Sort Options:

• Sort By Last Name
• Sort By Title

Other:

• Print All

Sort by time-scale:

• Short
• Intermediate
• Long
• SHOW ALL

Forecasts of Southeast Pacific Stratocumulus with the NCAR, GFDL and ECMWF Models

Forecasts of Southeast Pacific Stratocumulus with the NCAR, GFDL and ECMWF Models

Cecile Hannay, Dave Williamson, Jeff Kiehl, Jim Hack, Jerry Olson, Chris Bretherton, Steve Klein and Martin Koehler.

We examine forecasts of Southeast Pacific Stratocumulus at 20S, 85W during the East Pacific Investigation of Climate(EPIC) cruise of October 2001 with the NCAR, NCAR-UW, GFDL and ECMWF models. The NCAR-UW is a modified version of the NCAR model that uses the turbulence and shallow convection schemes developed at the University of Washington.

Observations during the EPIC cruise show a very stable and well-mixed boundary layer under a sharp inversion. The inversion height and the cloud layer have a strong and regular diurnal cycle. The forecasts are initialized from atmospheric conditions and each model is run for 5 days to determine the drift from the field data. This method allows us to diagnose parameterization deficiencies. The atmospheric initial conditions are obtained from ECMWF analyses, which provide a good estimate of the EPIC column state, although the height of the PBL and the strength of the inversion are underestimated compared to the EPIC radiosonde data.

A key problem common to the 4 models is that the forecasted PBL height is too low compared to observations. The ECMWF model shows a steady PBL with no significant decrease or increase of the inversion height. The LWP and its diurnal cycle are represented very well in the ECWMF forecasts. In particular, the daytime LWP is well captured while the other models unrealistically collapse the LWP during daytime. This affects the solar flux at the surface. The ECMWF forecasts also have some deficiencies: it produces a cloud layer too thick compared to observations. The NCAR forecasts show 2 typical behaviors: either the PBL is maintained or it collapses, while the observations during the same period do not show any shallowing of the PBL. Climate runs with the NCAR model also show an inability to maintain the proper PBL depth. When the PBL collapses, the model becomes very moist near the surface. The cloud fraction is poorly represented: the model produces an unrealistically thick layer of clouds that sometimes extends to the surface. The diurnal cycle is underestimated. The NCAR-UW forecasts show a similar maintenance or collapse of the PBL. When the PBL is maintained, the clouds are more realistic than in the NCAR model, occupying a single level. The model also better represents the diurnal cycle of the inversion height due to the entrainment of dry air at the top of the PBL. When the PBL collapses, the cloud fraction and cloud water in NCAR-UW go to zero. The GFDL forecasts have periods during which the boundary layer becomes shallower. The collapse is less dramatic than in the NCAR forecasts but it is more pronounced than in the ECMWF ones. We also present SCM simulations with the NCAR and NCAR-UW models to better understand the mechanisms that control the PBL.