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1. INTRODUCTION

Thunderstorms are frequently organized in lines or clusters much larger than the individual storms making up the group. These organized areas of convection are commonly referred to as mesoscale convective systems, or MCSs. Although the term "MCS" is sometimes reserved for groupings which satisfy certain spatial or temporal criteria (e.g., see Houze 1993), it may also be applied more generally to refer to any meso-alpha or meso-beta scale (Orlanski 1975) area of organized, moist convection. In this broader sense, MCSs include not only such "classic" systems as mesoscale convective complexes (Maddox 1980), mid latitude squall lines and tropical cloud clusters, but also a wide range of other systems including cool season convective plumes over the Great Lakes (Peace and Sykes 1966, Niziol 1987), self-propagating warm season thunderstorm clusters over the southeastern United States, and narrow cold frontal rainbands in mid latitude cyclones (Hobbs 1978, Houze and Hobbs 1982).

Because MCSs are associated with a disproportionate share of significant convective weather, and because the details of their evolution are often poorly handled by the numerical forecast models, correctly anticipating MCS development and motion remains one of the most important tasks facing operational meteorologists today. The challenge does not, however, end there. Once a system has formed, forecast success depends upon correctly anticipating the predominant organizational mode or modes that a convective system will assume during its lifetime. For example, a given kinematic environment might be supportive of both back-building squall line (Bluestein and Jain 1985) and forward-propagating MCS (e.g., bow echo) development. The vastly different sensible weather hazards posed by such disparate convective systems make it imperative that forecasters obtain not only the ability to anticipate MCS formation and movement, but also the skill to correctly identify the favored convective mode or modes that will be assumed during the life of an event.

2. PREVIOUS WORK

Recent efforts have significantly improved our ability to anticipate the development of "classic" mid latitude MCSs (see for example, Augustine and Caracena 1994). Problems remain, however, in forecasting the initiation of other important types of MCSs. In particular, while numerous studies have documented the observed evolution and radar characteristics of MCSs containing long-lived bow echoes and derechos (e.g., Fujita and Wakimoto 1981, Johns and Hirt 1987, Duke and Rogash 1992, Przybylinski 1995), and numerical studies have examined the mesoscale airflow in these systems (e.g., Weisman 1992, 1993), little if any literature is available on forecasting the onset of derecho development.

Similarly, mixed success has been realized in forecasting MCS movement. Merritt and Fritsch (1984), in a study examining the movement of more than 100 MCSs (most of which were MCCs), were amongst the first to recognize that while no true "steering level" exists for MCC motion, most systems move approximately parallel to the contours of the 1000-500 mb thickness. They also noted that the speed of MCC motion was in part modulated by the location of the area of maximum low-level moisture convergence relative to existing convection. However, they also noted that many systems did not move along lines of constant thickness, and that some moved downshear along the contours while others inexplicably moved upshear ("back-built") along them.

Corfidi et al. (1996) extended and generalized Merritt and Fritsch's work, showing that the propagation component of MCS motion in many cases may be estimated by the speed and direction of the low-level jet. This is a rather surprising finding given that storm propagation can be influenced a myriad of other factors such as the distribution of convective available potential energy (CAPE), convective inhibition, gravity waves, outflow boundaries and orographic effects (e.g., Juying and Scofield 1989, Moore et al. 1993). Based on this finding, Corfidi et al. developed a simple procedure for the predicting the short term (6-12 hour) motion of the meso-alpha scale cores of MCSs. In this scheme, MCS core motion is taken to be the vector sum of (1) a vector representing cell advection by the mean cloud-layer wind and (2) a vector representing storm propagation --- i.e., new cell development --- directed anti-parallel to the low-level jet (See Figure 1).

Because the "vector approach" requires knowledge of only the low-level jet and mean cloud-layer wind, and because the technique may be applied in any type of environmental wind regime (systems are not constrained to follow a given thickness contour), it is ideal for operational use. The vector approach is also of value in identifying those kinematic situations conducive to the development of quasi-stationary and back-building MCSs. The former occur when cell advection is completely offset by cell propagation, while the latter develop when propagation exceeds advection (Figure 2). Both types of systems are frequently associated with excessive rainfall and flash flooding (Chappell 1986).

Useful as the vector approach is, the assumption that new cell development (and, therefore, system propagation), necessarily occurs most rapidly in the direction of the low-level jet is not universally valid. The assumption is of course true in most "classic" warm-season MCC and MCS situations over the central United States, where boundary layer convergence is typically maximized in the direction of the low-level jet. The premise was also valid in the vast majority of the cases studied by Moore at al. (1993) and Corfidi et al. (1996). However, as will be shown in the following example, there is an important class of MCS for which the direction of the strongest low-level convergence is not necessarily the same as that of the low-level jet.

3. AN EXAMPLE

Figure 3 shows a proximity thermodynamic sounding and wind profile associated with the incipient stage of warm-season MCS that developed over northwest Ohio and subsequently moved east southeast across northeast Ohio and northern Pennsylvania. Using a mean wind vector of 260/35 and a low level jet of 250/32 (the strongest wind in the lowest 1.5 km), application of the vector approach yields a system movement toward the east southeast at approximately 5 kts. As Figure 4a shows, one would expect cell advection to be almost totally offset by cell propagation --- a kinematic setup conducive to excessive rainfall and flash flooding.

Unlike most flash-flood producing convective systems, however, this MCS was poised to move through a comparatively dry low- to mid-tropospheric environment (Figure 3) . Such an environment is favorable for the development of strong convective downdrafts and, therefore, the formation of a well-defined surface mesohigh (Rotunno et al. 1988). A mesohigh did indeed form beneath the mesosystem soon after the first convection developed (not shown). Because the associated downdrafts necessarily brought westerly mid-tropospheric momentum to the surface, the high and its associated gust front moved eastward. And, because of the presence of strong convective-scale downdrafts, the gust front moved east at a rate faster than that of the mean wind. Consequently, system-relative convergence was maximized on the downshear (eastern) side of the MCS, rather than in the direction of the low-level jet. Forced ascent led to new cell development along the gust front, and system propagation was therefore to the east. Because cell advection and cell propagation were nearly directly additive, the MCS did not remain nearly stationary, but rather became a forward-propagator --- i.e., a bow echo squall line (Figure 4b). The system raced east at a speed faster than that of the mean wind, and produced a swath of significant wind damage from Toledo, OH to New York City (Storm Data 1997).

4. A REVISED "VECTOR APPROACH" FOR PREDICTING MCS MODE AND MOTION

The above example illustrates the need to consider the direction of greatest system-relative convergence when applying a technique like the vector approach. Blind use of the 850 mb flow (or some other wind maximum in the lowest 1.5 km above ground level (Bonner 1968)) will yield erroneous results when low-level convergence is forced to occur in some other direction because of the development of storm outflow. In short, more accurately stated, MCS propagation does not necessarily occur most rapidly in the direction of the low-level jet but rather in the direction of the greatest system-relative low-level convergence. This direction may or may not be the same as that of the low-level jet.

But how can one anticipate those situations when storm propagation will occur in a direction other than that of the low-level jet, or more specifically, when bow echoes and derechos are favored instead of back-building MCSs? As Chappell (1986) has noted, some environments are kinematically supportive of both quasi-stationary MCSs and fast-moving squall-lines. Both MCS modes are associated with strong mean flow and relatively weak, unidirectional cloud-layer shear. This suggests that some aspect of the thermodynamic environment plays a major role in determining the predominant mode of convective development.

Indeed, a preliminary examination of proximity soundings from nearly 30 MCSs observed between 1980 and 1998 reveals that a characteristic common to those unidirectional shear environments which produce bow echoes and/or derechos is the presence of unsaturated air --- either at the mid levels or in the sub-cloud layer --- ahead of the developing convective system. Conversely, quasi-stationary and back-building MCSs were found to be associated with more nearly saturated lower tropospheric environments. This finding implies that it is the potential to produce strong downdrafts at the surface --- and, therefore, a strong mesohigh --- that distinguishes the bow echo/derecho environment from that more conducive to the development of back-building or quasistationary convection.

That dry air should be found to be related to the occurrence of derechos and bows is, of course, not surprising. Johns et al. (1990) noted the presence of large dew point depressions at 700 and 500 mb in the vicinity of long-lived derechos. The ingestion of dry air from the pre-storm environment is known to play an important role in the formation and maintenance of surface mesohighs in developing MCSs (Rotunno et al. 1988). Dry air at mid levels is also essential to the formation of strong rear inflow jets (Smull and Houze 1987). Model simulations have shown that rear inflow jets are especially important in fostering mesohigh development in systems which produce large stratiform precipitation areas (Houze 1993).

In the mid latitudes, where westerly momentum typically increases with height, the presence of dry air means that new convective development will necessarily be forced in the downshear ("forward") direction, where the mesohigh's gust front lifts parcels to the level of free convection (as in the case discussed in Section 3). In contrast, mesohigh development is weak or nearly absent in more saturated environments. As a result, the downshear gust front movement is minimal and low-level convergence will remain focused on the upshear ("back") side of a developing mesosystem. This idea is consistent with Maddox et al. (1979), Juying and Scofield (1989) and others who found that quasistationary MCSs typically occur within axes of maximum deep-layer theta-e or precipitable water.

Some of the cases used in the present study suggest that bow echoes and derechos can also be initiated by mesohigh development associated with the presence of dry air in the sub-cloud layer. This mechanism appears to occur most frequently in arid regimes (e.g., northern Utah, 31 May 1994), but has also been observed elsewhere (e.g., eastern Pennsylvania, 20 November 1989 and 4 April 1995). The systems which form in these environments typically display weak radar reflectivities but can produce very devastating winds (R. H. Johns, personal communication).

5. SUMMARY AND CONCLUDING REMARKS

An improved technique for estimating 6-12 hour MCS motion that builds on the work of Corfidi et al. (1986) has been presented. It has been shown that the preferred direction for new cell development relative to existing activity (that is, the direction of system propagation), is not necessarily toward the low-level jet, but rather in the direction of the greatest system-relative convergence. For typical mid-latitude bow echo and derecho situations, system-relative convergence occurs on the downshear or forward side of the convective system. Because the advective and propagation components of system motion are additive, MCSs of this type can move downshear at speeds greater than that of the mean wind.

The strong unidirectional kinematic fields associated with bow echoes and derechos are, however, also supportive of quasi-stationary and back-building convective systems. The presence of unsaturated air, either at the mid-levels or in the sub-cloud layer, appears to distinguish those environments supportive of bow echoes and derechos from those more conducive to the development of quasi-stationary or back-building convection. Since dry air is necessary to produce a surface mesohigh, this finding is consistent with Evans (1998), who found that long-lived bow echoes are associated with comparatively strong system-relative flow near the surface. It also suggests that the various forms of convective systems found in environments of unidirectional shear represent, in fact, a spectrum of MCS modes. The particular mode that a given system assumes is seen to be governed not only by the degree of shear present, but also by the potential of the system to produce convective-scale downdrafts at the surface.

Although the vector approach is generally reserved for use in situations involving large summertime MCSs over the central and eastern United States, the concept is also useful in determining the behavior and motion of other types of convective systems. For example, lake-effect snow plumes, cool season convective trains (Reynolds 1998), and narrow cold-frontal rain bands may now be viewed as regional and/or seasonal variations of a common kinematic and thermodynamic theme: weak, unidirectional cloud-layer shear in a nearly saturated environment. Likewise, the unusual propagation characteristics of thunderstorm clusters over the southwestern deserts (e.g., McCollum et al. 1995) can be more easily understood when system-relative convergence is considered in relation to the potential for downdraft development in an arid environment.

One important factor which remains to be considered is the influence of supercell convection on MCS motion and development. As Schmidt and Cotton (1989) have shown, the presence of a supercell can drastically alter the evolution of the convective system of which it is a member. In some cases, bow echo MCSs appear to be initiated by supercells (e.g., the Texas derecho of 4 May 1989 (Smith 1990) and 17 August 1994 Lahoma, OK storm). And, of course, many "classic" MCSs begin as isolated supercells. The influence of supercell convection on MCS structure and motion is certainly a topic worthy of further investigation.

6. REFERENCES

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