A LOWER MESO- SCALE HEAVY SNOW BAND
Christopher C. Buonanno and Kevin W. Brown
NWSFO, Norman, Oklahoma
During the morning and afternoon hours of February 1, 1996, a narrow band of moderate to heavy snow fell across western and northern Oklahoma. Snowfall amounts of 10 to 20 cm, confined to a band approximately 60 km wide, stretched from extreme west central to extreme northeast Oklahoma (Fig. 1). Development of this snow event was captured well by local WSR-88D radars. Absent of any significant synoptic weather features, extra attention was given to mesoscale forcing mechanisms and real-time data analysis.
The goal of this study will be to emphasize the importance and necessity of mesoscale analysis during short-term forecasting. It will be shown that the primary mechanisms at work during this snow event were a direct thermal circulation (DTC) induced by frontogenetical forcing, maximized in mid-levels (around 700 mb) and the presence of conditional symmetric instability (CSI).
The synoptic situation
At 1200 UTC February 1, an arctic airmass was entrenched across the southern plains with the surface arctic front stretching from northern Mexico, through the Gulf of Mexico, into central Florida. Surface temperature readings between minus 9 and minus 5 degrees Celsius were common across north and central Oklahoma. At 850 mb, a strong temperature gradient existed across southern Oklahoma and northern Texas, but over central and northern Oklahoma a very weak gradient in temperature existed. However, at 700 mb, a weak temperature gradient was seen over southern Oklahoma and northern Texas, while a strong temperature gradient existed over central and northern Oklahoma (not shown). At 500 mb, moderate west to southwest flow dominated with an open wave over Baja California (Fig. 2). Two distinct upper-level jet maxima were also evident over the Ohio River Valley and on the west side of the Baja open wave (not shown).
During the day on February 1, a reinforcement of arctic air overspread the southern Plains as a 1044 mb high pressure system moved southward from southern Canada behind the passage of a mid-level shortwave over the central Plains. Although weak cold air advection did allow for surface temperatures to decrease 2 to 4 C across central and southern Oklahoma by 0000 UTC February 2, little change occurred with the temperature gradient at 850 mb. At 700 mb, however, the temperature gradient did tighten slightly across northern and central portions of Oklahoma. The Baja open wave moved eastward through 0000 UTC, sheared out, and became positively tilted across New Mexico and West Texas. Most of the positive vorticity advection with this feature was confined to northern Texas and extreme southern Oklahoma.
By 1400 UTC, mid-level echoes were detected by the Oklahoma City (KTLX) WSR-88D radar across west-central Oklahoma, but several calls to observers in the vicinity of the echoes yielded flurries. By 1600 UTC, higher 30 dBZ values were evident on KTLX across west central Oklahoma. These echoes were detected at approximately 3 km MSL, and were arranged in an east-west oriented band (Fig. 3). The Vance AFB (KVNX) WSR-88D in northwest Oklahoma also detected the 30 dBZ echoes extending into the northeast Texas Panhandle (not shown). By 2000 UTC, a band of even higher reflectivity (30-40 dBZ) was detected from west-central through north-central Oklahoma (Fig. 4), where moderate to heavy snow was reported. The location of this higher reflectivity correlates well with the largest snow amounts reported. By 2300 UTC on February 1, the organized band of higher reflectivities had moved into extreme northern and northeastern Oklahoma, with the rest of western and central Oklahoma tapering off to snow flurries (Fig. 5).
Forcing mechanisms--vertical motion
An investigation into forcing mechanisms responsible for the vertical motion necessary to produce the heavy snow leads to an analysis of frontogenetical forcing. The process of frontogenesis (or strengthening of a thermal gradient) acts to create a thermally driven, direct transverse vertical circulation. This vertical circulation created by ageostrophic forces acts to restore the thermal wind balance which frontogenesis acts to destroy (Bluestein 1993, Holton 1992).
Upper level frontogenesis was investigated by using a two dimensional adiabatic frontogenesis equation (after Miller 1948) in pressure coordinates (after Boyle and Bosart 1986), which included contributions due to horizontal deformation, as well as differential vertical motions. Also, a two dimensional horizontal (neglecting a twisting term) adiabatic frontogenetic function (Petterssen 1956) was used.
Significant frontogenesis occurred in the lower and middle troposphere along the arctic boundary across parts of the central and southern plains by 1200 UTC February 1 (Fig. 6). Frontogenesis became maximized in mid-levels across northern and central sections of Oklahoma by afternoon (Fig. 7). By analyzing different terms of the (Miller 1948) frontogenesis equation, a large part of this frontogenetical forcing (and strengthening of the thermal gradient) was due to horizontal deformation acting on the thermal gradient. The region of maximum frontogenesis shifted to portions of eastern Oklahoma and Arkansas by 0000 UTC on February 2 (not shown).
Cross-section analyses, created normal to the frontal zone, reveal insight into the direct thermal vertical circulation created by frontogenetic forcing. Evidence of a developing direct thermal circulation (DTC) was found by 1200 UTC February 1, as shown by Fig. 8 (developing DTC noted by "D" in the figure). The circulation continued to develop during the day, and a well defined upward branch of the DTC was created (Fig. 9). The DTC still was evident after 0000 UTC, February 2, but had dissipated six hours later (not shown).
Instability analysis--conditional symmetric in stability
Convective stability analyses (not shown) revealed a convectively stable environment across northern Oklahoma on February 1, with surface based lifted indices of up to 16 C. The presence of the strong frontogenesis and resulting direct thermal circulation leads to an analysis of conditional symmetric instability (CSI). A parcel in areas of CSI are stable to horizontal or vertical motions, are but are unstable to slantwise displacements. Conditions necessary for the creation of regions or layers of CSI include a well mixed, nearly saturated layer, high baroclincity, and low static stability (Bluestein 1993, Moore and Blakely 1988, Snook 1992). Previous studies (including Emanual 1985, Shields et al. 1991) have shown that regions of conditional symmetric instability, or regions of slightly positive conditional symmetric stability, can intensify and focus the upward branch of a direct thermal circulation induced by frontogenetical forcing.
Using a technique presented by Snook (1992), cross section analyses were completed to help diagnose regions of CSI. In regions of CSI, slopes of absolute angular momentum (Mg) are less than the slopes of equivalent potential temperature, and these conditions occur in an area of near saturation. (Near saturation is defined as relative humidities greater than 80%, per Bennetts and Sharp 1982, and Moore and Lambert 1993.) The analyses (Fig. 10) reveal a region of CSI or slight moist symmetric stability, coinciding with the upward branch of the DTC. This region of instability enhanced and focused the vertical motions created by the upward branch of the DTC.
A narrow band of moderate to heavy snowfall occurred on February 1, 1996, across a section of northern and western Oklahoma. This intense snowfall occurred in a synoptically tranquil environment. However, an analysis of mesoscale factors revealed that strong vertical motions were created in the upward branch of a direct thermal circulation. This circulation was induced by strong frontogenetical forcing that was maximized in mid-levels of the troposphere. Furthermore, the strong vertical motions were enhanced and focused by a region of conditional symmetric instability. This event highlights the usefulness of higher resolution gridded data to display complex meteorological data in a short fuse operational setting.
The authors would like to thank David Andra (SOO) and Dennis McCarthy (MIC) at NWSFO Norman for their reviews of this paper.
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