Cloud microphysical responses to cloud condensation nuclei and ice nuclei concentrations in cloud resolving model simulation using CAIPEEX observations over India

With increasing awareness of human influence on the environment, growing attention has been paid to the possible effect of anthropogenic aerosols on cloud properties and precipitation formation. Water soluble aerosols that act as condensation nuclei (CN) for cloud drop formation are usually quite abundant in the troposphere, so increases of them usually leads to smaller cloud drops thus reduce the chances of coalescence to form raindrops. Again the effect of changing ice nuclei (IN) concentration received relatively little attention which has a crucial role in mixed-phase cloud processes. The measurements of CN and IN during Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) in India actually opens up the possibility of heterogeneous ice nucleation by IN and cloud-aerosol interaction over Indian peninsular.

Thus in this present endeavor, the microphysical responses of cloud and precipitation formation to the variation of both condensation nuclei concentrations (CNC) and ice nuclei concentrations (INC) were simulated using different numerical models like MM5 and WRF with two-moment warm-cloud microphysical process (CL scheme) coupled with the ice-phase parameterizations of Reisner (hereafter CLR-scheme). The NCEP FNL data with 1º x1º horizontal resolution are used to provide initial and lateral boundary conditions for our simulations. The simulations are carried out considering four nested domains with resolution of 27 km, 9 km, 3 km and 1 km grid spacing. The results of the innermost domain (at 1 km resolution) are presented here. The values of CN and IN concentrations were used based on in situ CAIPEEX study over India.

Under increasing CNC, snow formation becomes weaker in a clean (low-CNC) environment but becomes stronger in a polluted (high-CNC) environment. The formation of graupel/hail behaves just the opposite, becoming stronger at low CNC but weaker at high CNC. Identifying the ice-phase microphysical responses to CN and IN, we analyze snow and graupel/hail formation and how this contributes to surface rainfall. The results also indicate an important attribute of “tipping-point concentration” for both CNC and INC, below and above this point precipitation decreases. Cold-rain initiations by melting respond to CNC/INC in the formation of snow and graupel/hail, whereas warm rain initiation by auto-conversion becomes exponentially weaker with increasing CNC/INC. The net production of surface rainfall depends on the relative strength of the warm rain and the two cold-rain initiation processes (by snow and graupel melting). Main factors causing the increasing or decreasing trends are depended on (i) size of cloud ice initiated from frozen cloud drops; (ii) Wagner-Bergeron-Findeisen conversion; (iii) conversion from cloud ice, including aggregation; (iv) liquid water content; (v) collision efficiency, for either accretion or riming. The net response of surface rainfall to CN and IN variation depends on the relative strength of these rain initiation and growth mechanisms. The study suggests the importance of detailed knowledge of microphysical tendency for understanding cloud and precipitation formation with respect to various CN/IN concentrations.