Observations show conflicting relationships between sea surface temperature (SST) and deep convection in the tropics. Xiaoqing Wu (joint with CGD) and Mitchell Moncrieff showed that it is only when SST variations are positively correlated with the large-scale forcing that a positive correlation between SST variations and convection exists. Because large-scale dynamics has been shown to be paramount to both convective intensity and cloud amount, the ‘thermostat hypothesis’ as presently formulated is not universal, a result that quantifies an important issue concerning the role of clouds in climate.

  By comparing two-dimensional and three-dimensional cloud-resolving model simulations of cloud systems occurring in GATE and by examining the effects of horizontal resolution, Wojciech Grabowski, Wu, and Moncrieff showed that as long as high-frequency temporal variability is not of primary importance, two-dimensional simulations on a 2-km grid realizes the bulk properties of tropical cloud systems. Therefore, a relatively simple modeling framework can provide a physical basis for the large-scale role of cloud systems and thereby improve their parameterization in climate models.

  Analysis of measurements by Lynn Russell (ASP) and Donald Lenschow (joint appointment with ATD) from a Lagrangian experiment carried out by the NCAR C-130 aircraft during the Aerosol Characterization Experiment (ACE-1) southwest of Tasmania, Australia, shows that exchange of air can occur in both directions across the top of the boundary layer even in clear air (see figure). They demonstrated this primarily by using aerosol and dimethyl sulfide as tracers of vertical transport. The air above the boundary layer was intermittently turbulent due to wind shear, but was still distinctly different from the boundary layer. This process has important implications in understanding the evolution of trace constituents in the marine boundary layer.

  Juanzhen Sun and N. Andrew Crook (joint appointment with RAP) applied the adjoint data assimilation system to observations collected by an operational NEXRAD radar. Using single Doppler data from the NEXRAD radar in Memphis, Tennessee, the three-dimensional wind and buoyancy field in the boundary layer was retrieved. These retrieved fields were then used in a correlation analysis with storm growth. Decay and moderate correlation coefficients were found.

  In their work on flow over complex terrain and mountain wave activity, William Hall, Terry Clark, Robert Kerr (joint appointment with CGD and HAO), and Larry Radke (ATD) using new high resolution simulations (in collaboration with lidar observations by Ralph, Levenson and Banta (NOAA/ETL)) demonstrated new sources of vorticity and turbulence consistent with lidar and satellite data collected on 9 December 1992. A physical hypothesis for the DC-8 incident over Evergreen, Colorado was established.

  Clark and Janice Coen demonstrated capabilities of coupled atmosphere-fire modeling in explaining fire shape, fingers of flame shooting ahead of the fireline, and how negative wind shear may contribute to explosive wildfire behavior. Together with Radke and Donald Middleton (SCD), data collected with an infrared camera during prescribed burns in the Northwest Territories was analyzed to produce temperatures, velocities, heat fluxes, and time scales of features in the burning forest plot.

  Many observed features associated with coastally-trapped disturbances along the west coast of the U.S. were captured in recent idealized numerical simulations conducted by William Skamarock, Richard Rotunno, and Joseph Klemp. Simulations using a 3D primitive equation model show that an imposed offshore flow that is observed in the climatological synoptic-scale precursor flow advects the marine layer offshore, weakens the prevailing northernly flow in the marine layer, and lowers the pressure at the coast. The nearly-balanced marine-layer flow around this low pressure raises the marine layer to the south of the low as the flow encounters the coastal mountains. This flow propagates northward along the coast as a Kelvin wave and that may later steepen into a bore or gravity current.

  In a collaborative effort, NCEP, NOAA/FSL, CAPS, MMM, and university scientists have begun the development of a new Weather Research and Forecast (WRF) Model that will advance the understanding and prediction of important mesoscale weather, and promote closer ties between the research and operational communities. The model is intended to be accurate and efficient across a broad range of scales (cloud to synoptic), and to be well suited for applications ranging from idealized research to operational forecasting. The model will be freely distributed and supported as a "community" model for the benefit of university researchers and to facilitate the transfer of research advances into operational forecasting.

  Andrew Heymsfield, Larry Miloshevich, and Steven Aulenbach, along with Glen Sachse (NASA Langley) and Sam Oltmans (NOAA) found that the relative humidities with respect to water which are required to form ice crystals in cirrus clouds decline from almost 100% near 40 degrees C to 75 or 80% from -55 to -65 degrees C. This is consistent with their earlier measurements and the notion of homogeneous nucleation of solution droplets. But it is noteworthy that high relative humidities, approaching 90%, were measured in clear air at -52 degrees C off the coast of California and relative humidities approaching 100% were observed in orographic wave clouds between -62 and -65 degrees C. These results indicate that very high relative humidities can build up at low temperatures in instances with high vertical velocities and possibly with depletion of cloud condensation nuclei, thus retarding the formation of ice crystals. These regions provide conditions highly favorable for contrail formation by aircraft.

  Scientists within the Mesoscale Prediction Group of the MMM Division are developing a variational data assimilation system based on MM5. During the last year, development of the tangent linear and adjoint of a "dry" version of MM5 was completed; these models were released to the users community. Although still incomplete, this primary version of the MM5 variational data assimilation system already offers the possibility of performing minimization in a twin experiment configuration and carrying out useful sensitivity experiments (see figure). A tutorial was offered at the time of the release and was attended by more than 30 interested users from NCAR and the university community.