Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part I: The Critical Role of Vortex Enhancement in Deep Convection

Abstract

This is the second of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TC-LAPS). The primary TC-LAPS vortex enhancement mechanism (convergence/stretching and vertical advection of absolute vorticity in convective updraft regions) was presented in Part I. In this paper (Part II) results from a numerical simulation of TC Chris (western Australia, February 2002) are used to illustrate the primary and two secondary vortex enhancement mechanisms that led to TC genesis. In Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS.During the first 18 h of the simulation, a mature vortex of TC intensity developed in a monsoon low from a relatively benign initial state. Deep upright vortex cores developed from convergence/stretching and vertical advection of absolute vorticity within the updrafts of intense bursts of cumulus convection. Individual convective bursts lasted for 6-12 h, with a new burst developing as the previous one weakened. The modeled bursts appear as single updrafts, and represent the mean vertical motion in convective regions because the 0.15°grid spacing imposes a minimum updraft scale of about 60 km. This relatively large scale may be unrealistic in the earlier genesis period when multiple smaller-scale, shorter-lived convective regions are often observed, but observational evidence suggests that such scales can be expected later in the process. The large scale may limit the convection to only one or two active bursts at a time, and may have contributed to a more rapid model intensification than that observed.The monsoon low was tilted to the northwest, with convection initiating about 100-200 km west of the low-level center. The convective bursts and associated upright potential vorticity (PV) anomalies were advected cyclonically around the low, weakening as they passed to the north of the circulation center, leaving remnant cyclonic PV anomalies.Strong convergence into the updrafts led to rapid ingestion of nearby cyclonic PV anomalies, including remnant PV cores from decaying convective bursts. Thus convective intensity, rather than the initial vortex size and intensity, determined dominance in vortex interactions. This scavenging of PV by the active convective region, termed diabatic upscale vortex cascade, ensured that PV cores grew successively and contributed to the construction of an upright central monolithic PV core. The system-scale intensification (SSI) process active on the broader scale (300-500-km radius) also contributed. Latent heating slightly dominated adiabatic cooling within the bursts, which enhanced the system-scale secondary circulation. Convergence of low-to midlevel tropospheric absolute vorticity by this enhanced circulation intensified the system-scale vortex. The diabatic upscale vortex cascade and SSI are secondary processes dependent on the locally enhanced vorticity and heat respectively, generat...

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

“…This likely prevented further organization of convection and kept vorticity from consolidating. 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).…”

“…20) do not clearly indicate a direct pathway for low-u e air to enter the inner core of TCS025, the negative normalized Okubo-Weiss values in the low-and midlevels (Fig. 7b) indicates there was large horizontal shearing deformation surrounding the circulation, and suggests TCS025 lacked a quasi-closed circulation (see Tory et al 2013). As a result, the circulation was more susceptible to the entrainment of low-u e air to the west of the circulation following 0600 UTC 28 August (Figs.…”

“…Stratiform precipitation processes likely contributed to the strengthening and maintenance of the midlevel vortex at the expense of low-level spinup. As argued by Tory et al (2006), the divergence profile associated with stratiform precipitation (low-level divergence and midlevel convergence) would be detrimental to the spinup of the low-level circulation. A south-north vertical cross section of u e constructed from dropwindsonde data (Fig.…”

“…Values of N range from 21 to 11. Positive values indicate a larger rotational component of the flow and negative values indicate that horizontal shear is large, which may prevent the consolidation of vorticity and indicate the circulation of an incipient disturbance is less protected from the detrimental effects of dry air in the surrounding environment (Raymond et al 2011;Tory et al 2013). The evolution of average N (Fig.…”

“…Lussier et al (2014) provided further evidence for the importance of the wave pouch and its improved vertical alignment over time that supported the mesoscale spinup at low and midlevels in TY Nuri. Similarly, Tory et al (2013) analyzed several cases of tropical storm formation in the northern Australian region and found that the vertical alignment of regions of recirculating, low-deformation flow in the low and midlevels was important for further development.…”