UPPER AIR DYNAMICS (continued) MSC 243 Lecture #8, 10/22/09

Ridges and Troughs Aloft Mountains and valleys of warm and cool air Mountains and valleys of warm and cool air The height of the pressure level depends on the temperature of the column of air below it The height of the pressure level depends on the temperature of the column of air below it 500 mb Very warm column Cool column Warm column Very cool column Surface 500 mb Ridge Trough Increasing Height 700 mb 850 mb

1. Loop “250 Spd”, then stop it at the initial time and describe the general pattern (troughs/ridges) over and just offshore of the USA. What is the highest wind speed on the map, and where is it? Recap of FCST 4 2. Does the 500 mb pattern look similar to the 250 mb pattern at the initial time? Now wind the two loops forward to 18Z Friday. Carefully identify the relative locations of the trough axes at each level around the Great Plains. Which one is further upstream?

TROUGH RIDGE TROUGH

TROUGHS

3. Where is the surface low located relative to the 500 mb low (and is it upstream or downstream)? If the 250 mb trough/low is upstream of the 500 mb trough/low, and the 500 mb trough is upstream of the surface low/trough, then conditions are favorable for the surface low to develop. Is this the case here? 4. Now wind forward the surface loop through to 06Z Sunday. Note down the minimum sea level pressure every 6 hours. Did this development support your argument in Question 3?

TROUGHS

5. Note down the location of maximum vorticity (red) associated with our weather system at 12Z Saturday. 6. Where is the vorticity maximum located with respect to the 500 mb trough axis at this time? Also, where is the vorticity maximum located with respect to the surface low (upstream / same position / downstream?) 7. What direction are the 500 mb winds travelling in, over the surface low? Are the winds bringing in low vorticity or high vorticity aloft over the surface low, i.e. is there advection of negative or positive vorticity (or none)?

998 mb

996 mb

995 mb

8. Now go to “850 Theta-e” and identify the locations of the cold front. How does the location of the front differ from that at the surface (2m Theta-e)? (This question is just to get an idea of the vertical structure of a front that we learned in class) 850 mb level?

What to diagnose at different pressure levels 850 mb: useful for estimating surface temperatures, low level moisture, and determining precipitation type (rain, snow, sleet).

700 mb chart The 700 mb chart is used to determine cloud cover or rainfall, using the relative humidity field and the vertical motion field.

700 mb chart The 700 mb chart is also used to determine short-wave disturbances via the geopotential height field.

500 mb geopotential height The 500 mb chart is the forecasters’ favorite for depicting the motion of weather systems. It shows the large-scale flow (long waves) and jet streams, and also the small- scale flow (short- waves, low level storm systems) TROUGH RIDGE

250 mb Chart The 250 mb chart is used to locate the jet stream. Strong upper- level winds help develop surface low pressure in mid-latitudes.

Thickness (yellow lines): what is it related to? 5400m contour = first approx for rain/snow border

Convergence and Divergence

(Horizontal) Convergence: more air is entering an area than leaving it on a pressure surface (Horizontal) Convergence: more air is entering an area than leaving it on a pressure surface (Horizontal) Divergence: more air is leaving an area than entering it on a pressure surface (Horizontal) Divergence: more air is leaving an area than entering it on a pressure surface Because mass is conserved, horizontal divergence relates directly to vertical motion Because mass is conserved, horizontal divergence relates directly to vertical motion

What can we tell from a 500 mb chart? Convergence upstream of trough axis. Winds coming together, height contours narrowing. Speed increases following the flow. Divergence downstream of trough axis. Winds spreading apart, height contours widening. Speed decreases following the flow. TROUGH “Upstream of trough axis “Downstream of trough axis

Divergence aloft is associated with rising motion and surface low pressure Divergence aloft is associated with rising motion and surface low pressure Convergence aloft is associated with sinking motion and surface high pressure Convergence aloft is associated with sinking motion and surface high pressure Surface pressure patterns are offset from troughs and ridges aloft in developing systems Surface pressure patterns are offset from troughs and ridges aloft in developing systems Surface 500 mb Trough Ridge Sinking Rising Convergence Divergence High Pressure Low Pressure High Pressure

Development of Surface Low Net convergence west (upstream) of an upper air trough and net divergence east (downstream) of an upper air trough. For a surface storm to intensify, the upper air trough must be located upstream of the surface low. Divergence aloft, convergence below = “good upper-level support” As the upper air low moves closer to being directly over the surface low, upper air divergence lessens and the surface low stops deepening (intensifying). The surface weather often improves once the 500 mb trough axis has passed.

Conditions for surface low (L) to develop

Vorticity Divergence is tricky! It is difficult to accurately measure divergence, and nearly impossible to use the observed horizontal winds to diagnose vertical motion. Can it be related to something else – yes it can! Vorticity is a measure of the rotation of a fluid around a local vertical axis. Vorticity is a measure of the rotation of a fluid around a local vertical axis. Earth's vorticity Earth's vorticity The local vertical component of spin due to the rotation of the earth The local vertical component of spin due to the rotation of the earth Depends on latitude (greatest at poles, zero at equator) Depends on latitude (greatest at poles, zero at equator) Earth's vorticity = 2 x rate of rotation x sin(latitude) Earth's vorticity = 2 x rate of rotation x sin(latitude) Relative vorticity Relative vorticity Vorticity generated by air motions relative to the earth Vorticity generated by air motions relative to the earth Counter-clockwise flow is positive vorticity (spin) Counter-clockwise flow is positive vorticity (spin) Clockwise flow is negative vorticity (spin) Clockwise flow is negative vorticity (spin)

Vorticity at 500 mb

Total (Absolute) Vorticity Absolute vorticity = Earth's vorticity + Relative vorticity Absolute vorticity = Earth's vorticity + Relative vorticity Absolute vorticity is always positive (at the large scale) in middle and high latitudes Absolute vorticity is always positive (at the large scale) in middle and high latitudes Meteorologists focus on vorticity at the 500 mb level to diagnose (and predict) vertical motion and the converging and diverging areas above and below 500 mb Meteorologists focus on vorticity at the 500 mb level to diagnose (and predict) vertical motion and the converging and diverging areas above and below 500 mb Absolute vorticity on the large scale changes relatively slowly with time, and so discrete structures can be reliably tracked, from observations and in models Absolute vorticity on the large scale changes relatively slowly with time, and so discrete structures can be reliably tracked, from observations and in models

Finding divergence from vorticity maps S imple relation to qualitatively diagnose divergence from vorticity maps. Divergence is associated with Vorticity Advection Positive vorticity advection (PVA) east of the 500 mb trof often corresponds to POSITIVE DIVERGENCE aloft Negative vorticity advection (NVA) west of the 500 mb trof often corresponds to NEGATIVE DIVERGENCE, i.e. CONVERGENCE

Vorticity and Vertical Motion Downstream of Positive Vorticity = Rising Motion Downstream of Negative Vorticity = Sinking Motion 500-mb heights and vorticity for January 7, 2002 (00Z Jan 8). Heights are in meters (solid) and vorticity is in 1x10 -5 s -1 (dashed). The “N” denotes a local vorticity minimum and the “X” denotes a local vorticity maximum. 500-mb heights and vorticity for January 22, 2002 (00Z Jan 23). Heights are in meters (solid) and vorticity is in 1x10 -5 s -1 (dashed). The “N” denotes a local vorticity minimum and the “X” denotes a local vorticity maximum m 5700 m 5580 m N X m 5520 m 5640 m 5760 m 5280 m X N Negative Vorticity Positive Vorticity