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Title:Characterization of the role of precipitation in tropical cyclone intensification
Author(s):Harnos, Daniel
Director of Research:Nesbitt, Stephen W.
Doctoral Committee Chair(s):Nesbitt, Stephen W.
Doctoral Committee Member(s):McFarquhar, Greg M.; Sriver, Ryan L.; Wang, Zhuo
Department / Program:Atmospheric Sciences
Discipline:Atmospheric Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):cold clouds
convective bursts
cumulus congestus
deep convection
passive microwave
numerical modeling
precipitation features
radiative transfer
radius of maximum wind
rapid intensification
shallow cumulus
tropical cyclones
updraft features
warm clouds
warm rain
Abstract:Tropical cyclone (TC) intensity prediction remains a primary challenge facing the tropical meteorology community. A host of widely varying intensification paradigms have been put forth in recent decades associated with various dynamic or thermodynamic mechanisms. The one constant across these mechanisms is the role of the secondary circulation in terms of intensifying the vortex through the aggregation of moisture, vorticity, and angular momentum. The secondary circulation is governed by the latent heat release associated with hydrometeor phase transitions within its ascent branch in the TC inner-core. Disparate paradigms regarding the importance in TC intensification of solely liquid versus combined liquid and frozen hydrometeors have been put forth based upon space-borne passive microwave records. Furthermore, studies have suggested multiple archetypical structural modes dependent upon environmental wind shear associated with the most extreme intensification events, known as rapid intensification (RI). Here two cloud-resolving simulations are undertaken with the Weather Research and Forecasting (WRF) model to reproduce RI under low (Hurricane Ike, 2008) and high (Hurricane Earl, 2010) wind shear respectively. The simulations permit investigation of the explicit hydrometeor contributions towards latent heat release and critical dynamic and thermodynamic information unavailable to passive microwave sensors. Two novel approaches are developed: objective determination of the four-dimensional radius of maximum wind (RMW) and objective identification and characterization of three-dimensional updraft properties. The former algorithm is desirable due to the theorized greater efficiency for heating existing within the RMW to develop the TC warm core due to limited radial responses associated with the greater inertial stability, while the latter algorithm permits characterization of the full spectrum of convection for the first time during RI episodes. It is found that the high shear simulation is predominantly driven by convection penetrating the tropopause, whereas the low shear case features increased contributions from convection failing to reach the tropopause. Despite the different modes of intensification, each simulation exhibits an absolute maximum in the net diabatic heating over ±6 h of RI onset at subfreezing temperatures within the high inertial stability existing within the RMW. Each of these numerical model results underscores the importance of convection with associated ice phase processes in RI episodes. The high shear simulation exhibits greater hydrometeor asymmetry leading up to RI relative to the low shear case, in line with passive microwave archetypes that have been previously introduced. The low-shear simulation exhibits relatively even contributions towards diabatic heating and vertical fluxes from cumulus congestus, deep convection, and convective bursts near RI onset with a secondary heating role apparently associated with stratiform precipitation, whereas the high shear simulation is nearly-entirely CB driven. Within the low shear simulation congestus clouds do provide a noteworthy contribution to moisture convergence across the inner-core through shortly after RI has begun, and both simulations show that all types of convection possess a vortical component. Despite both simulations showing evidence of the vortical hot tower paradigm, the vortical nature of all modes of convection and heating forcings in the low shear simulation from multiple convective modes suggest the need for inclusion of the importance of less vertically-developed convection in the theoretical framework. Nevertheless, the majority of the heating and vertical fluxes for each simulation are associated with convection containing ice hydrometeors. In light of the WRF simulations, passive microwave records are analyzed in an attempt to quantify the relative prevalence of each structural mode within the observational record and further examination of the relative roles of liquid and ice in RI episodes. It is found that individual pixel brightness temperatures associated with axisymmetric ring-like features when recast into wind shear-relative coordinates are statistically significant in TCs undergoing RI at 37 and 85 GHz brightness temperatures, corresponding to signals from liquid and ice hydrometeors respectively. Frequency analyses reveal that the 85 GHz ring pattern is more likely to be distinguishable due to a stronger radial gradient, whereas 37 GHz product frequencies exhibit a slight radial gradient within the TC inner-core. When evaluating the full spectrum of coincident 37 and 85 GHz brightness temperatures, a relatively low prevalence of pixels associated with solely warm rain appears, particularly for RI cases, while the majority of the brightness temperature distributions across the TC inner-core are associated either with precipitation containing frozen hydrometeors or an absence of hydrometeors. The use of false color imagery at 37 GHz fails to distinguish between precipitative ring presence and RI episodes versus lesser intensification rates. Dual frequency analyses at 37 and 85 GHz reveal the lack of warm rain across the TC inner-core for all intensity changes, particularly for RI scenarios. In such RI scenarios, 37 GHz false color patterns that have been associated with warm rain show exhibit 85 GHz that are more commonly associated with frozen hydrometeor presence than lesser intensity changes. Idealized radiative transfer simulations are used to reveal substantial sensitivity at 37 GHz to environmental characteristics such as the surface winds and cloud liquid water even in the absence of precipitating hydrometeors, which can confound interpretations at 37 GHz. Due to the numerical model simulation results and passive microwave characterizations, the importance and roles of ice in TCs undergoing intensification, and particularly RI, should be a focus for the tropical meteorology community going forward.
Issue Date:2015-01-21
Rights Information:Copyright 2014 Daniel Sean Harnos
Date Available in IDEALS:2015-01-21
Date Deposited:2014-12

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