84th AMS Annual Meeting

Monday, 12 January 2004: 4:15 PM
Analysis of the Texas Norther: Case Study
Room 605/606
Frank P. Colby Jr., University of Massachusetts, Lowell, MA
Poster PDF (527.8 kB)

Weather and Forecasting Abstract – ID: 67449    PW:  813602 Analysis of the Texas Norther: Case Study The Texas Norther, according to the Glossary of Meteorology (2001), is “... a cold air outbreak associated with the southward movement of a cold anticyclone ... [which comes as] ... a rushing blast and brings a sudden drop of temperature of as much as 25 oF in one hour...”  During the Spring of 2003, there were several Texas Northers in the U.S.  One of these cases is examined here using observations and mesoscale model output from both the Penn State/NCAR Mesoscale Model, version 5 (MM5) and the National Centers for Environmental Prediction (NCEP) Eta model.  On March 5, 2003, Whichita Falls, TX, recorded a  temperature change of 11 oC in one hour, and 24 oC over a time span of 6 hours, as the winds changed from southerly at 10 knots to northerly at 20 knots.  The NCEP Eta model 24 hour temperature forecasts for the area near Wichita Falls, TX were in error by more than 10 oC.  In Figs. 1-4 below, taken from a MM5 simulation initialized at 12 UTC, 04 March, winds and temperatures at about 25 meters, as well as sea-level pressure are shown both before and after the cold surge reached the area near Wichita Falls, TX.  The sharp edge of the temperature gradient is evident.  Also, notice how the winds blow straight down the pressure gradient, a feature typical of Texas Northers.  Observations and MM5 model output will be used to show the mesoscale details of the leading edge of the cold air, including the temperature structure, horizontal winds, and vertical motion.  In addition, the movement will be compared to that expected for a gust front, to determine if Texas Northers are driven mainly by density differences, or if they propagate as edge waves.  Schultz, et al., 1997, noted in an extraordinary case they examined from 1993, that the  “… cold surge had characteristics reminiscent of a Kelvin wave, a tipped-forward cold front, a pressure-jump line, a bore, and a gravity current, but none of these conceptual/dynamical models was fully applicable. “  By studying this case from 2003, we hope to gain a better understanding of the structure and dynamics of these cold-season events.  

References: Shultz, D.M., W. E. Bracken, L. F. Bosart, G. J. Hakim, M. A. Bedrick,

M. J. Dickinson, K. R. Tyle, 1997:  The 1993 superstorm cold surge: frontal structure, gap flow, and tropical impact.  Mon. Wea. Rev., 125, 5-38.
Figure 1.  Output from MM5 run valid at 21 UTC, 04 March 2003.  Sea-level pressure contoured at 2 hPa intervals.  Winds are at about 25 m above ground, with the scale shown at lower right (m/s). Location of Whichita Falls, TX is shown by circled cross. Figure 2.  As in Fig. 1, except 25 m temperatures contoured at 5 oC intervals are shown instead of sea-level pressure. Figure 3.  As in Fig. 1, except valid for 09 UTC, 05 March 2003. Figure 4.  As in Fig. 2, except valid at 09 UTC, 05 March 2003.

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