Idealized Simulation Of The Latitudinal Dependence Of The Dry Air Effect On Tropical Cyclone Development

Relative humidity is an important parameter in controlling tropical cyclone(TC)development.Sudden intensity change related to the interaction between TC and environmental dry air generally leads to considerable forecasting errors.Due to the different distributions of relative humidity and the main development region between different ocean regions,the main latitudes at which TCs are subjected to dry air intrusion is also different.It has not been well understood how the varied latitudes modulate the dry air effect on TC.To this end,a series of idealized simulations are carried out to examine the latitudinal dependence of the interaction between TC and the dry air in the current study.It is found that the effect of low-level dry air layer in inhibiting TC development decreases with increasing latitude.When dry air is excluded,the greater boundary layer imbalance under an environment of lower background planetary vorticity results in a strong boundary layer inflow,which increases the mean radial vorticity flux and moisture convergence in the TC core region,and thus help the TC development at lower latitudes.However,when a low-level dry air layer is imposed,the dry air can easily penetrate into the TC inner-core region under the help of strong low-level inflow.The intruding dry air inhibits the inner-core deep convection and leads to marked asymmetric convective structure,which significantly suppresses the TC development.In contrast,at higher latitudes,the dry air can hardly intrude into the TC inner-core region due to a weaker radial inflow but can suppress the development of the outer spiral rainbands.The suppressed outer spiral rainbands lead to a weaker barrier effect to the boundary layer inflow and help the TC development.The effect of the dry air on TC also depends on its horizontal and vertical distribution.The farther the dry air resides from the vortex center,the less the TC development is affected.The effective radius of dry air is larger at lower latitudes.Moreover,the impact of dry air on TC intensification decreases with the increasing altitude,so does the latitudinal dependence.Analysis on the downdraft-induced equivalent potential temperature advection indicates that the boundary layer cooling induced by convective downdrafts is not promoted in the presence of midlevel dry air and thus the intensity change is limited.Through examining the distribution of the moisture field and the main development region of TCs,we expect that the mean latitude of dry air intrusion can be smaller in North Atlantic Ocean and therefore,has more effects on TC development.