The marine atmospheric boundary layer over the eastern Pacific and its simulation in climate models

The atmospheric boundary layer (ABL) height (h) is an essential parameter in weather and climate models. An analysis of almost 1000 soundings from 11 cruises between 1995 and 2001 in the eastern Pacific reveals large meridional, zonal, seasonal, and interannual variations in h. When compared with h produced by a couple of climate models (NCAR's CCSM and NCEP's Climate Forecasting System or CFS), the boundary layer height parameterization is shown to need improvement particularly in certain regions of the eastern Pacific. CCSM's h is generally underestimated especially north of 20°N in August and September, and CFS's h is overestimated in the same region. Directly applying the radiosonde data to these models' formulations for the determination of h at their respective vertical resolutions reveals that the CCSM formulation significantly underestimates h and that the CFS formulation almost always overestimates h. A simple adjustment of h to cloud top or bottom based upon cloud thickness is shown to improve CCSM's determination of h. When this adjustment of h to cloud top or bottom is implemented into the atmospheric component of the CCSM coupled to the land component, the simulated stratus and stratocumulus clouds are significantly changed. At the Conference, we will show how the model simulation of other parameters are changed by this adjustment as well.

We have also analyzed field experimental data over the eastern Pacific as well as other oceanic regions to document the probability distribution functions of cloud base, top, thickness, and cloud liquid water path (LWP). The relationship of stratus/stratocumulus cloud fraction and depth with LWP has also been established. In our presentation, we will also discuss these results and new results from the evaluation of the CCSM and CFS output using these data.