@article{hill_lackmann_2011, title={The Impact of Future Climate Change on TC Intensity and Structure: A Downscaling Approach}, volume={24}, ISSN={["1520-0442"]}, DOI={10.1175/2011jcli3761.1}, abstractNote={A comprehensive analysis of tropical cyclone (TC) intensity change in a warming climate is undertaken with high-resolution (6- and 2-km grid spacing) idealized simulations using the Weather Research and Forecasting (WRF) model. With the goal of isolating the influence of thermodynamic aspects of climate change on maximum hurricane intensity, an idealized TC is placed within a quiescent, horizontally uniform tropical environment computed from averaged reanalysis data for the tropical Atlantic Ocean. The analyzed tropical environment is used for control simulations. Changes between the periods 1990–99 and 2090–99 are computed using output from 13 GCMs from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), for the A1B, A2, and B1 emissions scenarios. These changes are then added to the reanalysis-derived initial and boundary conditions used in the control simulations. Some processes known to impact TC intensity, such as environmental vertical wind shear and sea surface wake cooling, are not considered in this study. Future TC intensity increased for 75 of 78 future simulations using 6-km grid length, with a 9% (~8 hPa) average increase in central surface-pressure deficit. For the 2-km simulations, the average increase was 14% (~14 hPa). The depth of the TC secondary circulation increases in future simulations, consistent with an increase in the height of the freezing level and tropopause. Inner-core precipitation increases of 10%–30% are found for future simulations, with large sensitivity to the emission scenario. The increase in precipitation is consistent with a stronger potential vorticity tower, a warmer eye, and lower central pressure. Enhanced upper-tropospheric warming in the GCM environment is shown to be an important mitigating influence on TC intensity change but is also shown to exhibit large uncertainty in GCM projections.}, number={17}, journal={JOURNAL OF CLIMATE}, author={Hill, Kevin A. and Lackmann, Gary M.}, year={2011}, month={Sep}, pages={4644–4661} }
 @article{hill_lackmann_2009, title={Analysis of Idealized Tropical Cyclone Simulations Using the Weather Research and Forecasting Model: Sensitivity to Turbulence Parameterization and Grid Spacing}, volume={137}, ISSN={["1520-0493"]}, DOI={10.1175/2008MWR2220.1}, abstractNote={Abstract The Weather Research and Forecasting Advanced Research Model (WRF-ARW) was used to perform idealized tropical cyclone (TC) simulations, with domains of 36-, 12-, and 4-km horizontal grid spacing. Tests were conducted to determine the sensitivity of TC intensity to the available surface layer (SL) and planetary boundary layer (PBL) parameterizations, including the Yonsei University (YSU) and Mellor–Yamada–Janjic (MYJ) schemes, and to horizontal grid spacing. Simulations were run until a quasi-steady TC intensity was attained. Differences in minimum central pressure (Pmin) of up to 35 hPa and maximum 10-m wind (V10max) differences of up to 30 m s−1 were present between a convection-resolving nested domain with 4-km grid spacing and a parent domain with cumulus parameterization and 36-km grid spacing. Simulations using 4-km grid spacing are the most intense, with the maximum intensity falling close to empirical estimates of maximum TC intensity. Sensitivity to SL and PBL parameterization also exists, most notably in simulations with 4-km grid spacing, where the maximum intensity varied by up to ∼10 m s−1 (V10max) or ∼13 hPa (Pmin). Values of surface latent heat flux (LHFLX) are larger in MYJ than in YSU at the same wind speeds, and the differences increase with wind speed, approaching 1000 W m−2 at wind speeds in excess of 55 m s−1. This difference was traced to a larger exchange coefficient for moisture, CQ, in the MYJ scheme. The exchange coefficients for sensible heat (Cθ) and momentum (CD) varied by <7% between the SL schemes at the same wind speeds. The ratio Cθ/CD varied by <5% between the schemes, whereas CQ/CD was up to 100% larger in MYJ, and the latter is theorized to contribute to the differences in simulated maximum intensity. Differences in PBL scheme mixing also likely played a role in the model sensitivity. Observations of the exchange coefficients, published elsewhere and limited to wind speeds <30 m s−1, suggest that CQ is too large in the MYJ SL scheme, whereas YSU incorporates values more consistent with observations. The exchange coefficient for momentum increases linearly with wind speed in both schemes, whereas observations suggest that the value of CD becomes quasi-steady beyond some critical wind speed (∼30 m s−1).}, number={2}, journal={MONTHLY WEATHER REVIEW}, author={Hill, Kevin A. and Lackmann, Gary M.}, year={2009}, month={Feb}, pages={745–765} }
 @article{hill_lackmann_2009, title={Influence of Environmental Humidity on Tropical Cyclone Size}, volume={137}, ISSN={["1520-0493"]}, DOI={10.1175/2009MWR2679.1}, abstractNote={Abstract Observations demonstrate that the radius of maximum winds in tropical cyclones (TCs) can vary by an order of magnitude; similar size differences are evident in other spatial measures of the wind field as well as in cloud and precipitation fields. Many TC impacts are related to storm size, yet the physical mechanisms that determine TC size are not well understood and have received limited research attention. Presented here is a hypothesis suggesting that one factor controlling TC size is the environmental relative humidity, to which the intensity and coverage of precipitation occurring outside the TC core is strongly sensitive. From a potential vorticity (PV) perspective, the lateral extent of the TC wind field is linked to the size and strength of the associated cyclonic PV anomalies. Latent heat release in outer rainbands can result in the diabatic lateral expansion of the cyclonic PV distribution and balanced wind field. Results of idealized numerical experiments are consistent with the hypothesized sensitivity of TC size to environmental humidity. Simulated TCs in dry environments exhibit reduced precipitation outside the TC core, a narrower PV distribution, and reduced lateral extension of the wind field relative to storms in more moist environments. The generation of diabatic PV in spiral bands is critical to lateral wind field expansion in the outer portion of numerically simulated tropical cyclones. Breaking vortex Rossby waves in the eyewall lead to an expansion of the eye and the weakening of inner-core PV gradients in the moist environment simulation. Feedback mechanisms involving surface fluxes and the efficiency of diabatic PV production with an expanding cyclonic wind field are discussed.}, number={10}, journal={MONTHLY WEATHER REVIEW}, author={Hill, Kevin A. and Lackmann, Gary M.}, year={2009}, month={Oct}, pages={3294–3315} }