Kinematic and moisture structure of a stationary cold front observed on 10 June 2002 during IHOP

The accurate prediction of the formation and evolution of boundaries that can initiate convection remains a forecasting challenge. One goal of the International H2O project (IHOP), conducted in the Southern Great Plains of the United States during May and June 2002, was to investigate processes that determine the time and location of convection initiation. Airborne and ground-based measurements were collected in regions where convection initiation was forecasted. This study focuses on processes leading to convection initiation along a surface cold front observed in western Kansas on 10 June 2002. The northeast-southwest oriented cold front propagated toward the southeast during the morning hours then became stationary in the early afternoon. A diffuse boundary characterized by discontinuities in moisture and wind was evident southeast of the cold front, with drier, southwesterly winds immediately ahead and moister, southerly winds further to the southeast.

The analysis is focused on observations collected by instruments on the NRL P3. In particular, the ELDORA airborne Doppler radar provides reflectivity and pseudo dual-Doppler-derived winds. Also, the LEANDRE II differential absorption lidar allows for the derivation of water vapor mixing ratios. Additional data used in the analysis includes in situ temperature, mixing ratio, and wind from sensors on the NRL-P3 and University of Wyoming King Air, and vertical profiles of these parameters from dropsondes released by the Flight International Lear Jet and upsondes released by NCAR and NSSL ground-based sounding systems.

Pseudo dual-Doppler winds showed that the horizontal wind shear across the cold front increased with the approach of the diffuse boundary from the southeast. Organized patterns of misocyclones, indicated by vertical vorticity maxima (> 2 x 10-3 s-1) developed along the leading edge of the cold front. Vertical velocity maxima were observed at locations adjacent to the misocyclones. Two hours prior to convection initiation, the mean updrafts extended to 3 km MSL. As a consequence of increasing magnitudes of updrafts and vertical vorticity, areas of enhanced radar reflectivity and kinks in the reflectivity field became evident as the kinematic structure of the cold front evolved. In addition, water vapor mixing ratios increased in the layer between 2-3 km MSL during this period, thus making thermodynamic conditions more favorable for convection initiation.