Observations of the convective environment in developing and non‐developing tropical disturbances

Abstract

Analyses of thermodynamic data gathered from airborne dropwindsondes released from the upper troposphere during the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment are presented. We focus on two systems that finally became hurricanes Karl and Matthew, and one system (Gaston) that attained tropical storm status, but subsequently weakened and never redeveloped during five days of monitoring. Data for all events show that the largest values of total precipitable water are collocated with the surface trough and with values of convective available potential energy that seem high enough to support convective organization. These values coincide mostly with low values of convective inhibition. Vertical profiles of virtual potential temperature show little variability between soundings on a particular day, but the system means from day to day show a slight warming. In contrast, vertical profiles of pseudo-equivalent potential temperature, θ e , show much more variability between soundings on a particular day on account of the variability in moisture.In all systems, there was is a tendency for the lower troposphere to moisten, but in the non-developing system, the troposphere became progressively drier in the height range between approximately 2 and 9 km during the five days of observations. In the developing systems, the troposphere moistened. The most prominent difference between the non-developing system and the two developing systems was the much larger reduction of θ e between the surface and a height of 3 km, typically 25 K in the non-developing system, compared with only 17 K in the developing systems. Conventional wisdom would suggest that, for this reason, the convective downdraughts would be stronger in the non-developing system and would thereby act to suppress the development. Here we propose an alternative hypothesis in which the drier air weakens the convective updraughts and thereby the convective amplification of absolute vorticity necessary for development. Copyright

Observations of a Nondeveloping Tropical Disturbance in the Western North Pacific during TCS-08 (2008)

“…Large horizontal flow deformation in the low-and midtroposphere indicates a deep recirculation region was not present for TCS025 (Tory et al 2013), which also increased the likelihood that low-u e air in the surrounding environment was entrained into the system. The TCS025 environment did not follow the steady progression of drying in the midlevels typical of the nondeveloping system examined by Smith and Montgomery (2012) and Davis and Ahijevych (2012). There was essentially no difference in midlevel u e between IOP-1 and IOP-3, and low-level u e was actually highest during IOP-3, although sampling coverage may have been partly responsible for this.…”

“…It is hypothesized that similar processes occurred for TCS025; strong downdrafts depleted boundary layer u e in precipitation regions, which subsequently reduced convective intensity. Alternatively, it is possible that dry air entrainment also weakened updraft intensity and limited the vorticity amplification process as proposed by Smith and Montgomery (2012). A convective lull occurred after MCS-G dissipated; deep convection was relatively absent at the convective maximum time late on 28 August, and the next episode of deep convection did not form until the system had moved a considerable distance to the north.…”

“…Davis andAhijevych (2012, 2013) found evidence of this stabilization from dropwindsonde data collected during the Pre-Depression Investigation of Cloudsystems in the Tropics (PREDICT) field experiment , as low-level cooling occurred prior to storm formation for two developing systems. However, a similar study of PREDICT cases by Smith and Montgomery (2012) concluded that there was slight warming in the lower troposphere for both developing disturbances and the nondeveloping disturbance (Gaston) examined. Davis and Ahijevych (2012) suggested the low-level warming found by Smith and Montgomery (2012) could be due to the inclusion of dropwindsondes outside of the central convective region, which would act to obscure a lower-tropospheric cooling signal if present.…”

“…However, a similar study of PREDICT cases by Smith and Montgomery (2012) concluded that there was slight warming in the lower troposphere for both developing disturbances and the nondeveloping disturbance (Gaston) examined. Davis and Ahijevych (2012) suggested the low-level warming found by Smith and Montgomery (2012) could be due to the inclusion of dropwindsondes outside of the central convective region, which would act to obscure a lower-tropospheric cooling signal if present. Raymond and López Carrillo (2011) and Lussier et al (2014) utilized high-resolution airborne Doppler radar and aircraft data to note that the level of maximum vertical mass flux lowered during the development of Typhoon (TY) Nuri (2008), however the low-level mass flux calculated by Lussier et al (2014) was stronger than that found by Raymond and López Carrillo (2011) earlier in the genesis process.…”

“…Davis and Ahijevych (2012) found that the midtropospheric moist static energy increased over time for the developing systems analyzed, while the nondeveloping system (Gaston) exhibited a decrease in moist static energy and an increase in downdraft convective available potential energy (DCAPE). Smith and Montgomery (2012) also found that the mid-and upper troposphere became progressively drier for Gaston and suggested that the midlevel dry air observed in nondeveloping systems might act to dilute updraft intensity and limit the amplification of lower-tropospheric vorticity by vortex stretching (Bell and Montgomery 2010).…”