Extended Abstract (264K)3.6
Effects of IFN entrainment from above the inversion on Arctic boundary clouds
William R. Cotton, Colorado State University, Ft. Collins, CO; and G. Carrió and H. Jiang
The Los Alamos National Laboratory sea-ice model (LANL CICE) was implemented into the Colorado State University-Regional Atmospheric Modeling System (RAMS). Various improvements have been made to those modules involved in the coupling, among them, the inclusion of iterative methods that consider variable roughness lengths (dependent on snow saltation) to evaluate momentum and heat fluxes. The radiation scheme previously implemented in RAMS was modified to take into account the cloud fraction. This version of the model also includes more complex microphysics whereby CCN and Giant-CCN are activated for the nucleation of cloud droplets, allowing the prediction of mixing ratios and number concentrations for all condensed water species. We performed two types of cloud resolving simulations to access the impact of the entrainment of ice freezing nuclei from above the inversion on Arctic boundary layer clouds. The first series of numerical experiments corresponds to a boundary layer cloud observed on May 4 1998 during the FIRE-ACE/SHEBA field experiment. For this case, a series of sensitivity numerical experiments has been performed modifying the initial IFN profile above the top of the boundary layer. The control run assumed a constant "clean" profile (4l-1) such as observed with the NCAR continuous flow diffusion chamber (CFDC) below cloud base. All other simulations consider the observed polluted profile multiplied by factors between 1/3 and 2 above the inversion (and a clean profile below). All simulations have been performed using the two-moment microphysical treatment that predicts mixing ratios and number concentrations for all hydrometeor species. Results indicate a significant impact of the entrainment of IFN's from above the inversion on the microstructure of the simulated clouds. The observed secondary IWC maximum just below the inversion could be reproduced only when assuming polluted initial profiles. The liquid water fraction of the cloud monotonically decreases when more polluted initial profiles are assumed. Droplets located in the top layer of the cloud are the most affected the corresponding concentration, as well as their mean diameters decrease. However, total condensate paths monotonically increase and downward IR tend to increase due to a significant increase in the ice water path. We performed a second set of cloud resolving simulations focused on the evaluation of the potential effect of high IFN concentration above the inversion on melting rates during spring-summer period. For these multi-month simulations, the IFN profiles were also initialized assuming clean and polluted concentrations within and above the boundary layer, respectively using the 4 May profiles as benchmarks. Aerosols were entrained as in the May 4 case although, IFN profiles were also nudged considering the time-varying altitude of the inversion Results suggest that increasing the IFN concentrations above the boundary layer increases sea ice melting rates.
Session 3, Short Temporal and/or Small Spatial Scale Processes
Tuesday, 13 May 2003, 8:30 AM-10:59 AM
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