Vertical Structure of Extratopical cyclones

Leila M. V. Carvalho

Upper level charts

• The objective of these analyses is to identify how a baroclinic wave is observed in upper level charts and the relationships with the evolution of the extratropical cyclones.

• The discussion is based on the case study presented previously

00UTC: geopotential height (black) temperature (red). Note that contour interval varies 30m for 850-700hPa, 60m for 150hPa, 120m for250 and 200hPa, and 60m for

100hPa. Temperature 4oC left, 2oC right. Shading position of the jet stream

Warm advection

Warm advection

Cold Advection

Click in the figure to animate the vertical structure of the trough

Some important conclusions

• Stronger height gradients imply in stronger winds (winds are stronger in upper levels)

• Westerly winds increase with height: thermal wind equation indicates a prevailing meridional temperature gradient in lower levels, with colder air to the north (NH)

V(p2) – V(p1)Cold

Warm

Conclusions -2

• Patterns in lower levels are highly baroclinic and in upper levels tend to be more equivalent barotropic (why?)

• The horizontal advection of temperature within the frontal zone weakens with height as the wind vectors come into alignment with the isotherms.

• Distinct patterns of temperature in the lower stratosphere (250-100hPa): air in the troughs is warmer and air in the ridges tend to be cold.

• At 100hPa the only vestige of the baroclinic wave that remains is the weak trough over Western USA (in this case)

• The 850hPa isotherms tend to be concentrated within the frontal zone extending from the Great Plains eastward to the Atlantic seaboard and passing through the surface low

• To the east of the surface Low (L) southerly winds are advecting warm temperature.

• Note that the contours that pass through the frontal zone exhibit strong cyclonic curvature.

Surface 00:00UTC

L L

The Tropopause: what are the differences observed in each situation?

Colorado: Located near the center of the 250hPa trough

Iowa:Located near the center of the 250hPa ridge

Texas: Located close to the axis of the jet stream

Answers: Located near the center of the 250hPa trough: sharp discontinuity in the lapse rate around 350hPa with transition to more isothermal conditions above

Located near the center of the 250hPa ridge:Colder and sharp tropopause ~180hPa (12.5km)

Located close to the axis of the jet stream: more gradual decline in the lapse rate. Divergence near surface and convergence upper levels (adiabatic warming)

Frontal Soundings

Lies within the segment of the frontal zone to the south of the surface low

Mean wind

V2

V1

V2-V1

VT=V2+(-V1)

Cold

Cold

Conclusion: Backing of the wind in lower levels: strong cold advection

Frontal Soundings

Lies in an analogous position within the frontal zone to the east of the surface low

Mean wind

V2

V1

V2-V1

VT=V2+(-V1)

Cold

Cold

Conclusion: warm air advected by the southerly winds

Vertical cross sections

s

T

fp

R

pgn

VThe thermal wind equation in natural coordinates can

be written as:

What happens in regions where ?0~s

T

0

pgnV and Vgn should

be maximum!

Vertical cross-section wind speed (isotachs - blue) and temperature (red). Red Shading: relative humidity >80%. Blue shading (humidity < 20%). Heavy lines:

tropopause and surface-based fronts

Tropopause

Tropopause

Front

Vertical cross-section wind speed (isotachs - blue) and temperature (red)

Tropopause

Tropopause

Front

Regions where the flow is barotropic the isotherms are horizontal and δT/ δs = 0 resulting in δVg/ δp = 0 and

therefore the existence of a Jet (note how horizontal the

isotherms are near the Jet (J)

Near the tropopause, the vertical wind shear and the

horizontal temperature gradient undergo a sign

reversal at the same levelNote the discontinuity of the vertical spacing of the isotherms

Wind and potential temperature θ (red contours) and isotachs (blue) for 12UTC, Nov 10, 1998

• Note the high values of the vertical gradient of θ in the stratosphere in virtue of the vertical gradient of diabatic heat at those levels

• The static energy is defined as –δθ/δp and is very high in the stratosphere.

Interesting to know:

• Not that the stratospheric air can be drawn downward into the troposphere, near the jet.

• The frontal zones are associated with tropopause folds and are indicative of extrusions of stratospheric air with high ozone content and high static energy.

• Sometimes this process is irreversible (this air becomes incorporated in the troposphere

Isentropical potential vorticity

• Is a conservative tracer that serves as a marker for intrusions of stratospheric air into the troposphere in the vicinity of the jet stream

• When the layer of the stratospheric air is drawn downward, columns are stretched in the vertical, pulling the potential temperature surfaces apart, causing the static stability to decrease

• Conservation of potential vorticity requires that the vorticity of the air within the layer becomes more cyclonic as it is stretched in the vertical!

p

fg

PV Static energy

Air trajectories around cold and warm fronts: two frontal positions: lower one is for an earlier time when the configuration is of an open wave and the upper one is for a later time when the cyclone is in its mature stage and exhibits an occluded front. Cloud shield is indicated. Width of the arrows indicate different levels

• Note that as parcels move in these trajectories they may saturate or may become insaturated (when they move downward, for instance)

what can influence cyclogenesis?• Numerical models have indicated that the nature of cyclone

development is almost always “top-down”: it is initiated and subsequently influenced by dynamical processes in the UPPER TROPOSPHERE: important to monitor waves

• The region of cyclonic vorticity (and potential vorticity) advection downstream of a strong westerly jet is a favored site for cyclogenesis, specially if these feature passes over a preexisting region of strong low level baroclinicity (poleward edge of a warm ocean current, the ice edge, or a weakening frontal zone left behind by previous storms)

• The extrusions of stratospheric air, with its high potential vorticity, into frontal zones at the jet stream can increase the rate of intensification of cyclonic circulation in the lower troposphere

Observe the variability of cyclones

Influence of latent Heat

• Latent heat contributes to the intensification of extratropical cyclones

• Latent heat release occurs preferentially in warm, rising air masses

• Acts to maintain the horizontal temperature gradients within the storm increasing the supply of potential energy available for conversion to kinetic energy (essential mechanism for hurricanes and tropical cyclones)

Precipitation Patterns in Extratropical cyclones

• Precipitation in extratropical cyclones is often widespread but inhomogeneous in space and time

• It is concentrated within elongated mesoscale rain bands with areas ranging from 103 to 104 km2 life time of several

hours.

Precipitation and sea level pressure

Mesoscale systems with enhanced precipitation

Types of mesoscale rain bands frequently observed in association with mature extratropical cyclones. Green light precipitation, red heavy precipitation

The position of the cold front at the surface coincides with the leading edge of the narrow cold frontal rainband, and the frontal surface tilts upward toward the west with a slope comparable to that or the air trajectory

High liquid content

Ice particles

Strong updrafts

Orographic effects

M

MMΘo+δθ

ΘoWesterly flow

y

x

const

H

f )(

φo

φo +δφ -> f>fo- ζ <0 fo

ζ <0 to compensate decrease in H

v

ζ >0 – H returns to normal and f has decreased

1) Lee Cyclogenesis

Rossby wave propagation along sloping Terrain

Suppose there is already a low pressure system present along the lee slope.

The downslope wind will increase relative vorticity: more intense in the equatorward flank: propagation of the low exhibit an equatorward propagation

Upslope winds against the flow: note this upslope flow is associated with snow

Cold air Damming by a Barrier

Cold air

p + 2Δp

p + Δp

p

Incursion of cold air into lower latitudes

y

x

Rock

ies

Example for the Southern Hemisphere: “friagem” phenomenon (cold temperatures in southern Amazon)

Ande

s

H

Cold air

July 24 2009

Next classe you will learn about mountain related wind storms