By: Jorel Torres and Jack Dostalek
With all of the attention given to the pending winter storm along the East Coast, this blog may seem a bit out of place. Nevertheless, high-profile blizzards aren’t the only dangerous cold-season weather phenomena of interest to forecasters.
During the winter months, especially at high latitudes, air temperatures at altitudes used by passenger aircraft can get cold enough to cause jet fuel to gel (known as “cold air aloft” situations by the National Weather Service). The air temperature at which the gelling of jet fuel becomes a concern is typically considered to be -65°C. The knowledge of the location of pockets of air this cold is of importance to weather forecasters. Model forecasts are used, but confirmation from observations is beneficial. Radiosonde measurements and aircraft observations are at times available, but the temporal and spatial sampling is too coarse to accurately delineate the cold air pockets. Polar-orbiting satellites can provide vertical temperature profiles with higher temporal and spatial resolution and thus help to monitor the atmosphere for air which is dangerously cold for aircraft travel.
Example from 9 January 2015
On 9 January 2015, the Center Weather Service Unit in Anchorage, AK issued the following Meteorological Impact Statement concerning the existence of cold air aloft near Barrow, AK:
The 1200 UTC Barrow radiosonde does indicate a thin layer of air with temperature below -65°C:
The S-NPP satellite passed over the region around 1220 UTC, allowing for a good matchup to the Barrow radiosonde. Two different satellite retrieval algorithms will be compared to the radiosonde data. First is the MIRS (Microwave Integrated Retrieval System), NESDIS’ current operational microwave-only retrieval algorithm, run on both the ATMS (Advanced Technology Microwave Sounder) and the AMSU (Advanced Microwave Sounding Unit). The second is NUCAPS (NOAA Unique Combined Atmosphere Product System), which is a combination microwave/infrared retrieval algorithm which uses the ATMS and the CrIS (Cross-track Infrared Sounder) aboard the S-NPP satellite.
The map below shows the positions of Barrow and the MIRS and NUCAPS retrievals, as well as a GFS forecast sounding (6-hr forecast of the 0600 UTC run of 9 January 2015).
The full sounding (upper panel below) shows good agreement among all of the temperature profiles, with the low-level inversion somewhat higher in the MIRS data. The cold air aloft occurred around 200 hPa and the lower panel shows that altitude more closely. All temperature profiles reach -65°C or colder near 200 hPa, but the exact location and extent of the cold layer varies: the MIRS is at the lowest altitude followed by the Barrow radiosonde and the NUCAPS retrieval, with the GFS temperature minimum at the highest level. The MIRS and NUCAPS profiles also show a broader layer of cold air. This is expected, as satellite retrievals are usually quite smooth in the vertical.
All of the profiles were able to capture the cold air aloft event of 9 January 2015. The determination of the overall performance of the satellite retrievals and model forecasts, however, cannot be determined without many more cases. Of additional interest would be a study documenting the dynamical nature of these regions of wintertime cold air aloft.
Display of Near Real-Time Data
As the study of the utility of the MIRS and NUCAPS retrievals continues, a web page has been developed at CIRA to display in near real-time the existence of cold air aloft over the Artic (http://rammb.cira.colostate.edu/ramsdis/online/cold_air_aloft.asp). The page currently displays only AMSU/MIRS data from NOAA-18, NOAA-19, MetOp-A, MetOp-B, and DMSP-18. ATMS/MIRS and NUCAPS data are not yet ready for near-real time display.
The page currently contains two links, one to an Arctic view and the second to a Bering Sea regional view. The color scheme denotes the coldest temperature at each footprint of a satellite swath:
If the layer of cold air exists below FL450 (Flight Level 450 is approximately 45,000 ft.) a ‘+’ is displayed. Additionally on the regional view, the extent of the layer of cold air, measured in units of flight level is also displayed. The upper figure below shows the swath view of a MetOp-A pass on 19 February 2015, with most of the cold air aloft over Russia and Greenland. The lower panel is an example of the Bering Strait view from 22 January 2015, a day when cold air aloft was reported by an aircraft at FL360 near the International Dateline and latitude 55°N.
Cold Air Aloft (1/21/16)
There is currently an extensive area of cold air aloft over the Arctic, and prompted the issuance of the following Meteorological Impact Statement from Anchorage yesterday (Jan. 21, 2016):
The following image from CIRA’s cold air aloft web site gives an idea as to the extent of the cold air.
Additional retrievals, particularly those from ATMS/MIRS and NUCAPS will be added to the web displays. In addition, other regional loops will be added as needed. Finally, CIRA is collaborating with SPoRT, CIMSS, GINA, and the National Weather Service to supply satellite information on the location of areas of cold air aloft to forecasters via AWIPS-II.