Vertical Structure of Thunderstorm Outflows

Cold air outflow from thunderstorms has been observed from a multi-level 461 m tower in central Oklahoma. Four life-cycle stages are determined from 20 outflows sampled. Each outflow is affected by the behavior and character of the parent thunderstorm and by the flow and stratification in both the warm ambient air and the cold outflow air. This produces much variation from case to case and makes difficult the description of a representative outflow model. However, cases believed to depict each of the outflow stages are chosen and discussed in detail. The study is confined to the tower layer depth and concentrates on the characteristics of the outflow's leading edge. Outflow origin is not discussed. The leading edge of the outflow is characterized by strong shears in horizontal and vertical winds. Updrafts, which occur in a 1.5 km wide band ahead of the outflow, often exceed 6 m s−1. Maximum low-level horizontal wind gusts are found to be directly related to the translational speed of the front. Frontal slopes up to 75° are commonly observed. The windshift zone precedes the outflow thermal boundary by about 1 km and in strong outflows, both coincide with a pressure jump. Outflow gust front may be observed up to 45 min before measurable precipitation commences and are frequently characterized by multiple surges. Low-level stratification affects the slope of the gust front. If a strong surface inversion exists in the pre-outflow airmass, the outflow may have difficulty dislodging the dense air or may override it. In the case of an inversion base elevated off the ground, inflow air to the thunderstorm comes entirely from above the inversion and no well-defined gust front is observed at low levels. Evidence presented shows the possibility of cyclic formation and collapse of the gust front nose and the apparent related importance of low-level stratification. The paper concludes with a qualitative discussion of forces affecting the outflow. Surface drag and pressure forces are found to he most important. Near the front pressure forces dominate, elsewhere drag forces. The outflow friction (separation) layer thickness is shown to be related to the pressure gradient force.