Cold Air

January 26th, 2007 |

Because of relatively mild and snow free conditions in the first half of January, many stories appeared in the media about the lack of winter and mild temperatures. In a classic case of Beware What you Wish For, cold winter temperatures have struck back with a vengeance. Dangerously cold weather covered eastern North America this morning, spilling out over the Atlantic Ocean. Under clear skies overnight, temperatures in eastern Canada plunged below -30 (F) at many locations. The infrared cloud image from 12:45 UTC this morning bears testament to the cold air, with brightness temperatures colder than 240 K (-33 C, or -27 F) in the clear air. These cold temperatures match those of high clouds associated with a storm moving towards the north Atlantic at the eastern edge of the image.

coldairgreyscale.GIF

A common feature observed in images recorded when Arctic air spills out over the ocean is the development of a band of stratocumulus clouds. The edge of the cloudbank roughly parallels the coast, and the distance between the coast and the cloud edge tells you something about the air-sea temperature difference and the initial moisture content of the air. As dry, cold air leaves the continent, heat and moisture are added to the boundary layer from the ocean surface. The warming and moistening will reduce the stability of the lowest part of the atmosphere, allowing the development of shallow convection. At some point from shore, sufficient moisture is added that the convection becomes visible. That is, clouds develop. Note in the image how air flow down Delaware Bay and across Chesapeake Bay is likely adding moisture to the offshore flow. Clouds develop downstream of these features much closer to shore than over adjacent waters where the airflow emerges unmoistened from the Continent.

colorenhancena.GIF
The color-enhanced version of the grey-scale image highlights several features. Note that the coldest air (dark blue) over central New Hampshire and southern Maine is just upstream of a region where clouds develop very close to shore. This is a small region of mid-level cloudiness, with a surface echo in marginally higher dewpoints (-12 F at Portland vs. -15 to -17 at points north and south alont the coast) In adjacent, drier, areas, cloud development occurs farther offshore. Clouds extending to Cape Cod in this image were producing road-slickening snow showers. The enchancement also highlights the relative warmth of rivers, such as the ice-free Susquehanna (and its North Branch) in Pennsylvania and the Hudson in New York, and the ridge and valley geography of south-Central Pennsylvania.

Topography effects: warm ridge tops, and chinook warming

January 26th, 2007 |

AWIPS MODIS IR image

Not every part of the US was experiencing the wrath of winter on 26 January 2007 (see the previous  Blog post); an AWIPS image of the 1-km resolution MODIS 11.0 µm InfraRed (“IR window”) channel (above) revealed an interesting distribution of brightness temperatures that were strongly influenced by the topography of the Black Hills region of western South Dakota. Matthew Bunkers (Science and Operations Officer at the Rapid City SD National Weather Service forecast office) provided a nice high-resolution topography image (below) and offered insight toward the interpretation of the MODIS IR image. The most obvious feature evident on the IR image was the zone of darker red enhancement (brightness temperature values of 0º to +10º C, or 32º to 50º F) seen along the northern and eastern periphery of the Black Hills — this was a result of westerly/southwesterly “chinook winds” to the lee of the highest terrain, where downslope winds were warming the air adiabatically (MODIS IR image with surface METAR reports). Several hours after this 04 UTC MODIS image, the temperature at Ellsworth Air Force Base (KRCA) rose from 39º F at 09 UTC to 50º F at 10 UTC, with northwesterly winds gusting to 25 knots (29 mph) in the wake of a cold frontal passage. Another item of interest to note on the MODIS IR image is the narrow filaments of warmer brightness temperatures (red enhancement) that extended from the foothills of the Black Hills eastward across the plains — this signature indicates that the ridge tops in that area remained well-mixed (due to higher wind speeds) and stayed relatively warm compared to the adjacent river valleys (which decoupled, and experienced stronger radiational cooling). Note that the winds were still light (around 5 knots) at Rapid City Regional Airport (KRAP) and Ellsworth Air Force Base (KRCA) at 04 UTC, prior to the arrival of the stronger post-frontal winds. The daytime winds eventually gusted to 47 knots (54 mph) at the Rapid City airport, which prompted the issuance of an air pollution alert for blowing dust in Rapid City (a MODIS true color image from later that day showed that there was no snow on the ground in the Rapid City area and the eastern foothills — only the higher elevations of the northern and western Black Hills still had some residual snow pack). While the corresponding 4-km resolution GOES-12 IR imagery did show the “chinook warming” signature to the lee of the Black Hills, the smaller scale details such as the warmer ridge tops were not apparent.
topography image of western South Dakota