4C.6 A CLOSER LOOK AT VORTICAL HOT TOWERS IN A TROPICAL CYCLOGENESIS ENVIRONMENT

Andrea B. Saunders∗ Colorado State University, Fort Collins, Colorado

1. INTRODUCTION This experiment ceased to have any convective ac- tivity after t = 4 h and failed to develop a mesoscale Over the past several decades there has been surface circulation after 72 h. Here we investigate considerable interest in determining what mecha- which conditions, present in the control simulation nisms are responsible for the transformation of a yet absent in the experiment run without an initial midlevel mesoscale convective vortex (MCV) into a MCV, are responsible for deep convective activity. . It is a generally accepted fact that convection plays an important role in tropical cy- clone intensification, yet convective-scale processes 3. PRELIMINARY HYPOTHESES FOR VHT are absent from many current tropical cyclogenesis LONGEVITY theories. Given the numerous observational studies indicating that deep convective blow-ups are a pre- There are two factors that contribute to the con- cursor to tropical storm formation (eg. Zehr 1992), tinuous deep convective activity noted above: long- we believe one must consider the role of emergent lived individual hot towers and/or continual convec- convective structures in the spin-up process. tive redevelopment. Let us consider the first influ- encing factor. A typical hot tower lifetime in our control simulation is found to be on the order of 2. A VORTICAL HOT TOWER PATH TO TROP- an hour, with some individual VHTs being tracked ICAL CYCLOGENESIS: AN OVERVIEW for as long as 3 h. Upon hot tower formation, the updraft tilts the ambient horizontal of the Using the non-hydrostatic CSU RAMS, Mont- initial MCV into the vertical, and a vorticity dipole is gomery et al. (2003, hereafter M03) ran near - observed centered about the updraft core. As the hot resolving experiments to investigate tropical cyclo- tower intensifies, vortex tube stretching by the up- genesis from a single initial midlevel MCV embedded draft core greatly intensifies the positive convective within a moistened tropical environment. The time vorticity anomaly. Our model hot towers were found period before tropical cyclone formation (t < 24 h) to have positive absolute vertical vorticity (f + ζ ≈ was dominated by nearly continuous deep convective 30–40 ×10−4 s−1) values associated with them. These activity. Using a modified version of the Sawyer- “vortical” hot towers, or VHTs, were also observed Eliassen equations, M03 determined the toroidal cir- in the MM5 model simulations of Hurricane Diana culation that would be induced by diabatic heat re- documented in Hendricks et al. (2004). leased by deep convection, as predicted by Eliassen’s M03 discusses several consequences that height- balanced vortex theory. This toroidal circulation con- ened vorticity can have on hot tower structure. First, sists of an azimuthal mean radial inflow at low- and the increased vorticity in the vicinity of the hot tower mid-levels. This mesoscale inflow acts to converge will increase to local inertial stability. This increased planetary and MCV-related vorticity, leading to an stability will help keep the latent heat released in increase in the azimuthal mean tangential wind field the updraft core from propagating away via inertia- at low levels and thus the development of a sur- gravity waves, and hence should increase the effec- face concentrated mesoscale vortex. These predicted tiveness of the hot tower as a local heat source. The changes in the mesoscale wind fields are qualitatively heightened inertial stability should also reduce mi- and quantitatively similar to the changes observed in dlevel entrainment of cooler, drier air. If these pre- the model’s azimuthal mean vortex fields. dicted impacts hold true, we would hypothesize that M03 used quasi-balanced dynamics to describe VHTs should experience longer lifetimes than their the system-scale evolution. This theory, however, less vortical counterparts. hinges on the assumption that convective latent heat release is occurring in a quasi-steady fashion on the system scale. Given the fact that the first 24 hours of 4. MESOSCALE ENVIRONMENT’S IMPACT ON the control simulation are dominated by nearly con- VHT STRUCTURE AND LONGEVITY tinuous deep convective activity, this is a reasonable assumption. However, a variety of sensitivity trials We tested the above hypothesis that vorticity will were run in which this is not the case. For exam- increase a VHT’s longevity through a series of sensi- ple, an experiment was run without the initial MCV. tivity trials examining the first convective burst ini- tiated at t = 20 min. The sensitivity trials were run ∗Corresponding author address: Andrea B. Saun- with the same thermodynamic profiles as the con- trol. The first trial omitted the initial MCV and ders, Department of Atmospheric Science, Colorado the second omitted the MCV and increased plane- State University, Fort Collins, CO 80523-1371; saun- tary vorticity over the domain by a factor of 10 (ie, [email protected] f = 3.77×10−4 s−1) . The initial updraft in the control w, z = 1km, incr = 0.5 ms-1 theory as to why these developments occur in the 08-22-1300 08-22-1300 MCV environment is coldpool lifting. The MCV wind field results in the relative flow across the VHTs at 15 5 low levels, which may be converging at the coldpool

10 0 boundary and spawning new convection. However, the coldpools in our study are relatively weak, with y (km) y (km) 5 -5 potential temperature deficits ranging from 0.6–1.0 K below the environmental value. In looking at a few

0 -10 cases early in the simulation, it appears that convec-

40 45 50 55 40 45 50 55 tive redevelopment is favored downshear-left of an x (km) x (km) existing updraft. Although on a much smaller scale, this observation is consistent with the preferred con- vective region for a hurricane-like vortex (Reasor et Figure 1: Vertical velocity at z = 1 km for initial up- al. 2003, Braun et al. 2003). Further investigation draft just after peak in control (left) and 2 sensitivity is necessary to determine if these mechanisms are trial (right). Solid = positive, dashed = negative, con- acting in our simulations. tour intervals 0.5 ms−1. Arrow in upper righthand corner is the average vertical shear vector (control only). 6. CONCLUSIONS

The results presented here indicate that vortic- simulation experienced a significantly longer lifetime ity influences the structure of convective hot towers. than in the first trial without and initial MCV. At The exact nature of this influence is complex, and first glance, this finding appears to confirm our hy- dependent upon a number of other environmental pothesis. The second trial developed a highly vortical factors. We have presented several ways in which the hot tower, yet this VHT had a much shorter lifetime MCV environment may modulate deep convective ac- than either of the previous two hot towers. Hence, tivity, and thus alter its role in the tropical cycloge- it appears that vorticity alone is actually detrimental nesis process. Further investigation, through higher- to the updraft’s longevity. Could it be that the same resolution model simulations and an observational vorticity-induced stability that serves to protect the field campaign, is needed to test these hypotheses. updraft core at midlevels is inhibiting low level in- Until we fully understand the role of deep convec- flow that the hot towers need to sustain themselves? tion, or more specifically the vortical hot tower, in These results suggest that the relationship between the tropical cyclogenesis process, we cannot consider vorticity and hot tower longevity is not as simple as any theory complete. we had originally thought. ACKNOWLEDGMENTS I would would like to thank 5. HOW DOES THE MCV ENVIRONMENT CON- M. T. Montgomery, M. E. Nicholls, and T. A. Cram TRIBUTE TO QUASI-STEADY CONVECTIVE at Colorado State University for their diligent work on ACTIVITY? the RAMS tropical cyclogenesis project. This work was supported in part by the National Science Foun- The abundance of VHTs in the control simula- dation through grant NSF-ATM-0101781. tion suggests that the combination of the horizon- tal wind and vorticity fields of the initial MCV is 7. REFERENCES conducive to the formation and sustenance of long- lived VHTs. The most notable difference between the physical structure of VHTs in the MCV environ- Braun, S. A., M. T. Montgomery and Z. Pu: High- ment versus the still environments is the presence of resolution simulation of (1998). a downshear tilting with height. This tilt displaces Part I: The organization of vertical motion. J. At- the downdraft downshear of the updraft core (Figure mos. Sci., , submitted Sept. 1, 2003 1, left). The VHTs in the still environments appear Hendricks, E. A., M. T. Montgomery and C. A. Davis, to be vertically upright, so their downdrafts occur in 2003: On the role of “vortical” hot towers in tropical the center of their updraft cores (Figure 1, right). The former orientation reduces contamination of the cyclone formation. J. Atmos. Sci., , to appear updraft core by the colder, drier air of the downdraft, Montgomery, M. T., M. E. Nicholls, T. A. Cram and A. and is thus the preferred scenario for increasing up- B. Saunders: A vortical hot tower route to tropical draft lifetimes. cyclogenesis. submitted Dec. 4, 2003 Until now, we have focussed on the effects of vorticity on individual hot tower lifetimes. Another Reasor, P. D., M. T. Montgomery and L. D. Grasso, 2004: factor contributing to quasi-steady convective activ- A new look at the problem of tropical cyclones in ity is convective redevelopments. In the control, new vertical shear flow: vortex resiliency. J. Atmos. Sci., convection is continually forming in the vicinity of , 61, 3–22 existing convection. The still-environment sensitivity Zehr, R., 1992: Tropical cyclogenesis in the western trials experience at most one convective redevelop- North Pacific. NOAA Tech. Rep. NESDIS 61, 181 ment, with no further deep convection. The first