Midlatitude Surface systems

Midlatitude surface systems consist of continental-scale high and low pressure systems.

1. Can you summarize the three most important aspects of the wind circulation around high and low pressure systems?

2. What are the results of these wind circulations for cloud and precipitation formation of these characteristics associated with high and low pressure systems?

3. What are some the consequences of temperature advection associated with each of these systems?

Solution:

	High (anticyclone)	Low (cyclone)
1.	a) clockwise b) outward c) downward	a) counterclockwise b) inward c) upward
2.	downward circulation associated with clear conditions	upward circulation associated with cloud/precipitation
3.	warm advection to west (left in picture) cold advection to east (right in picture)	warm advection to east (right in picture) cold advection to west (left in picture)

Passage of cold front: Local Maps

Local maps for 18Z (2PM) and 0Z (8PM) show the eastward passage of the occluded front over Vermont. It transforms into a cold front during this period. The winds shifts from southerly ahead of the front to westerly behind the front.

The second cold front approaching the U.S. from Canada weakens during this period. The dashed cold front symbol means that the front is dissipating. Winds behind this front are strong, ranging from 10 to 20 knots. As temperatures are also considerably colder in this region (in the 10's and 20's) than over New England (40's), we can expect cold advection by the westerly winds to cause temperatures to fall at least to these levels.

Sounding data for Albany, NY at 0Z 4 mar, 12Z 4 Mar shows that the daily temperature variation is in fact restricted to the very lowest layer of the atmosphere. At 0Z, the surface temperature at Albany is just about -8 C, whereas at 12 Z it has decreased to about -11 C. Note however, that above 950 mb, the temperature has barely budged. For example, at 950 mb the temperature remains constant at about -16 C.

This is typical of diurnal (daily) temperature fluctuations. These are restricted to a region of the troposphere called the boundary layer. Although the height of this layer can vary depending on weather conditions, it typically is restricted to below 850 mb. It's common to use temperature data from above this layer (usually 850 mb maps) to determine temperature trends based on temperature advection to remove the effect of the sun.

Data is taken from the University of Wyoming website.

Warm layers and precipitation type near Portland, Maine

The image at left illustrates how a warm layer (above freezing) aloft affects precipitation type. Where temperatures are below freezing everywhere, snow falls. Where the layer is above freezing near the surface, rain falls. In between, the type of frozen precipitation depends on teh depth of warm and cold layer. Warm layers need to be deep enough to melt the snow, and cold surface layers need to be deep enough to freeze the melted precipitation.

The images below show the evolution of the warm layer ahead of the coastal warm front for 24 hours on Friday, 12 December, 2008 near Portland Maine. Note that an extra radiosonde was launched at 6UTC (1 AM EST) to document this exceptional event. The freezing line is marked by a bold blue line at 0C (32 F).



At 0Z (7PM Thursday), a small warm layer above freezing can be seen forming at 850 mb. An inversion can be seen through most of the layer below this level. This would likely produce snow mixed with some ice pellets and freezing rain.

By 6Z (1 AM Friday morning) the warm layer has expanded to include the entire 900-700 mb layer. Southwesterly winds keep this layer warm, whereas the cold layer below 900 mb is sustained by cold northeasterly winds.

By 12Z, the warm layer reaches temperature of 10 C (50 F) while surface temperatures hover around freezing. Saturated conditions can be observed throughout the entire troposphere during this entire period. At rhis point on Friday morning, the warm layer and associated frozen precipitation extended all the way across the Northeast Kingdom, albeit for a very brief period.

By 0Z 13 December (Friday 7PM), cold advection and northwesterly winds are pushing out the warm layer, ending the ice storm. Note that temperatures everywhere are zero, and that conditions are far drier than before.

Warm fronts and frozen precipitation

The structure of warm fronts (stationary fronts are similar) is illustrated at left. Warm, humid air from the warm sector of the system is advected over a wedge of cold air. The lift leads to cloud formation, and a layer of warm, humid air above the cold wedge. However, precipitation falls into the colder air, and will freeze if the air is below freezing. Click here for a more detailed explanation of warm fronts.

Vapor channel from tropics

Warm front advances
The northeastern frontal map for 12Z Thu. 24 July, 2008 (left) shows that the stationary front that was over southern New England on Monday is being pushed northward by southerly winds circulating in a counterclockwise direction around a low pressure system over Lake Ontario. Cloudy conditions. Dewpoints in the high 60's almost everywhere in New England indicate humid conditions. Overcast conditions keep temperatures int he 70's all day.

Transport of humidity
The main weather story today (and all of this week!) is the rain and humidity. This is largely because of a channel of water vapor (i.e. humidity) that has opened up between New England and the Gulf of Mexico. This occurs along the narrow band of clouds ahead of a cold front along the Atlantic coast in the image at left (valid 12Z Thu. 24 July, 2008). A cold front extends southward from the low pressure system over southeastern Ontario all the way to the Gulf of Mexico. Red arrows in the image at left show the direction of the warm, humid flow along the Atlantic coast of the U.S. ahead of the cold front. This flow is pushing the warm humid air northward. The cold front is pushed eastward by a surge of cooler, drier westerly winds behind the front (blue arrows).

Warm conveyer belt
This is pretty typical of a mature phase of mid-latitude cyclone development shown at left (see Jetstream section of cyclone model for details). The image shows a warm, humid conveyer belt (red arrows) overrunning cooler air to the north where the warm flow meets the warm front. This rising motion leads to large amounts of precipitation ahead of the warm front. Note that the occluded front (purple at left) does not appear on todays map. Occluded fronts are rare in the summertime because weather systems evolve more slowly, with fronts moving much more slowly. Your text discusses this in more detail in Chapter 10.

Vapor channel in satellite imagery
The vapor channel is easy to see in satellite animations. Click here to see an animated loop of the warm conveyor belt (or vapor channel) discussed above. The cold front can be discerned along the border between clear and cloudy skies in the eastern half of the U.S. The thunderstorms and clouds along the eastern seaboard ahead of the front all moves northward in the fast moving southerly flow ahead of the front. Note also Tropical Storm Dolly along the Gulf coast of Texas/Mexico.

Air mass advection and fronts

The image at left from 21 Z (5 PM EDT) Tuesday 10 June 2008 will show how air mass advection and fronts around midlatitude cyclones can change the weather in New England.

A low pressure center can be seen in Quebec to the northwest of Vermont. Wind barbs indicate a counterclockwise and inward circulation around the low that is usually seen around a cyclone. A cold front extends soutwestward and a stationary front extends southeastward from the system center.

Three airmasses can be identified in the map. Over Vermont and most of New England, a marine tropical air mass predominates. It's this air mass that brought us near-record high temperatures between the weekend and Tuesday. It is bordered by the cold front to the west and a stationary front to the east, forming a wedge south of the low pressure center. Temperatures are in the 80's and 90's F and dewpoints are in the high 60's. Southerly winds in this air mass advect warm moist air northward in this region all the way from the Gulf of Mexico.

West and northwest of Vermont lies a continental polar air mass, with both temperatures and dewpoints in the 50's and 70's, the cooler temperatures residing to the north. It's this air mass that has moved in today, giving us cooler and drier conditions outside. Winds all have a westerly, with those over Canada having a more northerly component. They advect this airmass eastward. You probably noticed that it cooled off considerably yesterday evening as the front moved through with the storms yesterday.

Finally, over Maine and the Atlantic Ocean, a marine polar air mass had been destroying the hopes of anyone looking for a summer day at the beach. Temperatures and dewpoints along the coast of Maine and the adjacent ocean are all in the 40's and 50's, indicating the strong influence of cool Atlantic waters. It's these air masses that typically lead to ocean fog. Easterly and south easterly winds advect this cooler air onshore. These winds are enhanced by the afternoon seabreeze circulation.

The final feature to notice on the map is the dashed brown line, representing a trough. You can see this also in the isobar looping southward from the center of the low. This was associated with the line of thunderstorms that swept across northern Vermont and southern Quebec yesterday evening and knocked out our power.

You can access a 24 hr loop of New England fronts and weather station data at the NWS-HPC surface analysis website.