HR: 1340h
AN: A13B-0923    [Abstracts]
TI: Incorporating Fractional Cloud Cover in Chemical Transport Models
AU: * Neu, J L
EM: jneu@uci.edu
AF: Department of Earth System Sciences, University of California, Irvine, Croul Hall, Irvine, CA 92697-3100 United States
AU: Prather, M J
EM: mprather@uci.edu
AF: Department of Earth System Sciences, University of California, Irvine, Croul Hall, Irvine, CA 92697-3100 United States
AB: Photolysis rates are one of the key elements in the accurate simulation of atmospheric chemistry, and they are highly sensitive to the presence of clouds. We present a new quadrature-based method for incorporating fractional cloud cover that enables accurate calculation of photolysis rates within current 3D models. We use a maximum-random overlap scheme to represent the vertical coherence of the fractional cloud field and calculate the grid box-averaged photolysis rates for the full set of permutations of cloud overlap (100s-1000s of plane parallel atmospheres per grid area) using the UCI Fast-J full multiple scattering photolysis scheme. Our quadrature algorithm optimally selects four representative plane parallel atmospheres from the full set and uses those to calculate approximate grid box-averaged photolysis rates. The quadrature method reproduces the 24-hour average photolysis rates calculated with the full set of maximum-random overlap atmospheres with less than 1% bias and less than 10% root mean square error in the tropics, middle-, and high-latitudes. We also present comparisons between the maximum-random overlap photolysis rates and those calculated using a common linear scheme (in which the optical depth is scaled by cloud fraction) and a linear approximation to random overlap. Both of these schemes give large negative biases in the boundary layer and large positive biases in the upper troposphere, particularly in the tropics. We assess the importance of these biases on ozone photochemistry. We also investigate the consequences of averaging photolysis rates over the spectrum of realizations of cloud distribution in the maximum-random overlap scheme and discuss the results in the context of horizontal resolution.
DE: 0321 Cloud/radiation interaction
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005