Midlatitude Weather Systems ATMS 301

The Large Scale Picture

A jet stream (or jet) is a narrow current of strong winds. Can exist at several levels, but most often applied to the high velocity winds in the vicinity of the midlatitude tropopause

Midlatitude Jet Stream Facts Can reach 250 mph (or more) Centered on the upper troposphere…around 250 hPa Stronger in winter. Generally, weakens and moves northward during summer. Closely associated with a tropospheric temperature gradient (thermal wind!) Not uniform zonally

Wasaburo Oishi, Japanese Discoverer (1920s)

Fu-Go Balloon Weapon During WWII

Strahlstromung (German) for “jet stream” first used in 1939 Big effects on bomber flights during WWII

B-29s flying westward to Japan

Jet Stream winds are not uniform

250 mb isotach

Shaded 70—110 knot

Jet Streams are NOT uniform

Main Upper Tropospheric Jets Closely Associated with the Tropopause

Tropopause

There are TWO types of upper tropospheric jet streams Polar front jet: associated with main midlatitude frontal/baroclinic zone. Typically 30-45N, ~250-300 hPa Subtropical jet: associated with the northern portion of the tropical Hadley circulation. Typically around 30N, ~200 hPa

Subtropical Jet Stream

Actually Two Jet Streams 10-16km 7-12km http://www.srh.noaa.gov/jetstream//global/jet.htm

Subtropical Jet Stream Often Associated with a cloud band

http://itg1. meteor. wisc http://itg1.meteor.wisc.edu/wxwise/AckermanKnox/chap7/subtropical_jet.html

Jet Stream’s Relationship to Temperature Is Expressed Through Thermal Wind Arguments

Thermal Wind Equation

250 mb isotach

1000-500 mb thickness-large thickness gradient and thus thermal wind near jet core

Or alternatively…

The Jet Stream Recall the horizontal temperature effects on the pressure: 500 mb 700 mb 850 mb Psurface Warm Cold

The Jet Stream Consider the balance of forces at each level: Co PGF 500 mb 700 mb 850 mb Ps Warm Cold

Air Masses

Maritime Tropical (mT) Source Region

Continental Polar (cP) Source Region

Continental Tropical (cT) Source Region

Maritime Polar (mP) Source Region

Bering Sea Cloud Streets (cP to mP)

An Important Midlatitude Weather Feature Fronts An Important Midlatitude Weather Feature

Definition You Should NEVER Forget A front is a boundary between relatively uniform warm air and a zone in which temperatures cools rapidly

Four Main Types of Fronts

Warm Front

Stationary Fronts

Occluded Front (a hybrid)

As a front passes there are changes in: Temperature Dew point Wind direction Pressure cloudiness

Fronts and Pressure Fronts are associated with troughs of low pressure

Fronts are associated with bands of clouds

Vertical Structure of Fronts

Cold Front Slope ~1:50, moves fast (20-30 mph), convection on leading edge

Warm Front Smaller slope (~1:200), slower (1—15 knots), more stratiform clouds

Stationary Front similar structure to warm front, but without movement

There is a typical progression of clouds as cold and warm fronts approach and pass by Cirrus Cirrostratus Altostratus Nimbostratus Cumulus after cold front

There is another type of front: the occluded front But to understand this this front, you need to learn about the life cycle of fronts and cyclones.

For much of the 20th century the dominant paradigm for cyclone/frontal evolution has been the Norwegian Cyclone Model (Bergen School) Bjerknes, 1919

Concept of Evolution of Cyclones Bjerknes and Solberg 1922

Stationary Polar Front Wave Forming on Polar Front

Wave Amplifies Occlusion as Cold Front Catches Up to Warm Front

Occlusion Lengthens and System Weakens

Warm and Cold Occlusions

In the real world, only the warm occlusion is observed

During the 1930s-1950s we learned the relationship between cyclones and fronts and upper level flow Upper troughs associated with surface lows. Usually lagging to the west. Upper ridges asociated with surface highs. Usually lagging to the west.

In front of Ridges associated with sinking, in front of troughs with rising motions

https://atmos.washington.edu/~ovens/wxloop.cgi?h500_slp+/-168//

What is the energy source of midlatitude cyclones?

The answer: warm air rising and cold air sinking

Warm (less dense) air rising and cold (more dense) air sinking lowers the center of gravity of the atmosphere Like dropping a weight. Potential energy (energy inherent in being aloft) is converted to kinetic energy (energy of moving air)

The conversion to kinetic energy is enhanced by having large differences of temperatures (large horizonal temperature gradients)

No accident that cyclones grow in regions of large temperature gradients