Introduction
The 1993 Storm of the Century broke low pressure records and brought large amounts of snowfall and rainfall, as well as high winds, tornadoes and thunderstorms, across the East coast of America. Surface winds speeds reached 144mph in Mount Washington and 60 inches of snowfall was recorded over Mount LeConte. The cost of damage from these severe weather conditions is estimated around $6-10 billion and the storm was responsible for more than 300 deaths.
This case study examines some of the dynamical mechanisms that led to the storm’s explosive cyclogenesis, aided by figures 1-4, developed using GEMPAK.
Fig.1. Analysis at 1200 UTC 12 March 1993. (a) Mean sea level pressure is labelled in hPa and contoured every 4hPa. (b) The 300hPa geopotential height (solid orange lines) is labelled in metres and contoured every 120m. The 300hPa wind speeds is shaded at intervals of 15ms-1. The jet streaks are identified as “A” and “B”. The 300hPa divergence (solid blue lines) is labelled in 10-5s-1 at intervals of 2x10-5s-1. (c) The 300-700hPa thickness (solid pink lines) is labelled in metres and contoured every 60m. The 500hPa absolute vorticity is shaded at intervals of 10x10-5s-1. The 500hPa positive vorticity advection by the thermal wind (solid purple lines) is labelled in 10-8mkg-1 at intervals of 3x10-5mkg-1. (d) The 500hPa geopotential height (solid orange lines) is labelled in metres and contoured every 60m. Omega is shaded at intervals of 10x10-3cPa s-1, with the shading corresponding to upward vertical motion.
Fig.2. As in Fig.1 but for 0000 UTC 13 March 1993.
Fig.3. As in Fig.1 but for 1200 UTC 13 March 1993.
Fig.4. As in Fig.1 but for 0000 UTC 14 March 1993.
Possible storm deepening mechanisms
a) Effect of upper-level jet streak-induced circulations
Throughout the time period of 1200 UTC 12 March to 0000 UTC 14 March, the location of the low pressure system, in comparison to the location of the upper-level jet streaks A and B (noted on figures 1-4), was favourable for cyclogenesis.
At 1200 UTC 12 March, the low pressure system is located in the left jet exit region of jet streak B, particularly for the north-western side of the low pressure system. Our knowledge of the direction of the ageostrophic wind vector in the exit region of a jet streak, in association with the rapid deceleration of the geostrophic winds, tells us that there will be upper-level divergence of the air in the left jet exit region. The upper-level divergence provides a forcing for upward vertical motion throughout the column, by the mass continuity equation. Negative omega at 500hPa, a variable representative of upward vertical motion throughout the tropospheric column, is plotted in Fig.1d. The location of the large values of negative omega off the southeast coast of Texas relates well with the large upper-level divergence present in Fig.1b. In turn, the low pressure system experiences a reduction in its surface pressure and hence indicates that jet streak B is providing a cyclogenetic forcing at this time. The effect of this forcing can be observed on the plot of MSLP 12 hours later at 0000 UTC 13 March (Fig.2a). The circulation of the low pressure system has strengthened, as seen by the tightening of the isobars. The circulation has strengthened most notably in the northwest quadrant of the system, which corresponds to the location of the strongest forcing for ascent at 1200 UTC 12 March, so it is reasonable that this is where the low pressure system notably deepened.
At 0000 UTC 13 March, the cyclone is now located in the right jet entrance region of jet streak A (Fig.2b). This is another favourable position for upper-level divergence of the ageostrophic winds, due to the rapid acceleration of the geostrophic winds in the entrance region of the jet streak. Upper-level divergence suggests there is low-level convergence and hence upward vertical motion. Comparison of Fig.2d and Fig.2b shows that the strongest ascent at 500hPa is directly in the region of strongest upper-level divergence at 300hPa. This sets up a thermally direct ageostrophic circulation in the entrance region. The reduction of mass in the column that results from the ascent is favourable for continued deepening of the cyclone. The significantly greater strength and stubbiness of jet streak A at 0000 UTC 13 March compared to jet streak B at 1200 UTC 12 March - and hence much more rapid changes in speed in the respective jet entrance and exit regions - provides much larger values of upper-level divergence close to the cyclone at this time compared to the divergence values present 12 hours before. As a result, the upward vertical motion is much stronger at this later time, and so the deepening that occurs in the 12 hours after this is much more explosive. This explosive deepening causes the MSLP to drop from 992 hPa to 976 hPa at 1200 UTC 13 March (Fig.3a). The isobars are now much tighter around the cyclone – the cyclone has a much greater intensity.
Jet streak A remains extremely stubby through 1200 UTC 13 March, with great acceleration of the geostrophic winds in its entrance region, and so a thermally direct ageostrophic circulation remains. The upper-level divergence of the ageostrophic winds in the right entrance region of jet streak A is over a much greater area (Fig.3b) at this time and the surface cyclone centre is located just to the south of the right jet entrance region. However, jet streak B has now combined with jet streak A to a certain extent, and so the effect of jet streak A is not the only upper-level feature to be considered. The surface cyclone is also located in close proximity to the left exit region of jet streak B, another favourable position for upper-level ageostrophic divergence. Furthermore, the surface cyclone is downstream of an upper-level jet stream trough. This is yet another desirable location for upper-level divergence. The positioning of the surface cyclone among these three different factors results in the total upper-level divergence remaining very large over the surface cyclone. Once again, this upper-level divergence corresponds to location of large negative omega (Fig.3d). The removal of mass via upward vertical motion causes a strong deepening of the cyclone, as observed 12 hours later at 0000 UTC 14 March (Fig.4a) by the further tightening of the isobars.
b) Effect of midlevel short-wave troughs
Investigation the vorticity advection by the thermal wind around the location of the low pressure system can give insight into the rapid cyclogenesis. Regions of positive vorticity advection (PVA) by the thermal wind are directly associated with upward vertical motion, as seen using the Trenberth form of the quasigeostrophic omega equation.
At 1200 UTC 12 March, the low pressure system is located at the base of a mid-level short-wave trough, where a maximum in absolute vorticity is positioned (Fig.1c). The PVA downstream of the thermal wind is over the eastern side of the low pressure system, and so the PVA may be providing some degree of cyclogenetic forcing for the low pressure system. By comparing the location of the PVA by the thermal wind with the area of large, negative omega in Fig.1d, it is evident that they are co-located. Therefore, the PVA is likely providing a forcing for ascent at this time. Ascent is favourable for cyclonic deepening, and so this region of PVA by the thermal wind most probably caused some of the storm’s development in the following 12 hours, to 992hPa at 0000 UTC 13 March.
At 0000 UTC 13 March, there is a region of large PVA by the thermal wind over the northern side of the low pressure centre (Fig.2c), over the south coast of New Orleans. 12 hours later the surface cyclone has deepened from 992 hPa to 976 hPa (Fig.3a), as expected for strong ascent. Most notably, the circulation in the northern quadrant of the cyclone has strengthened, as seen by the tightening of the isobars 1200 UTC 13 March. This part of the cyclone is where the greatest forcing for ascent was located at 0000 UTC 13 March, so it is reasonable that this is where the cyclone notably deepened.
At 1200 UTC 13 March, there is now an area of strong PVA by the thermal wind located over the southern side of the cyclone centre and there is a corresponding region of negative omega to the south of the surface cyclone in Fig.3d.
c) Other cyclogenetic effects
Latent heat release may have played an important role in the deepening of the cyclone. It is likely that the warming produced by condensation processes will have provided a large forcing for cyclogenesis. The convectively unstable pre-storm environment would also have had a large role in the development of the storm. These effects are not analysed here.
Conclusions
The amplification and evolution of the 1993 superstorm was partly driven by two midlevel short-wave troughs merging and then interacting with a thermally direct ageostropic circulation in the entrance region of an upper level jet streak.
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