Possible Triggering Mechanisms for Severe Storms in SESAME-AVE IV (9–10 May 1979)

Abstract

The SESAME-AVE IV (9-10 May 1979) rawinsonde data were analyzed to uncover possible triggering mechanisms for severe storms that developed over western Oklahoma and Texas. The high frequency of observations (at 3 h intervals) and high vertical resolution of reported data (at 25 mb intervals) at all stations permitted investigation of the diurnal variation of the planetary boundary layer on the synoptic scale. Thunderstorms developed first just ahead of a stationary front over the Texas panhandle on the afternoon of 9 May. This area was characterized by the absence of a strong inversion (or "lid") that represented an interface between very warm and dry air aloft, and warm moist tropical air below. Apparently, mesoscale low-level ascending motion associated with frontal lifting and/or the inland sea breeze effect led to the removal of the lid. Another noteworthy feature in this storm event was the strong vertical wind shear at low and middle levels over the storm area. When combined with the development of a deep boundary layer with weak stratification during the daytime, the Richardson number became less than one in the boundary layer in the prestorm situation. The results of our numerical linear stability analysis indicate that the observed basic states were indeed symmetrically unstable. This may suggest that the triggering processes were argumented by symmetric instability. Although a well defined dry line was present, it does not seem to have contributed directly to initiation of storms in this case. It also was observed that, as the thermal low began to weaken in the early evening, the cold air behind a stationary front started advancing eastward and helped to extend the line of thunderstorms deep into central Texas. This may be another process whereby some storms prefer to develop in the late afternoon or early evening.

Climatology of the premonsoon Indian dryline

“…Near the dryline boundary, severe thunderstorms are often triggered during the premonsoon (Weston, 1972;Akter and Ishikawa, 2014). However, upward motions occur in the dry air to the west of the dryline, offsetting the formation of convection in the moist air to the east of the dryline, as noted in previous studies (Koch and McCarthy, 1982;Ogura et al, 1982). Further studies using higher resolution numerical model are necessary to understand the triggering mechanisms of the convection and to help identify the differences between convection on the western and eastern sides of the dryline.…”

Mesoscale convective systems and nocturnal rainfall over the West African Sahel: role of the Inter-tropical front

“…This sharp meridional moisture gradient in close proximity to the genesis region of southern Niger has a diurnal cycle analogous to drylines observed over the south-central Great Plains of the U.S. (Schaefer 1986). While thunderstorms tend to develop in their vicinity, drylines are not the triggering mechanism for convection (e.g., Ogura et al 1982;Carlson et al 1983;Trier et al 2015). However, the dryline's proximity can be an important environmental factor because convective systems that form near them tend to become stronger and better organized (Rhea 1966).…”

“…While drylines are typically associated with atmospheric conditions favorable for the development of convection, it does not necessarily mean that it is the triggering mechanism (Ogura et al 1982;Carlson et al 1983;Benjamin and Carlson 1986). As discussed in the above-mentioned studies, the primary role of the dryline is to provide a restraining inversion for lower-tropospheric turbulence that suppresses the release of convective instability too early in the day.…”

“…It varies diurnally, similar to drylines observed in other regions of the world including the U.S. Great Plains of the United States (Schaefer 1974(Schaefer , 1986. Such features have been associated with the development of organized convection (e.g., Hamilton and Archbold 1945;Ogura et al 1982).…”

Is a Cold Pool Necessary for the Maintenance of a Squall Line Produced by a Cold Front Aloft?

“…parent surface frontal feature, but is collocated with the leading edge aloft of a forward-tilted eastward-moving Pacific cold front. Many studies have shown that convection can be initiated when a cold front overtakes a dryline, or lee trough (Koch and McCarthy 1982;Ogura et al 1982;Shapiro 1982;Schaefer 1986;Neiman et al 1998;Neiman and Wakimoto 1999;Stoelinga et al 2000). However, in cases where the cold front proceeds aloft after overtaking a lee trough, and an extensive squall line develops and remains collocated with the CFA, the dynamical role of the CFA in maintaining the squall line, rather than just initiating it, has not been well established.…”