Idealized Study of Land Surface Impacts on Tropical Cyclone Intensity Predictions Using the HWRF Modeling System

Although a favorable synoptic environment is the most important factor in the maintaining a tropical cyclone and the upward transfer of enthalpy fluxes from the ocean surface to the atmosphere and the downward transfer of momentum from the atmosphere to ocean surface eventually controls the evolution and intensification of a Tropical Cyclone (Anthes and Chang 1978; Emanuel 1995; Ooyama 1969; Tuleya and Kurihara 1978), several TCs are known to sustain over land and some of them have even been found to intensify over land. Tropical Storm Erin (2007) and Tropical Cyclone Abigail (2001) are two examples. Given that most of tropical cyclone's impact is felt on land – Rainfall, inland flooding, storm surges, wind damage etc., understanding the problem of sustenance/ re-intensification of tropical cyclones over land is not only important but critical to plan, coordinate and execute mitigative efforts.

The objective of this study is to understand all the land surface conditions in which a storm may sustain/re-intensify over land and the processes responsible for re-intensification/ sustenance. To answer these questions, forecasts are performed using the idealized framework of the operationally adopted Hurricane Weather Research and Forecasting system (HWRF) with similar setting as that of Gopalakrishnan et al. (2012). To simulate landfall half of the domain was land masked with different soundings specified for land and ocean. a comprehensive study of the energetics of the storm over land with special focus on (1) storm parameters (storm size and translational speed); (2) sensitivity studies involving soil moisture changes; (3) changes in soil temperature in relation to its thermal properties of soil and its effect on enthalpy flux changes and hurricane intensity changes. Boundary layer effects and the role of inflow layer over land will also be analysed. Figure 1 shows the effect of different soil and vegetation type on the evolution of the storm. Preliminary results on the impact of land temperature on intensity changes are illustrated by Figures 2 and 3. It clearly shows that at lower temperatures, the storm decay quickly over land owing to the lack of surface fluxes to feed the storm. Figure 2 shows the evolution of storm where the surface temperatures were modified and land surface at 22°C is dramatically more intense and sustained itself for longer over land than the runs were the temperature was much lower. Sandy soil was the soil type used to define land on this domain. Though there is an apparent lack of latent heat fluxes over land, it is hypothesized that it is the total enthalpy that contributes to the sustenance and in this case, surface heat fluxes are high enough to sustain the warm core over land. More analysis on to test the hypothesis will be conducted.

References

 

1.      Anthes, R. A., and S. W. Chang, 1978: Response of the Hurricane Boundary Layer to Changes of Sea Surface Temperature in a Numerical Model. Journal of the Atmospheric Sciences, 35, 1240-1255.

2.      Emanuel, K. A., 1995: Sensitivity of Tropical Cyclones to Surface Exchange Coefficients and a Revised Steady-State Model incorporating Eye Dynamics. Journal of the Atmospheric Sciences, 52, 3969-3976.

3.      Gopalakrishnan, S. G., F. Marks, J. A. Zhang, X. Zhang, J.-W. Bao, and V. Tallapragada, 2012: A Study of the Impacts of Vertical Diffusion on the Structure and Intensity of the Tropical Cyclones Using the High Resolution HWRF system. Journal of the Atmospheric Sciences, 70, 524–541.

4.      Ooyama, K., 1969: Numerical Simulation of the Life Cycle of Tropical Cyclones. Journal of the Atmospheric Sciences, 26, 3-40.

5.      Tuleya, R. E., and Y. Kurihara, 1978: A Numerical Simulation of the Landfall of Tropical Cyclones. Journal of the Atmospheric Sciences, 35, 242-257.

 

Figure 1: Hovemoller of asymmetric mean tangential winds at 10-m averaged azimuthally for different soil and vegetation type. (a) Sandy soil (desert condition), (b) wetland, (c) grassland – savannah and (d) Herbaceous wetlands.

 

Figure 2: Maximum sustained wind (left) and minimum sea level predicted (Right) for three different land temperatures (290K, 293K, 295K)

Figure 3: Hovemoller of asymmetric mean tangential winds at 10-m averaged azimuthally for 290K (left), 295K (center) and 295K (right)