R. Cianflone, UCAR/COMET/NWS
Introduction
Conditions conducive to aircraft icing occur quite often across much of the United States during the winter months. These conditions are often the result of several synoptic-scale processes that include the large-scale flow, moisture, temperature and vertical motion regimes. Icing conditions are also closely tied to several mesoscale and microscale interactions such as cloud liquid water content and distribution, cloud glaciation, precipitation and supercooled large drop (SLD) formation processes. During March 6, 1996 several of these large and smaller scale processes combined to produce a large area of significant icing conditions which included portions of the Mid-West to Northeast and the Northwest.
This case study will be an exercise in examination of both the synoptic scale forcing as well as some of the mesoscale interactions that came into play to cause the extensive icing areas. Although some of the synoptic features of this case are ‘classic’ for aircraft icing, several smaller scale processes interacted with the larger scale regime to locally enhance or decrease the severity of the icing.
GOAL
The goal of this lab is to identify the conditions favorable for aircraft icing that occurred in this case. Then from these conditions, assess areas of potential icing and a subset of these areas where smaller scale processes may have enhanced or decreased the icing severity.
OBJECTIVE
The objective of this exercise is to gain an appreciation for the complexity of factors that come into play during an icing event. Through the versatility of GARP and several icing diagnostic tools, some of these factors can be identified and used to improve icing forecast accuracy.
The data available includes model guidance, national radar, satellite imagery, upper-air plots, profiler data and some icing diagnostic tools. There is meso-eta data that provides forecasts of cloud liquid water content and a new experimental icing diagnostic algorithm. More information about these data is available from RAP.
OUTLINE
Section I: Synoptic Overview Use GARP to display model data, upper air, surface, radar and infrared satellite data. Additional satellite imagery is also available.
Section II: Icing Assessment Tools and Parameters Use GARP to display model data including cross-sectional data of temperature, RH, and omega. Look at and compare soundings for freezing versus frozen versus liquid precipitation. Look at surface data, etc. Display visible, water vapor, reflectance imagery, and IR satellite imagery to estimate where ice versus non-ice clouds exist.
Section III: Additional Diagnostic and Forecast Tools Display satellite icing and RAP temperature/RH algorithm products and ETA LWC products. More information on these data is available from RAP.
Section IV: Identify Potential Icing Areas Assimulating all data, identify those areas where icing conditions are most likely and a subset of those areas where more severe icing is possible.
SECTION I
Synoptic Overview:
Using GARP and other data sets made available for this exercise, review the evolution of the synoptic pattern during the period 00 UTC 6 March to 00 UTC 7 March 1996. Data available includes several model runs including the ETA, NGM, RUC and meso-ETA. Data plots from surface reports and from upper air soundings are available. The areas to concentrate on are from the Mid-West/Great Lakes to Northeast, and the Northwest.
As you investigate the model and plotted data consider the following:
Synoptic scale features from the surface to the mid levels locating:
As you investigate the satellite and radar data consider the following:
(View satellite data which includes IR, VIS, REF and WV data. IR data also available on GARP under GOES8_UNI, 8km. REF is the shortwave reflectance product. This data only looks at the reflectance contribution from clouds and as a result can distingish liquid cloud from ice cloud. Also, it can be used to determine where cirrus is located and may overlay lower cloud)
SECTION II
Icing Assessment Tools and Parameters
Assessing icing potential involves an assessment of the evolution of the synoptic pattern using many of the typical meteorological parameters and then investigating several atmospheric parameters that have a direct or indirect impact on icing potential. Although some of these can be investigated through the use of GARP in displaying model data, some can not be measured directly and can only be inferred. For example, model-generated soundings while useful for looking at vertical temperature structure, are often smoothed and are missing important details such as inversions or warm air intrusion that may be important in determining precipitation type and SLD formation.
In this case a large area of in-flight icing was reported over much of the central United States and the Northeast. The time of maximum icing was between 18 UTC and 21 UTC, so concentrate on this time frame. There were also icing reports in the Northwest. As you go through the data and examine the ETA, Meso-eta and NGM model outputs, look for reasons to support this widespread icing event. In the process consider the following:
Hint: For the following look at vertical cross sections from the meso-eta 12Z run valid for 18Z and 00Z , particularly between OKC and DET, STL and ALB and DCA to BOS . Use parameters such as RH, omega, temp, geostrophic and ageostrophic circulation.
There was also some icing reported in the Northwest. Evaluate the temperature and moisture structure over Idaho and eastern Washington.
It is difficult to fully define the limits of the icing in this short time but you should be starting to get a feel for the areas where icing may be a significant problem.
SECTION III
Additional Diagnostic and Forecast Tools
With hindsight, the available numerical model guidance, satellite, radar and plotted reports, one can begin to gain some insight into the processes producing the extensive icing. However, there is also another set of tools that can help to support the assessments made through analysis of the data thus far. Some of these can only be used in a diagnostic mode.
These tools include currently experimental techniques such as satellite icing algorithms, the RAP temperature/relative humidity product and the eta cloud liquid water content (LWC) forecast product. Each have their strengths and weaknesses and should be used with caution but they do show merit in helping to assess a potential icing event.
In this section review the satellite and RAP algorithms and ETA product provided for this case and consider the following:
Use the outputs from these products to help define your suspected icing areas and possible more severe icing regions.
SECTION IV
Identify Potential Icing Areas
Assimilating all the data viewed thus far and incorporating information about the satellite algorithms and the Eta LWC product define the general icing areas and those areas likely to experience more severe icing by outlining the areas on the map provided. Include estimates of icing bases and tops as well. When developing this assessment be sure to consider the following:
Useful Web Sites
Research Applications Program (RAP) of the National Center for Atmospheric Research (NCAR)
http://www.rap.ucar.edu/largedrop/ottawa.html
Includes icing products from Ottawa including the Stovepipe algortihm output on a realtime basis.http://www.rap.ucar.edu/weather/satellite.html
Realtime satellite products used for icing detection, turbulence detection, reflectance products, etchttp://www.rap.ucar.edu/weather/aviation.html
Realtime forecast and analysis aviation products throughout the U.S.
National Weather Service
http://www.nws.noaa.gov/adds/
This is the aviation digital data service which provides at digital meteorological data, analysis and forecast products for aviation interests