Lecture7_101311 - UPPER AIR DYNAMICS UPPER MSC 243 Lecture...

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Unformatted text preview: UPPER AIR DYNAMICS UPPER MSC 243 Lecture #7, 10/13/11 Forecast Discussions Forecast Main ingredients Local geography (brief) Climatology (brief) Broad synopsis of main weather systems currently Broad over the USA, and how they are forecast to evolve over Next day: GFS MOS numbers, NAM MOS Next numbers, USL numbers, NWS numbers numbers, Based on these numbers and also the surface and Based upper-air model maps (look at GFS and NAM/WRF), give reasoning and ballpark ranges for high, low, winds and precipitation amount. for Low pressure and frontal system in Midwest early next week in How are the upper air dynamics affecting this system? Ridges and Troughs Aloft Ridges Mountains and valleys of warm and cool air The height of the pressure level depends on the The temperature of the column of air below it temperature Increasing Height Ridge Ridge 500 mb 500 mb Trough Trough 700 mb 850 mb Surface Very warm column Cool column Warm column Very cool column Convergence and Divergence Convergence Convergence and Divergence Convergence (Horizontal) Convergence: more air is entering (Horizontal) an area than leaving it on a pressure surface an (Horizontal) Divergence: more air is leaving an (Horizontal) area than entering it on a pressure surface area Because mass is conserved, horizontal Because divergence relates directly to vertical motion divergence What can we tell from a 500 mb chart? What Convergence upstream of trough axis. Winds coming together, height contours narrowing. Speed increases following the flow. TROUGH Divergence downstream of trough axis. Winds spreading apart, height contours widening. Speed decreases following the flow. How about the surface low beneath? How There is a developing surface low (1002 mb) moving eastwards. How does the general flow aloft and the divergence aloft affect the motion and development of the surface low? How about the surface low beneath? 6 hours later … The surface low moved eastward (since the whole system is carried eastward through the upper air). The surface low also strengthened to 997 mb and is exhibiting frontal characteristics. How about the surface low beneath? 6 hours later … The surface low moved eastward (since the whole system is carried eastward through the upper air). The surface low also strengthened to 997 mb and is exhibiting frontal characteristics. Why did this low strengthen? Divergence aloft is associated with rising motion and surface Divergence low pressure low Convergence aloft is associated with sinking motion and Convergence surface high pressure surface Surface pressure patterns are offset from troughs and ridges Surface offset aloft in developing systems aloft 500 mb Ridge Convergence Divergence Convergence Trough Surface Sinking Rising Sinking High Pressure Low Pressure High Pressure Development of Surface Low Development 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 develop NVA/ PVA/ Vorticity 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. around Earth's vorticity The local vertical component of spin due to the rotation of the earth Depends on latitude (greatest at poles, zero at equator) Earth's vorticity = 2 x rate of rotation x sin(latitude) Earth's Relative vorticity Vorticity generated by air motions relative to the earth Counter-clockwise flow is positive vorticity (spin) Clockwise flow is negative vorticity (spin) Shear vorticity and curvature vorticity. Total (Absolute) Vorticity Total Absolute vorticity = Earth's vorticity + Relative vorticity Absolute vorticity is always positive (at the large scale) Absolute in middle and high latitudes in Meteorologists focus on vorticity at the 500 mb level to Meteorologists diagnose (and predict) vertical motion and the converging and diverging areas above and below 500 mb converging Absolute vorticity on the large scale changes relatively Absolute slowly with time, and so discrete structures can be reliably tracked, from observations and in models reliably Finding divergence from vorticity maps Finding Simple relation to qualitatively diagnose divergence imple from vorticity maps. from Divergence is associated with Vorticity Advection Divergence Vorticity Positive vorticity advection (PVA) east of the 500 mb trof Positive often corresponds to POSITIVE DIVERGENCE aloft often Negative vorticity advection (NVA) west of the 500 mb Negative trof often corresponds to NEGATIVE DIVERGENCE, i.e. CONVERGENCE i.e. Case Study: Storm 10/26/10 Case http://www.crh.noaa.gov/images/mpx/sfclowanimation.gif U.S. National Weather Service New record set today for the lowest pressure in a non-tropical storm in the mainland U.S. The massive storm system barreling across the central U.S. had a minimum central pressure of 28.24" or 956 mb (equivalent to the minimum pressure of a Category 3 hurricane). This breaks the old record of 28.28" (958 mb), set on Jan. 26, 1978, during the Blizzard of 1978 (aka the Cleveland Superbomb). This is also lower than the March 1993 Superstorm (aka "The Storm of the Century"), or the "Witch of November" storm that sank the Edmund Fitzgerald in 1975, or even the Columbus Day Storm of Oct. 1962. 1975, Conditions for surface low (L) to develop develop NVA/ PVA/ ...
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This note was uploaded on 01/08/2012 for the course MSC 243 taught by Professor Majumdar,s during the Fall '08 term at University of Miami.

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