There are several factors that are hypothesized to affect TC size, including the environmental relative humidity, the size of the initial disturbance, and perhaps the background value of absolute vorticity (thereby affecting the Rossby radius of deformation). Subjective observations by the authors of relatively small TCs forming in relatively dry environments led us to formulate a hypothesis for the role that environmental relative humidity plays in regulating TC size.
Our hypothesis can be presented using a potential vorticity (PV) view of TC dynamics. The strength and size of the TC PV tower is clearly linked to the balanced rotational wind field of the storm. In a moist environment, when heavy showers form in outer bands surrounding of TC, lower-tropospheric, diabatically produced cyclonic PV anomalies will form at a larger radius from the storm center. Although these PV filaments spiral inward towards the storm center, by forming at a radius beyond the initial eyewall, the TC PV tower is broadened, and a larger wind field and correspondingly larger radius of maximum wind results. In contrast, TCs forming in relatively dry environments exhibit less precipitation outside the eyewall, and the PV tower remains more concentrated, with a smaller radius of maximum wind.
This hypothesis has been tested using a large number of WRF simulations, all of which produce consistent results: Dry environments favor smaller storms. Idealized initial vortices were placed in homogenous tropical environments with constant relative humidity ranging from 80% down to 20%. Simulations utilizing 2-km horizontal grid spacing were performed using an entirely oceanic model domain. More rapid initial intensification was found in the more moist environments, but by the end of the 10-day model integrations minimum central pressure values varied by < 10 hPa in simulations using 40, 60 or 80% initial relative humidity values. The simulated TC with 20% initial relative humidity took longer to become organized, and reached a minimum central pressure that was ~30 hPa higher than for the TCs in the more moist simulations. TCs in relatively moist environments developed more precipitation outside the eyewall, while TCs in dryer environments produced precipitation over a much smaller area. The radius of maximum wind (RMW) was larger in simulations with higher initial moisture values, varying between ~80 km and ~20 km in simulations with 80% and 20% relative humidity, respectively. The radii of hurricane force and tropical storm force winds were also significantly larger in simulations with higher environmental moisture. The results were insensitive to the size of the initial vortex, provided that a realistic range of sizes was used.