Geog 166: weather systems

Prof. Leila M. V. Carvalho

objectives

• The weather analysis will be focused first on the formation and evolution of extratropical cyclones

• We will discuss a case-study that caused strong winds and heavy precipitation to parts of the central United States

• It was unusually intense but typifies many features of winter storms in middle and high latitudes

Plotting conventions synoptic charts

Extratropical cyclones: case study example: November 1998

500hPa chart for 00UTC

Polar Vortex is split into two regional cyclonic features (Russia and Northern Canada)

Two ridges separate the two troughs

Solid red lines denote axes of ridges and dashed red lines are axes of troughs

Typical distance between two troughs : 50o of longitude or ~ 4000km

500hPa charts (contours 60m intervals (dKm) and relative vorticity (blue shading 10-4s-1). Right sea level pressure 4hPa interval and 1000-500hPa thickness (colors)

Baroclinic waves travel ~ 10m/s: wintertime climatological wind at 700Pa (steering level)Successive ridges (or troughs typically ass at a fixed point on Earth at roughly 4 days (but it can be sometimes 1 or 2 days apart depending on the steering flow)Baroclinic waves tend to be stronger over the oceans (as we have seen many times) but they can develop over land

Frontal surfaces

Note the intensification of the Low pressure system and how it “splits off the westerly to form a CUTOFF LOW (ISOLATED MINIMUM OF GEOPOTENTIAL HEIGHT FIELD)That implies the existence of a closed cyclonic circulationNote the tightening of the contours that imply the intensification of the winds around the low

Note the intensification of the low pressure at the surface

Amplification of the wave

The surface low is the EXTRATROPICAL CYCLONENote that pressure gradients increase with time showing the intensification of the low as the cold air moves southward and the warm air moves northward

In the beginning the surface Low is eastward of the trough but as the 500hPa and low level cyclone amplify they come into vertical alignment

Advection of temperature, amplification of the wave and the extratropical cyclone

Vorticity analysisyu

xv

indicates cyclonic vorticity related to the horizontal wind shear (u varies rapidly with y and v varies rapidly with x)

these maxima move eastward 9 hours later

Surface Low pressure system drops from 998 hPa at 00UTC to 978 hPa at 18UTC (bottom) to 968 at 00UTC (not shown) Nov 11=> 30hPa in 24h!!! 3x more rapid than the typical cyclone

500 hPa (thick black), 1000hPa height (thin black), 1000-500hPa thickness (dashed) Arrows: sense of the geostrophic wind

Bottom: from text book: Atmospheric Circulation Systems: Palmen and Newton (p. 326)

Amplification of the wave in the thickness field: advection of temperature by the windNote that the strengthening of the east-west gradient of temperature leads to the weakening of the North-south temperature gradient as the wave is growingSurface low aligned with the 500hPa trough: weakening of the horizontal temperature advection

In summary

• The geostrophic wind field evolves from a highly baroclinic pattern (high gradients and strong vertical wind shear) into a more equivalent varotropic pattern (less directional shear of the lower tropospheric geostrophic wind field in fully developed baroclinic waves

• Strong winds but weaker temperature gradients marks the end of the intensificaiton phase in the life cycle of the cyclone

Vertical velocity and 500hPa chartsRapid Intensification

Rising of the northward air in the region of warm advection sinking air in the region of southward cold advection to the rear of the cycloneRising of warm air and sinking of cold air is indicative of a conversion of potential energy into kinetic energy In baroclinic waves the potential energy is associated with the east-west temp gradients and the kinetic energy with the meridional wind component

• Note that the juxtaposition of this inward-spiraling rising and subsiding air (“yin-yang” pattern) influences the pattern of rain and clouds

Frontal Zones

• Narrow bands marked by sharp horizontal gradients and sometimes outright discontinuities in wind and temperature

• Frontogenesis: development of frontal zones (initiated by large-scale horizontal deformation fields

• Mesoscale circulations in the plane perpendicular to the fronts are instrumental in increasing temperature contrasts and organizing the distribution of precipitation into bands parallel to the fronts

Winds and pressure: cold frontBelow: sea level pressure, and surface winds

• Pronounced wind shift, winds exhibit strong westerly component. To the east southerly wind component are dominant. Isobars bend sharply along the front

• As a cold front pass in the NH there will be a shift in an anticyclonic sense (veering) from southerly to westerly

Winds and pressure: warm frontBelow: sea level pressure, and surface winds

• Is a more subtle feature • Shows signal of advection around the surface low• As a warm front pass in the NH there will be a shift in an anticyclonic sense (veering) from

southeasterly to southerly• Late stage there will be a junction of the two fronts: occluded front extends from the center

of the surface low to a triple point when it meets the warm and cold front• When an occluded front pass a station the surface wind veers from southeasterly to

southwesterly

Secondary cold front

• A front which forms behind a frontal cyclone and within a cold air mass, characterized by an appreciable horizontal temperature gradient

• Cause also winds to veer as it pass over a station

Temperature structure

• The zone of relatively uniform temperature to the southeast of the surface low is referred to as the warm sector of a cyclone

• The cold front marks the leading edge of the advancing colder air from the west

Close-up in the temperature and winds• To the east of the cold front the

temperatures are relatively homogeneous

• Proceeding westward from the front, temperatures drop by 10oC within the first few hundred of km

• A cold front can be defined as the boundary of a frontal zone that is advancing in the direction of the warmer air.

• A cold front at a station marks the beginning of a period of falling temperatures due to the wind shift

Note that both fronts are embedded within troughs of low passage and their passage is marked by wind shifts and further decrease in temperature.

The passage of a warm front is associated with a local increase in temperature

Stationary fronts• Fronts that exhibit little movement in either direction and are

indicated on synoptic charts as dashed lines with alternating read and blue line segments

• When an occluded front approach a station, surface air temperature rises, and after the front passes the station, the temperature drops (for a stationary observed it is like he/she was experiencing a back to back warm and cold fronts)

Occluded Front:

Cold air moves faster than the warm air

Frontal systems have a tridimensional feature

What is the relationships Among Frontal Boundaries, Troughs and Ridges, Vorticity and Low and High Pressure Systems at the

surface?

Northern Hemisphere

Cold

Warm

Ascending AirDescending air

Low pressure systems are observed east of the upper level TroughsHigh pressure systems are observed east of the upper level RidgesCold and warm fronts are related to low pressure systems at the surface. Frontal boundaries extend upward as we will see in the next slides

Negative vorticity

Positive vorticity

Vertical Cross Section:

Cold air: denser than warm air

As a cold Front moves toward a warmer region it lifts warmer air ahead of the front

If the atmosphere is unstable or conditionally unstable cumulonimbus clouds and heavy precipitation may occur

Div

Conv.

Cold

Vertical Cross Section:

As a warm Front moves toward a colder region warmer air is lifted and shallow clouds are formed

Div

Conv.

Difficulties to locate fronts on the basis of gradients of surface air temperatures

• Over Oceans: air temperature is strongly influenced by sea surface temperature

• In mountainous terrain, large differences in station elevations mask the temperature gradients

• Terrain effects, patchy nocturnal inversions, convective storms, urban heat islands effects can raise or lower the temperature and create gradients that can confuse the location of frontal systems

Moisture• Frontal zones tend to be marked by strong gradients in dew point

and equivalent potential temperature, specially when the cold air is of continental origin and the warmer air is of marine origin (e.g. eastern USA).

• During spring and summer, the moisture gradient is often a more reliable indicator of frontal positions

• During summer, under conditions of southerly low level flow, there often exists a sharp contrast between humid air advected northward from the Gulf of Mexico and much drier air that has subsided along the eastern slopes of the Rockies: the boundary between these marine and continental air masses is referred to as Dry Line

Example of Dry line observed in a Td map: http://www.tornadochaser.net/dryline.html

• Tornados can form along a dry line, in the presence of other triggering such as strong high level jets

• http://www.crh.noaa.gov/lsx/?n=ap0206qlcs

Vertical Cross Section:

Dry air is: denser than moist air

As dry air moves toward a region with moist air it lifts humid air ahead and may form intense storms

If the atmosphere is unstable or conditionally unstable cumulonimbus clouds and heavy precipitation may occur, including tornados

Div

Conv.

Hourly observations: cold frontColdFront

SecondaryFront

Gage,Oklahomarain

Hourly observations: warm front

Bowling Green, Kentucky

Hourly observations: occluded front

Marquette, Michigan

975hPa!

veering

rain snow

Surface weatherSnow rain

00:00UTC 09:00UTC 18:00UTC

Snow rain

00:00UTC 09:00UTC 18:00UTC

Snow rain

WV 00:00UTC WV 09:00UTCWV 18:00UTC

Precipitation as seen by radar

0620UTC

Composite: 15:35 UTC

Some additional ideas about the evolution of frontal systems, cyclones and baroclinic waves:

Surface high- and low-pressure centers can migraterelative to the Rossby wave aloft.

In (a), the surface low exists below the area of upper-level divergence, and the resultant uplift maintains or strengthens the cyclone.

Surface systems are generally guided by upper-level winds,so eventually the center of the low might be displaced to the

zone where upper-level convergence occurs (b).

The sinking air fills into the low, causing its demise.This process occurs as the cyclone undergoes the

transition from its mature to occluding stages.