The Decay of Tropical Storm Gaston (2010)

During the field campaign PREDICT, post-tropical storm Gaston (2010) was observed over the course of 6 days. This disturbance emerged from Africa as a mesoscale convective system embedded in an easterly wave. It attained tropical storm status for only 12 hours, after which it was downgraded to a tropical depression. The first mission into the disturbance was conducted at this time. The dropsonde data revealed a deep layer of closed circulation and an especially strong mid-level vortex. In spite of these characteristics, Gaston kept decaying. Previous studies attributed the decay to vertical wind shear and dry air intrusion. Though ingestion of dry air was clearly important in later stages, our analysis suggests that a different mechanism was responsible for Gaston's initial decay. Even though dry air surrounding Gaston was evident on satellite images, our results show nearly identical relative humidity profiles in the core of the system on day 1 and day 2 of observation (Gaston 1 and Gaston 2, respectively). It appears that the closed circulation in Gaston 1 prevented dry air intrusion at the altitudes where SAL resided. The most remarkable change was a sharp decrease of the mid-level vorticity from Gaston 1 to Gaston 2.

According to our previous hypothesis for tropical cyclogenesis, which posits that a strong mid-level vortex is necessary for cyclogenesis to take place, we ascribe the rapid decay of Gaston to the rapid decrease of its mid-level vorticity. Why did the latter occur? Our analyses show that the vertical profile of the vertical mass flux during Gaston 1 was bottom heavy, with a maximum at 4 km, and a large negative vertical gradient above this level. The negative gradient of the mass flux profile implies mass divergence at these levels, and thus decrease of the mid-level vorticity was expected. This raises another question. Why deep convection and stratiform clouds were absent in Gaston 1? In the western part of the disturbance we found evidence of a temperature inversion in the lower troposphere and relatively low sea surface temperatures. The soundings also revealed low relative humidity above the inversion layer. We hypothesize that the observed trade wind inversion inhibited convection and thus resulted in the observed bottom-heavy mass flux profile.