A Thermodynamic Mechanism Analysis on Enhancement or Dissipation of Convective Systems from the Mountains under Weak Synoptic Forcing

A preliminary analysis of thermodynamic mechanisms of two well-defined convective systems over Beijing area and its vicinity was studied with a three-dimensional cloud-scale numerical model and a rapid-update cycling 4DVar assimilation technique using data from four of China's new-generation weather radar stations (CINRADs). The two convective systems were both under weak synoptic forcing and precipitation stratification in the low-middle layer. The cooperation of the cold pool and wind field act as trigger and strengthening mechanisms for the storm, which could propagate from the mountains to the plains. Originally, the cold pool was generated due to the uneven distribution of the thermodynamic field, and blocks wind propagated at the southern edge of the cold pool. The mechanism resulted in relatively high convergence, relatively vertical wind shear and helicity. In the first case, which occurred on June 26, 2009, a relatively strong cool pool located to the south, cut off the warm southeastern inflow that caused distinct divergence from the plains to the mountains, causing storms to continually weaken on the mountains. However, outflow from the dissipating storms moved over southeast winds, resulting in high shear and helicity, and therefore new storms formed at the edge of the original cool pool. Due to low shear over the plains, the cool pool extended more quickly than the storms, causing the storms to dissipate. For the second case on August 1, 2009, the cold pool was located to the north. Veering winds that were forced and blocked by the cool pool and mountains formed distinct and strong convergences via. When storms reached the foothills, the original long-term cool pool still provided relatively high convergence, shear and helicity for the storm spreading from the mountains to the plains. New and original cool pools squeezed each other, resulting in an intensified northern storm. Storms drifted toward each other, eventually leading to linearly organized echoes. As linear echoes spread over the plains, the perturbation temperature shows the cold pool further intensified and expanded. Gust fronts intensified, tilted forward, and moved away from the storm. Thermodynamics of the linear echoes showed some characteristics of a squall line. However, the weak wind shear in the path of storm propagation resulted in disequilibrium with the cold pool. The gust front blew out the convergence line away from the original storm, which became weaker. These data, combined other investigations, imply that simulated helicity and shear are useful to indicate development of the storms. Finally, a conceptual model was developed using observed data and simulation results, showing low-level dynamic and thermodynamic collocation significantly affects development and evolution of these storms from the mountains to the plains.