Analysis of Convergence Boundaries Observed during IHOP

There have been important advances in short-term forecasts (nowcasts) of thunderstorm initiation during the warm season. The improvements in our understanding of thunderstorm formation are largely attributed to the recognition that storms frequently develop near boundary layer convergence zones that are often detected by Doppler radars and satellite imagery. There have been a number of individual case studies that have examined the detail structure of convergence boundaries and their relationship to convection initiation. However, there has been no systematic attempt to perform a comprehensive analysis of a number of convergence boundaries using analogous data sets. Such an analysis would result in generalized conclusions concerning the thermodynamic and kinematic characteristics of the boundaries and the relationship to thunderstorm formation.

The current study presents airborne dual-Doppler wind syntheses and thermodynamic analysis based on soundings for six convergence boundaries during the International H2O Project (IHOP). Three cases were associated with deep convection while no convection developed on the other three days (i.e., null cases). The aircraft flew a box pattern around the boundaries with along-boundary legs ~100 km long. The Doppler radar data collected allowed for an assessment of both the along-frontal variability but also the mean characteristics of the convergence zone over an extended region. The flight legs also resulted in a data set with analogous spatial resolution so that direct comparisons between the case studies could be made. In addition, the kinematic and thermodynamic structure of all of these boundaries were well-documented with a series of dropsondes deployed by a jet flying at ~500 mb. The spatial and temporal resolutions of the sounding data were comparable which facilitated comparisons between the cases.

The results show that fine lines vary in width and that the maximum radar reflectivity value is not related to the peak updraft speed. This is in contrast to past studies that suggested that higher echo values imply stronger horizontal convergence. There is also a pronounced reduction in the mean dBZ values (4-5 dBZ) of the fine line from early to late spring suggesting a seasonal dependence of the clear air return over the IHOP domain. There was no correlation between maximum updrafts associated with the convergence boundary and convection initiation suggesting that other factors (e.g., stability) played a critical role. Surprisingly, there was no relationship between the magnitude of the horizontal gradient of the temperature/moisture discontinuity across the boundary and the strength of the kinematic discontinuity (e.g., horizontal convergence).

The dropsonde data providing an important opportunity to analyze the thermodynamic structure in the vertical plane approximately perpendicular to the convergence boundary. Well-defined moist, cold pools that resembled density currents were apparent in all cases. There was no relationship between frontogenesis/frontolysis and convection initiation. Indeed, the largest value of frontogenesis during IHOP was associated with a null case. The calculation of the tendency of horizontal vorticity; however, did reveal an intriguing relationship. The dominant term in the vorticity calculation was the horizontal gradient of buoyancy. The plot of this term revealed that the days when convection developed were characterized by couplets of positive and negative tendencies of horizontal vorticity that would promote vertically erect updrafts. The couplets were absent on the null days. This strongly implies that the solenoidal term is important in creating favorable conditions for convection initiation. A plot of the level of free convection (LFC) and convective inhibition (CIN) suggest that the null days are associated with large CIN values and high LFCs as would be expected. Other distinguishing features between the days when convection did and did not develop are highlighted.