Effect of Precipitation and Clouds on Ocean Surface Fluxes at Different Spatial Scales

A number of observational studies have shown the significant modulation of surface fluxes by precipitation convection using the GATE data over the eastern tropical North Atlantic and using the pre-COARE and COARE data over the western Pacific warm pool region. Similar results were also obtained using cloud-resolvable models. Despite its importance, most of the current bulk algorithms do not consider the mesoscale enhancement of surface fluxes due to precipitation and clouds.

In order to consider the above mesoscale enhancement in surface flux parameterization, Redelsperger et al. (1998) introduced a mesoscale gustiness as a logarithmic function of precipitation rate or cloud mass flux. However, the above parameterization didn't consider the impact of clouds, mean wind, or spatial scales.

Recently, we have developed a comprehensive bulk algorithm for ocean surface fluxes that considers the effect on wind gustiness of boundary layer large eddies and mesoscale enhancement due to both precipitation and clouds (Zeng et al. 1999). Furthermore, we have considered the dependence of our scheme on spatial scales. For instance, mesoscale gustiness increases rapidly with horizontal scale L when L is less than 100 km, and does not change much when L is greater than 200 km.

We have developed the above algorithm based on the analyses of the two-dimensional Goddard Cloud-Ensemble (GCE) model output for three cases over the tropical Oceans. We will analyze both 2-D and 3-D GCE outputs over different regions and use observational data to further evaluate our algorithm. These results will be presented at the Conference.