A Model Study on Tropical Cyclone Structural Changes in Response to Ambient Moisture Variations

Recent studies had emphasized the important role of moisture in altering tropical cyclone (TC) vortex structure. Latent heating in rainbands could change TC size and thus affect its intensity through hydrostatic adjustment. This study is to further explore TC vortex behavior with the presence of commonly observed asymmetric moisture variations. TC Talim (0513W), with shear-induced asymmetric rainbands during its mature stage, was simulated using the Weather Research and Forecasting (WRF) model, and sensitivity experiments were conducted by artificially modifying the amount and distribution of moisture around TC vortex. Outer rainbands in simulated TCs are concentrated in the down-shear left sector. Results showed that these rainbands are sensitive to their upstream moisture supply. TC with enhanced moisture supply into its outer rainbands will grow larger in size. However, its core intensity and strength are weakened because (i) outer rainbands block moisture inflow to TC core, (ii) rainband heating warms up the atmosphere and reduces radial pressure gradient in TC core. On the contrary, drier environment inhibits rainband convection, causing TCs to contract in size but increase in core strength and intensity. TCs in moister environment have lower intensification rate than those in drier environment, but they tend to have longer life cycle.

The upstream moisture supply to outer rainbands was found to be an important ingredient for eyewall replacement cycle. Secondary eyewall could not form if (i) outer rainband is too weak and gets advected into TC core rapidly; (ii) outer rainband is too strong and propagates radially outward.

Moreover, ambient moisture indirectly influences TC structure by altering synoptic-scale flow patterns. Pressure redistribution due to latent heating could sometimes lead to increased vertical wind shear, which is detrimental to TC intensification.