Aerosol impacts on tropical cyclones

As part of the DHS sponsored Hurricane Aerosol Microphysics Program(HAMP) we have been investigating possible influence of aerosols on the intensity of tropical cyclones. The focus of this research is on aerosols acting as cloud condensation nuclei(CCN). It is hypothesized that in the outer rainbands, increasing CCN concentration results in reduced collision/coalescence, increased supercooled water transported aloft, enhanced convection (latent heat of freezing) and ultimately enhanced precipitation and low level cooling (evaporation). The increase in low level cold-pool coverage in the outer rainband region blocks the flow of energy into the storm core inhibiting the intensification of the TC. However, the amount of suppression of the strength of the TC depends on the timing between the transport of CCN to the outer rainbands and the intensity and lifecycle stage of the outer rainband convection. The outer rainband convection needs to be strong in order for the transport of supercooled liquid water aloft to take place.

Our modeling studies repeatedly demonstrate that the areal extend and strength of low-level cold-pools are an important regulator of tropical cyclone intensity. Cold-pool strength in TCs can be regulated by environmental factors such as the presence or absence of a deep-moist layer, strong vertical shear, or the presence of air polluted with high concentrations of CCN either by man-made pollutants or dust. Strong cold-pools during the genesis stages of a TC can lead to decoupling between mid-level mesoscale convective vorticies and surface-based mesocyclones. During the mature stages of a TC, decoupling can also result, but also, large areal cold-pools in the outer rainband region can inhibit the flow of enthalpy into the storm interior. We propose that monitoring cold-pools in TCs may provide a method of diagnosing future variations in TC intensity.