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By Edward I. Tollerud, Fernando Caracena, Adrian Marroquin, Steven
Koch, Jennie L. Moody, and Anthony Wimmers
IntroductionGOES water vapor images frequently show exceptionally dark filaments, ranging in size from short "dry slots" curling into cyclonic centers to equally narrow but very long regions extending longitudinally for many hundreds of kilometers (Figure 1). The implied midlevel dryness within these filaments suggests air with a history of subsidence. The largest of these longitudinal filaments indicate the presence of a jet stream immediately to the south, and recent studies show them to be collocated with similarly shaped filaments of enhanced potential vorticity ("PV streamers") on the cyclonic side of the jet. Occasionally there is dramatic convective development over the central U.S. within and toward the nose of the streamers or filaments. Figure 1 illustrates such a filament which persisted across the western U.S. for six days and spawned large mesoscale convective systems (MCSs) on four of those days. As the winds and the contours of pressure on the 2 PV-unit surface in Figure 2 demonstrate, the filament coincides with an undulating PV surface that is slightly subsided relative to its surroundings over the southern borders of Utah and Colorado in the western U.S. Three distinct regions of strongly lowered PV are shown to the east. The westernmost of these regions, over eastern Kansas, coincides with the western edge of a large MCS that produced heavy rainfall and flooding in the Kansas City area. The other two regions, one over southern Illinois and another extending from central Alabama into Georgia, are also associated with areas of convection. Both appear to be the result of residual midlevel PV from the previous day that drifted eastward over night and redeveloped during the next heating cycle. This particular case of serial MCSs and a long-lived streamer suggest that such filaments and their implied PV streamers may have prognostic value as an indicator of likely MCS development.
Figure 1. GOES-8 water vapor image. White indicates clouds; the gradient from yellow to blue passes from high water vapor content to low.
Figure 2. Atmospheric pressure (hPa) and winds on the 2PV-unit
The value of this particular kind of mesoscale "trigger" is of course dependent on the frequency with which it occurs. Thus, a first step toward determining forecast utility for dry filaments in water vapor imagery is at least a qualitative assessment of their ubiquity. To assess this, we follow water vapor imagery during several months of 2001 and present representative images of the exceptionally large or persistent filaments that also seemed related to significant MCS development. In addition to becoming a seasonal inventory of sorts, these cases also serve as a set of days for which future detailed analyses of the associated meteorological fields may help to explain the elusive physical mechanisms that relate the dry filament, the PV streamer, and subsequent MCS development. Studies of MCSs and FilamentsFrom April to July 2001, there were roughly six episodes of a persistent (two days or longer) continental-scale dry filament over the United States with associated MCS development. One of these episodes (15 – 20 July) persisted for six days. Images of several of these cases are displayed in Figures 3 – 5.The early-season (mid-April) example shown in Figure 3 extends essentially east-west across virtually the entire United States. Its orientation and the coinciding PV contours are reminiscent of the 1999 case discussed in the introduction and displayed in Figure 1.
Figure 3. As in Figure 1, except for 0000 UTC 16 April 2001. Contours of potential vorticity (PV units; K kg-1 s-1 10-6) are superposed. In contrast, the filament on 20 July (Figure 4) has a pronounced latitudinal component, following the upper-level flow around the persistent midcontinent ridge that developed during this period. This streamer produced MCSs at its terminus in Minnesota and southern Canada for several consecutive nights.
Figure 4. GOES-8 water vapor imagery from 1630 UTC 20 July 2001. The third example (Figure 5) from 22 April is displayed using an altered water vapor product developed by Jennie Moody, Tony Wimmers, and J. Davenport at the University of Virginia. The larger domain in the figure reveals that the filament is an extension of a much larger feature of the large-scale flow pattern. At the time of this observation, organized convection in the center of the thin filament is forming over Texas, Oklahoma and Kansas.
Figure 5. Mid-to-upper tropospheric specific humidity at 0000 UTC 22 April 2001 displayed using an altered water vapor product. The filament in the larger domain is an extension of a much larger feature of the large-scale flow pattern.
Tropical Storm AllisonAnother fascinating feature of water vapor and upper-level potential vorticity fields during 2001 is revealed in Figure 6. This GOES-8 water vapor imagery (0000 UTC of June 2001) shows that Tropical Storm Allison and, to a lesser extent, Barry were located downstream from regions of dry air with high PV. Although not as clearly a dry filament as the other examples, these regions were nonetheless prominent features during much of the lives of these storms. As with earlier studies of Hurricane Floyd, there is strong circumstantial evidence to suggest that the surprising longevity of and rainfall in these storms derived in part from the existence of these upstream regions. Confirmation of this hypothesis awaits closer analysis of the corresponding wind and thermodynamic fields.
Figure 6. GOES-8 water vapor imagery and 250 hPa potential vorticity (PV units; K kg-1 s-1 10-6) at 0000 UTC 7 June 2001. Potential vorticity contours are shown at .5 PV-unit intervals.
ConclusionsThis research shows the possibility that MCSs form preferentially within long, narrow filaments of dry, and PV-rich, subsiding air. The examples shown do not in themselves, however, offer a physical explanation for how this works. On the contrary, it might seem more intuitively likely that MCSs would form at the terminus of filaments (more often called plumes) of moist air. A clue to how the MCCs might have evolved in the dry air is provided by the Fort Worth radiosonde profile (Figure 7) denoted by a white asterisk within the dry filament of Figure 8 shortly before a convective system developed. Lifting of parcels into the nearly adiabatic layer between 450 and 550 hPa could have been the critical factor leading to strong convective development, particularly in this region of very favorable vertical wind shear.
Figure 7. Radiosonde observation at 0000 UTC 21 May 2001 at Fort Worth, Texas.
Figure 8. GOES-East water vapor imagery at 0115 UTC 21 May 2001. White asterisk indicates location of radiosonde ascent made at Fort Worth, Texas (see Figure 7). Among other mechanisms that may play a role in these cases of MCS development are dynamical processes involving jet interactions and potential vorticity (e.g., circulations driven by superposed upper-level and low-level jet streams). Another possibility is that the critical factor promoting MCS development is simply the dryness and location of the descending jet as it interacts with precipitation. Diagnostic analyses including potential vorticity budgets of these cases may help to resolve these possibilities. In any event, the cases inventoried here suggest the usefulness of dry filaments in water vapor imagery as precursors of MCS development. Note: More information and a complete list of references on this and related topics are available at the main FSL Website, www.fsl.noaa.gov, under "Publications" and click on "Research Articles." Drs. Jennie Moody and Anthony Wimmers are with the University of Virginia, Charlottesville, VA. (Dr. Ed Tollerud is a meteorologist in the Forecast Research Division, and can be reached at e-mail: Edward.Tollerud@noaa.gov.)
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