On the Response of Hailstorms to Enhanced CCN Concentrations

This study is an idealized exploration of cloud nucleating aerosol impacts on hail storms. A previously studied case was used as a benchmark to design a rather large set of cloud-resolving simulations varying both cloud-condensation nuclei (CCN) concentrations and cloud base heights. A three moment-scheme developed by Loftus (2012) was implemented into the Regional Atmospheric Modeling System (RAMS v. 6) to perform this exploration of the parameter space.

This new triple-moment bulk hail microphysics scheme (3MHAIL) predicts the relative dispersion parameter for a gamma distribution function via the prediction of the sixth moment (related to the reflectivity factor) of the distribution in addition to the mass mixing ratio and number concentration (third and zeroeth moments, respectively) thereby allowing for a fully prognostic distribution function. The well-known narrowing of the hail size distributions as the precipitation core descends makes intuitive the limitations imposed by two-moment schemes that assume constant width parameters. Moreover, a significant improvement in the representation of sedimentation, melting, and formation processes of hail compared to lower-order moment schemes (Loftus, 2012) was seen when simulating two storms; one of them was the case used for the experimental design of the present study.

RAMS was configured to cover an area 100x100km with a horizontal grid spacing 500m (Nx=Ny=200) and a vertically stretched grid (Nz=50) with the first model level at an altitude of approximately 40m. In addition to the aforementioned recent improvements, RAMS microphysical modules considered the explicit activation of CCN (and giant CCN), a bimodal representation of cloud droplets, a bin-emulation approach for computing hydrometeor number concentration and mixing ratio changed due to all collisional mechanisms, including droplet collection, and ice-particle riming. CCN concentrations (a prognostic variable) were initialized with values between 150 and 3000 cm-3 at low levels, exponentially decreasing above 3km. The sounding of a hailstorm for which 1-inch diameter hail was reported was systematically perturbed by enhancing vapor contents at low levels up to 25%. More than a hundred numerical experiments have been performed covering a simulation period of 90 minutes each.

This study is not focused on the time evolution and behavior of individual storms but rather concentrates on a more macroscopic perspective; the analysis is similar to that of Carrió and Cotton (2011) for urban storms, although varying cloud base heights instead of instability levels. The response of many integral quantities was non-monotonic when CCN concentration was increased. Among them, maximum updraft speeds occurred at slightly higher altitudes and were more pronounced for more “moist”runs. Maximum surface winds and low-level divergence linked to downdrafts showed a similar response. The integral mass of hail precipitation, maximum local accumulations and the corresponding precipitation rates behaved similarly for the “dryer” runs. In the case of liquid precipitation, however, the integral mass of precipitation, the maximum local accumulations, and precipitation rates tend to monotonically increase with CCN, especially for more the “moist"runs.