# Presentation: 425553

## Upper Level Analysis/Forecasting

## Why we look at the upper levels...

- Practically all weather we experience occurs in the troposphere.
- Data from levels other than the surface is needed to provide the most complete 3-D distributions of winds and temperature in the atmosphere.

## The atmosphere...

- We generally think of the atmosphere in terms of being gaseous.
- It reacts in many ways similar to that of a fluid.

## Standard Levels...

## 850 mb	

- Locate frontal positions
- Determine representiveness of surface winds
- Determine depth of moisture patterns in the winter
- Serve as surface charts in mountainous and plateau area where mean elevation is around 5000 ft.

## 700 mb

- Determine vertical extent and structure of fronts and pressure centers
- Analysis moisture patterns in the summer (moisture extends to greater height due to convective activity)
- Short waves are a predominate feature

## 500 mb	

- Primary features are warm highs and cold lows.
- Long waves are identified at this level, but short waves have lost their identity.
- It represents the mean state of the atmosphere.

## 300 mb

- Primary features are permanent and semi-permanent highs and low, certain dynamic lows, long waves, and polar jet stream in the winter.
- Primary uses to determine characteristics of long waves and analyzing/forecasting jet stream.

## 200 mb

- Uses are same as the 300 mb.
- Used to locate the polar front jet in the summer.
- In the winter, its principal use is estimating changes in temperature advection pattern in the stratosphere.

## The Jetstream

- A band or belt of strong winds of 50 kts or more with a westerly component.
- At times it is a continuous band, more often it is broken up into several discontinuous segments.
- Generally labeled as the polar front and subtropical jet (and the controversial Arctic or Polar Night jet).

## Polar Front Jet

- Associated with the principal frontal zones and cyclones of middle and sub-polar latitudes.
- The PFJ lies vertically above the max temperature gradient of the middle troposphere.
- The PFJ axis at 500 mb coincide with the -17 deg C isotherm (STJ with -11 deg C isotherm).

## Thermal Field around the Jet

- The core of the Jetstream is located directly above, or nearly so, the thermal concentration of the 500 mb surface.
- The jet core will live between 200 and 300 millibars directly above the strongest meridional temperature gradient at 500 mb.

## Jetstreams and Fronts

## Vertical cross section of Jetstream

## Slide 17

## PFJ and Surface Front Relationship

- The jetstream will be perpendicular to an occlusion and to cold fronts oriented north-south with no associated warm fronts.
- The jetstream will remain north of an unoccluded wave.
- The jetstream will be south of the low associated with an occluded front.
- In a series of lows of a cyclone family each low will be associated with a jetstream maximum, but every jet maximum is not necessarily associated with a low.

## Relationship of PFJ to surface lows/front

- Although not every jet max will have an associated low, each low embedded in the westerlies will be associated with a jet max.
- The jetstream should parallel the direction of the warm sector isobars of a surface low.
- The jetstream will roughly parallel the isobars around the southern periphery of a cold surface low.

## Relationship of PFJ to surface lows/front

## Contour-isotach pattern for shear analysis

-  No curvature of the    streamlines
-  Shear alone determines the relative vorticity
-  Shear downstream in region I and IV becomes less cyclonic
-  Shear downstream in region II and III becomes more cyclonic
-  Region I and IV favorable for deepening downstream.

## Contour-isotach pattern for shear analysis

-  In region I, both cyclonic shear and curvature decreases downstream thus highly favorable for deepening.
-    In region III, both cyclonic shear and curvature increase downstream thus unfavorable for deepening.
-    In region II the cyclonic curvature decreases downstream, but shear increases - situation determined by prominent term.
-    In region IV, the cyclonic curvature increase downstream, but cyclonic shear decreases - situation determined by prominent term.

## Contour-isotach pattern for shear analysis

-   Region I, cyclonic shear decreases downstream and the cyclonic curvature increases - situation determined by prominent term.
-   Region II has increasing cyclonic shear and curvature downstream and is quite unfavorable.
-   Region III, the shear becomes more cyclonic downstream and the curvature becomes less cyclonic - situation determined by prominent term.
-   Region IV, the shear and curvature becomes less cyclonic downstream and the region is favorable for deepening.

## Long and Short Waves

- Waves are classified according to their length, amplitude, and speed. Wavelength is the measured distance (in degrees longitude) between successive waves.  Measurement taken from trough to trough, ridge to ridge, or from any point on one wave to the same corresponding point on the next wave.  The amplitude is measured from the peak of the ridge to the base of the trough.  The longer the wave the slower they move.

## Isotherm-contour patterns

## Long Waves

- Barotropic/Vary between 50 to 120 deg longitude
- Outline by the movement of the short waves through the polar front jet
- They can be progressive, retrogress, or remain quasi-stationary
- Usually 3-7 in the atmosphere
- Moves an average of 1 deg per day

## Long wave trough contour/isotherm relationship (in phase)

- Contour and thermal trough same amplitude (barotropic), long wave remains quasi-stationary with no change in amplitude.
- Thermal trough amplitude greater than contour trough, long wave will progress eastward and fill (CAA ahead of trough and WAA begin).
- Thermal trough amplitude is less than contour trough, long wave will retrogress (1-2 deg) and deepen (due to WAA ahead of trough and CAA behind).

## Short Waves

- Baroclinic
- CAA into troughs; WAA into ridges
- Up to 10 major short waves in the northern hemisphere
- Moves an average of 8 deg per day in the summer and 12 deg per day in the winter
- Short waves are progressive--never retrogress
- Height rises to the rear of a short wave trough/advance of short wave ridge.
- Height falls in advance of short wave trough/to the rear of short wave ridges.

## Short wave trough contour/isotherm relationship

- Phase relationship tend to remain the same with time.
- Evaluate temp advection into a trough or ridge from the system axis to the upstream inflexion point.
- When isotherm/contour are 90 deg out of phase, short wave will move at 50 % of 500mb flow/70% of 700mb flow.
- When isotherm/contour are 180 deg out of phase, short wave will move rapidly - 70 to 100 % of 700mb flow. Trough will eventually outrun isotherm/contour relationship and fill rapidly.

## Guide to moving short waves

- Move with the long wave pattern.
- Short waves will intensify/deepen as the move in the long wave trough; fill as they move into long wave ridge.
- Large amplitude waves generally will move slower than minor short waves.
- The more out of phase the isotherm/contour relationship, the faster the short wave will move.

## Slide 31

## Effects of super/sub-gradient winds on troughs

- When strongest winds aloft are the North-westerlies on the western side of the trough, the trough deepens.

## Effects of super/sub-gradient winds on troughs

- When the strongest winds aloft are the south-westerlies between the trough and the down-stream ridge, the trough decrease in intensity.

## Effects of super/sub-gradient winds on troughs

- When the strongest winds aloft are the westerlies in the southern quadrant of the trough, the trough moves rapidly eastward and does not change in intensity.

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## The END

- Questions????