Type of Document Dissertation Author Guimond, Stephen Author's Email Address email@example.com URN etd-09242010-154512 Title Tropical Cyclone Inner-Core Dynamics: A Latent Heat Retrieval and its Effects on Intensity and Structure Change; and the Impacts of Effective Diffusion on the Axisymmetrization Process. Degree Doctor of Philosophy Department Earth, Ocean & Atmospheric Science, Department of Advisory Committee
Advisor Name Title Mark Bourassa Committee Chair Ming Cai Committee Member Robert Hart Committee Member Xiaolei Zou Committee Member Michael Navon University Representative Keywords
- Doppler Radar
- Latent Heat
- Numerical Modeling
Date of Defense 2010-08-25 Availability unrestricted AbstractDespite the fact that latent heating in cloud systems drives many atmospheric
circulations, including tropical cyclones, little is known of its magnitude and structure
due in large part to inadequate observations. In this work, a reasonably high-resolution (2
km), four-dimensional airborne Doppler radar retrieval of the latent heat of condensation
is presented for rapidly intensifying Hurricane Guillermo (1997). Several advancements
in the retrieval algorithm are shown including: (1) analyzing the scheme within the
dynamically consistent framework of a numerical model, (2) identifying algorithm
sensitivities through the use of ancillary data sources and (3) developing a precipitation
budget storage term parameterization. The determination of the saturation state is shown
to be an important part of the algorithm for updrafts of ~ 5 m s-1 or less. The
uncertainties in the magnitude of the retrieved heating are dominated by errors in the
vertical velocity. Using a combination of error propagation and Monte Carlo uncertainty
techniques, biases were found to be small, and randomly distributed errors in the heating
magnitude were ~16 % for updrafts greater than 5 m s-1 and ~156 % for updrafts of 1 m s- 1.
The impact of the retrievals is assessed by inserting the heating into realistic numerical simulations at 2 km resolution and comparing the generated wind structure to the Doppler radar observations of Guillermo. Results show that using the latent heat retrievals outperforms a simulation that relies on a state-of-the-art microphysics scheme (Reisner and Jeffery 2009), in terms of wind speed root-mean-square errors, explained variance and eye/eyewall structure. The incorrect transport of water vapor (a function of the sub-grid model and the numerical approximations to advection) and the restrictions on the magnitude of heat release that ensure the present modelís stability are suggested as sources of error in the simulation without the retrievals.
Motivated by the latent heat retrievals, the dynamics of vortex axisymmetrization from the perspective of thermal anomalies is investigated using an idealized, non-linear atmospheric model (HIGRAD). Attempts at reproducing the results of previous work (Nolan and Grasso 2003; NG03) revealed a discrepancy with the impacts of purely asymmetric forcing. While NG03 found that purely asymmetric heating led to a negligible, largely negative impact on the vortex intensification, in the present study the impacts of asymmetries are found to have an important, largely positive role. Absolute angular momentum budgets revealed that the essential difference between the present work and that of NG03 was the existence of a significant, axisymmetric secondary circulation in the basic-state vortex used in the HIGRAD simulations. This secondary circulation was larger than that present in NG03ís simulations. The spin-up of the vortex caused by the asymmetric thermal anomalies was dominated by the axisymmetric fluxes of angular momentum at all times, indicating fundamentally different evolution of asymmetries in the presence of radial flow.
Radial momentum budgets were performed to elucidate the mechanisms responsible for the formation of the physically significant secondary circulation. Results show that explicit (sub-grid) diffusion in the model was producing a gradient wind imbalance, which drives a radial inflow and associated secondary circulation in an attempt to re-gain balance. In addition, the production of vorticity anomalies from the asymmetric heating was found to be sensitive to the eddy diffusivity, with large differences between HIGRAD and the widely used WRF model for the exact same value of this uncertain parameter.
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