MEA 443 Synoptic Weather Analysis and Forecasting Fronts and Frontogenesis

Monday, 23 November 2009 Gary Lackmann

IV. Fronts and Jets A. Frontogenesis 1.) General frontal properties 2.) Review of kinematic frontogenesis mechanisms 3.) QG frontogenesis 4.) Transverse circulations: The Sawyer-Eliassen Equation 5.) Frontal collapse and frontal dynamics B. Types of fronts 1.) Cold frontal structures a.) Katafront b.) Anafront c.) Arctic fronts and other variations 2.) Warm fronts 3.) Occluded fronts (Bluestein Vol. II, 273-277) 4.) The coastal front (Bluestein Vol II, 277-282) C. Upper Fronts and Jets

1. Cold Frontal Structures Fronts of the same “type” are not always accompanied by similar weather conditions. We have seen numerous examples of this already this semester in forecasting—some cold fronts are accompanied by heavy precipitation, other frontal passages are completely dry. Also, stronger fronts are not necessarily accompanied by more “weather” than weak ones. Last week, we discussed some of the factors that dictate the extent of clouds and precipitation along a front, and today we will explore some of these factors a bit more. For example, cold-frontal weather is sensitive to the pattern of air flow in its vicinity. The storm (front) relative isentropic flow framework is a good one to use in understanding how airflow in a frontal system and weather are related.

Cold-frontal characteristics: • By definition, cold air is advancing • Usually located within low-pressure trough, accompanied by wind shift, cyclonic vorticity • In some cases, a trough may precede surface front (“pre-frontal trough”) • Front is often most intense at surface, weakens with height • Frontal zone is marked by large static stability • Frontal zone accompanied by strong vertical wind shear

In addition to the basic characteristics listed on the previous page, there are different types of front-relative precipitation. These depend on the front-relative orientation of the cold front and the “warm conveyor belt” (the main flow of warm, moist air that is typically located in the warm sector of a cyclone ahead of the cold front). The two basic structures are the katafront and the anafront.

a.) Katafront structure:

• The warm conveyor belt is oriented such that the primary lift is ahead of the surface cold front • Often, a push of dryer, cooler air aloft has advanced ahead of the surface cold front • The leading edge of this low-θe or θw air is known as a “cold front aloft”, or CFA • These are also known as a type of “split front” • Precipitation is heaviest at or ahead of surface cold front, can be well ahead of it. • The surface cold front may pass with only light precipitation, or none at all. • There is forward-sloping ascent; see Browning and Monk (1982) for details

a.) Katafront b.) Anafront

“Split front” “Narrow cold-frontal rainband (NCFR)”

b.) Anafront: • Rearward-sloping ascent with warm conveyor belt overrunning cold air • Heaviest precipitation at or behind surface front • Obvious implications for precipitation-type forecasting in winter • Low-level jet (LLJ) often intense immediately ahead of cold front • Often a narrow cold-frontal rainband (NCFR) is located right at the front with lighter precipitation behind

c.) Arctic fronts, “back-door” cold fronts, and other structures • Very shallow, may not be accompanied by precipitation • Movement may be consistent with that of a density current • What the heck is a density current?

Idealized cross section of clouds and precipitation in an anafront. Vertical lines represent precipitation, and arrows represent airflow relative to front. IPC stands for ice-particle concentrations in # per liter, and liquid water content (lwc) is given in g/m3. Figure from Matejka et al. 1980.

New England back-door cold front

2.) Warm Fronts • By definition, warm air is advancing • Because warm air less dense, mixing at frontal interface contributes to frontal advance • Warm air is advancing, sometimes discontinuously, veering wind with height • Shallow nature of frontal zone makes for weaker pressure trough there • Often more subtle than cold fronts, and may not be present for some cyclones. • In some cases, trough may precede surface front to mark location of warm front aloft • With anticyclone to N or NE, front will be most pronounced • “Easterlies north of warm front” - a good analysis rule for locating the cold front

a.) “Back-door warm front”: Warm maritime polar wraps around north side of

cyclone, WAA NW of cyclone center:

The diagram above was included on a past quiz… see point “B” northerly warm advection?? 3.) Occluded fronts a.) The occlusion process We discussed the frontal evolution accompanying a typical midlatitude cyclone earlier in the semester. Two different evolutions were presented: 1.) The “Norwegian cyclone model”, and 2.) The more recent “Shapiro-Keyser” model. Here are the key points from the comparison:

Surface 850-mb level

• In both models, initial development takes place along a stationary front • As the frontal wave amplifies, cold air retreats to the east & pushes southward to the west • At the point when the occluded front develops, the cyclone is near peak intensity • Local topographic features & other influences can cause departures from the idealized case The nature of the occlusion process continues to be debated. The original Norwegian cyclone model explained occlusion as a faster-moving cold front overtaking a warm front. However, the Wallace & Hobbs textbook states “…there are few, if any, well-documented examples of cold fronts overtaking warm fronts to form occlusions. Rather, it appears that most occluded fronts are essentially new fronts which form as surface lows separate themselves from the junctions of their respective warm and cold fronts and deepen progressively further back into the cold air.”

More recently, Schultz and Mass (1993) and Schultz et al. (1998) undertook a careful analysis of the occlusion process, and found that frontal catch-up was observed.

We can conclude that in at least some cases, the catch-up process appears to be important, but that the front lengthens due to the known tendency of the surface cyclone to deepen back towards the cold air. What phenomenon might be influencing the frontal evolution in the images above? b.) Warm and cold type occlusions Wallace and Hobbs, along with other texts and articles, point out that if the air behind an occluded front is colder than the air ahead of the boundary, then a “cold-type” occlusion structure will result. The counterpart to this would be a warm-type occlusion structure.

Schultz and Mass also scoured the literature to find examples of warm and cold-type occlusions, and could find no clear examples of cold-type occlusion structures. c.) Instant occlusions Another non-classical frontal evolution is referred to as “instant occlusion”. This process is described by the schematic below, taken from McGinnigle et al. 1988:

Schultz, D. M., D. Keyser, and L. F. Bosart, 1998: The effect of large-scale flow on low-level frontal structure and evolution in midlatitude cyclones. Mon. Wea. Rev. 126, 1767–1791.

Cold and warm-type occlusion structures (from Schultz and Mass 1993 at left, and Wallace and Hobbs 1997 below).