ATSC 5007 – Problems in Synoptic Meteorology: An introduction to mid-latitude cyclones and fronts Historical Background A. 19th century (pre-Bjerknes) – Epsy (1840) recognized that most clouds form from expansion of air, and also recognized the role of release of latent heat. He suggested that the storm was basically a thermally-direct circulation driven by release of latent heat during condensation. – Throughout the 19th century, the “thermal theory” of cyclones persists: buoyancy driven, dependent on release of latent heat, and basically a vortex. A “disturbance” of the general circulation. – Ferrel (1878): established that most midlatitude storms develop in statically stable conditions from the thermal wind equation, he deduced how the upper and lower wind fields must be related. hypothesizes that the storm’s energy is derived from UL kinetic energy, as a way to explain storms in statically stable conditions. Historical Development B. The Norwegian School – Geophysical Institute at Bergen: founded 1917 included Vilhelm Bjerknes, Jacob Bjerknes, Thor Bergeron, C.G. Rossby, Erik Palmen, Petterssen established observing network; analysis of the observations led to the Norwegian cyclone model. – Their concept of the structure and evolution of baroclinic disturbances (the “Norwegian cyclone model”) was pivotal and remains very influential today At first they coined “broad moving rain stripe”, caused by gradual flow of warm air over cold. Similarly, they defined a narrow moving rain stripe. Later they came up with frontal labels as we know it. Jacob Bjerknes Vilhelm Bjerknes From Bjerknes 1919: Warm Front (“Broad moving rain stripe”) tracked as it moved across Norway. 6 h later Note 1. The broad area of light precipitation (shaded) 2. The discontinuity in airflow (and airmass properties) behind the rainband. From Bjerknes (1919): Cold Front (“Narrow moving rain stripe”) tracked as it moved across Norway. Cold air advances,lifting warm air. Result: short but intense precip over a narrow region. Note intersecting flows, narrow area of precipitation (shaded) that moved across Norway. Now the discontinuity in airmass properties is at the leading edge of the rain band. From Bjerknes (1919): vertical sections for warm and cold fronts Warm front Cold front From Bjerknes (1919): large portion of cyclone in observing network Noted that the bands often alternated, with broad band followed by narrow band. Warm front Cold front Bjerknes cyclone model. Dark shading: precipitation. Light shading: middle/high clouds. Note cloud types in vertical sections through the storm. Fronts in a baroclinic disturbance What is a front? A front is a zone of pronounced horizontal temperature contrast. A dividing line between different airmasses (note: fronts are always analyzed on the warm edge of the transition zone) • • • • Cold Front • Cold more dense air displaces warm less dense air • Slope of the front is much steeper, so in warm unstable air there is significant lift and storms Warm Front • Cold air retreats and warm air advances • Widespread steady precipitation ahead of front • Light drizzle and fog along the front Stationary Front • No lateral movement • Wind blows approximately parallel to isobars • If precipitation occurs it is light and it occurs on the cold side Occluded Front • Cold Occlusion • Air behind the advancing cold front (cP) is colder than the cool air ahead of the warm front (mP) • Warm Occlusion • Air behind the advancing cold front (mP) is relatively mild compared to cold air ahead of the warm front (cP) • Neutral Occlusion • No temperature change, but showers present and a shift in winds Air masses Warm Front Cold Front Occluded Front Identification of Fronts Where is a cold front? What characteristics should we look for? • • • • • • • • Temperature gradient at surface Trough of surface low pressure (cyclonic circulation – convergence) Wind shift at surface (clockwise shift) Dew point temperature (colder dew point temperatures behind front) Pressure tendency (rising pressures after FROPA) Temperature gradient at 850 mb (for sea level stations – 700 mb for us) 1000-500 mb thickness Weather and clouds (clearing behind cold front, convective activity ahead) The Development of Cyclones (as deduced, Norwegian school) initial stage – Most start as waves on polar front – Solberg first suggested that this is a near-continuous feature – Cyclonic shear across polar front, even before the development of a wave Shear exists even if flow is westerly on the cold side – Frontal surface is tilted towards the cold air From Petterssen 1952, Weather Analysis and Forecasting, p. 218 The Development of Cyclones (as deduced, Norwegian school) Open wave stage: – deepening of low – strengthening of circulation and thermal advection – formation or amplification of upper-level wave (with crest over warm front and trough over cold front) From Petterssen, Weather Analysis and Forecasting, p. 218 The Development of Cyclones (as deduced, Norwegian school) Temperature structure at 500 mb during the open wave stage: Uniform temperature south of front; cont’d cooling north of front Frontal zone aloft 500 mb temperature contrast is less sharp at this level than at the surface Height contours tend to align with isotherms From Petterssen, Weather Analysis and Forecasting, p. 229 The Development of Cyclones (as deduced, Norwegian school) Occluded stage – Cold front overtakes warm front, or rather, LL vortex moves into the cold air – Warm air is lifted by intersection of two cold-air wedges. The final deepening of the low is often enhanced by strong latent heat release aloft. – This stage also ushers in decay, as diff CVA and WAA vanish, at least at the location of the low Trough forms north of tripple point (often with heavy precip), Cold front tends to remain frontogenetic, but becomes more shallow (katafront rope cloud) From Petterssen, Weather Analysis and Forecasting, p. 218 Frontal cyclone lifecycle Frontal Cyclone lifecycle cross section through fronts cross section through fronts Idealized Mature Frontal Cyclone 3D conveyor belts Surface isobars (solid) and 1000-500 mb thickness (dashed) 1 Note: • thickness contours 3 concentrated between sfc front and upper level frontal zone 2 4 From Petterssen, Weather Analysis and Forecasting, Vol. I, pp. 230-231 500 mb height contours Note displacement of upper-level trough to he west of surface low 1 3 2 4 From Petterssen, Weather Analysis and Forecasting, Vol. I, p. 231 Relationship to Upper-Level Structure 500 mb & 1000 mb height (thick & thin lines) and 1000-500 mb thickness (dashed). The deflection of the upper-level wave contributes to deepening of the surface low. Palmen and Newton, p. 326; cf. Houze p. 448 Why do developing baroclinic systems tilt westward with height? Source: Holton (2004) chap 6 Westward tilt with height implies QG uplift over surface low, i.e. development Source: Bluestein (1993) p. 134 temperature advection differential vorticity advection vertical motion eastward tilt with height implies QG sinking over surface low, i.e. decay Source: Bluestein (1993) p. 148 temperature advection differential vorticity advection vertical motion QG perspective: synergy between low-level & upper-level flow WAA CVA AVA CAA A new model for the evolution of baroclinic lows (Shapiro et al.) 316 K high PV blob example: Cyclone evolution 4 Jan 1989 340 K low PV area SLP and 950 mb z q, wind speed, and PV>2 PVU (shaded) q=340K wind speed and 950 mb z Shapiro et al 1999 Observed mesoscale low level T structure (T-bone structure) Shapiro et al 1999 18 Z on 4 Jan observations 350 m AGL qe and winds 950 mb q model grey: z>1 10-4s-1 black: z>2 10-4s-1 SLP and abs vort Shapiro et al 1999 T-bone seclusion slp, fronts, precip frontal fracture incipient bent-back warm front fracture seclusion 850 mb temperature, & LL jets Shapiro 1990 Conceptual model of lifecycle Idealized cyclone evolution, Shapiro et al 1999 305K PV (shaded) and 3 PVU contour at 340 K surface q and q’ za and winds at the surface Idealized cyclone evolution, Shapiro et al 1999 305K PV (shaded) and 3 PVU contour at 340 K surface q and q’ za and winds at the surface ATSC 5007 – Problems in Synoptic Meteorology: An introduction to mid-latitude cyclones and fronts Benjamin Franklin: In 1735, "Poor Richard," aka Ben Franklin, wrote: “Some are weatherwise, some are otherwise.” When he was 37 (1743), Benjamin Franklin observed that northeast storms begin in the southwest. He thought it was odd that storms travel in an opposite direction to their winds. After further observations and performing studies of storms, he predicted that a storm's course could be plotted. He then printed weather forecasts in his Poor Richard's Almanac. Thomas Jefferson, Notes on the State of Virginia, 1781 A change in our climate however is taking place very sensibly. Both heats and colds are becoming much more moderate within the memory even of the middle-aged. Snows are less frequent and less deep. They do not often lie, below the mountains, more than one, two, or three days, and very rarely a week. They are remembered to have been formerly frequent, deep, and of long continuance. The elderly inform me the earth used to be covered with snow about three months in every year. The rivers, which then seldom failed to freeze over in the course of the winter, scarcely ever do now. This change has produced an unfortunate fluctuation between heat and cold, in the spring of the year, which is very fatal to fruits. In an interval of twenty-eight years, there was no instance of fruit killed by the frost in the neighborhood of Monticello The accumulated snows of the winter remaining to be dissolved all together in the spring, produced those overflowings of our rivers, so frequent then, and so rare now. Mark Twain (Samuel T. Clemens - 1835–1910): “The coldest winter I ever spent was a summer in San Francisco.” “Of course weather is necessary to a narrative of human experience.” “Thunder is good, thunder is impressive; but it is lightning that does the work.” “Climate is what we expect, weather is what we get.” “It is your human environment that makes climate.” And, of course: “Don't let school interfere with your education.” “I could never learn to like her, except on a raft at sea with no other provisions in sight.” Sir Arthur Conan Doyle, “His Last Bow” (1917) "There's an east wind coming, Watson.“ "I think not, Holmes. It is very warm." "Good old Watson! You are the one fixed point in a changing age. There's an east wind coming all the same, such a wind as never blew on England yet. It will be cold and bitter, Watson, and a good many of us may wither before its blast. But it's God's own wind none the less, and a cleaner, better, stronger land will lie in the sunshine when the storm has cleared."