2004
Alan Berman Research
Publication Awards
Last Updated: Tuesday, 15-Jul-2008 11:05:01 PDT
Recent Modifications of
the Emanuel convective Scheme in the Navy
Operational Global Atmospheric Prediction System
MELINDA S. PENG, JAMES A. RIDOUT AND TIMOTHY
F. HOGAN
Marine Meteorology Division, Naval Research Laboratory
Monterey, California
ABSTRACT
The convective parameterization of Emanuel has been employed in the forecast model of the Navy Operational Global Atmospheric Prediction System (NOGAPS) since 2000, when it replaced a version of the relaxed Arakawa-Schubert scheme. Although in long-period data assimilation forecast tests the Emanuel scheme has been found to perform quite well I NOGAPS, particularly for tropical cyclones, some weaknesses have also become apparent. These weaknesses include underprediction of heavy-precipitation evens, too much light precipitation, and unrealistic heating at upper levels. Recent research efforts have resulted in modifications of the scheme that are designed to reduce such problems. One change described here involves the partitioning of the cloud-base mass flux into mixing cloud mass flux at individual levels. The new treatment significantly reduces a heating anomaly near the tropopause that is associated with a large amount of mixing cloud mass flux ascribed to that region in the original Emanuel scheme. In another modification, the selection of the updraft source level is buoyancy at cloud-base level in the Emanuel scheme. Test results suggest that the modified scheme may in some cases better represent precipitation during the middle and latter stages of convective events. The scheme has also been modified to eliminate cloud-top overshooting. The parameterization changes are supported in part by diagnostic tests, including semiprognostic model tests using observed data and sing-column model tests using cloud-resolving-scale simulation data. The modifications showed significant positive impacts in forecast experiments over the original designs and have been implemented into the operational NOGAPS.
Estimation of observation impact using the NRL atmospheric variational data assimilation adjoint system
ROLF H. LANGLAND AND NANCY BAKER
Marine Meteorology Division, Naval Research Laboratory, Monterey, California
ABSTRACT
An adjoint-based procedure for assessing
the impact of observations on the short-range forecast error in numerical
weather prediction is
described. The method is computationally inexpensive and allows observation
impact to be partitioned for any set or subset of observations, by instrument
type, observed variable, geographic region, vertical level or other category.
The cost function is the difference between measures of 24h and 30-h global
forecast error in the Navy Operational Global Atmospheric Prediction System
(NOGAPS) during June
and December 2002. Observations are assimilated at 00UTC in the Naval
Research Laboratory (NRL) Atmospheric Variational Data Assimilation System
(NAVDAS). The largest error reductions in the Northern Hemisphere are
produced by rainsondes, satellite wind data, and aircraft observations.
In the Southern Hemisphere, the largest error reductions are produced
by Advanced TIROS Operational Vertical Sounder (ATOVS) temperature retrievals,
satellite wind data and rawinsondes. Approximately 60% (40%) of global
observation impact is attributed to observations below (above) 500 hPa.
A significant correlation is found between observation impact and cloud
cover at the observation location. Currently, without consideration of
moisture observations and moist processes in the forecast model adjoint,
the observation impact procedure accounts for about 75% of the actual
reduction in 24-h forecast error.
