A high-resolution Large Eddy Simulation (LES) that utilizes a recently-developed nested grid method is being used to study entrainment processes in the capping inversion of Planetary Boundary Layers (PBLs). Recently, Peter Sullivan and Chin-Hoh Moeng have been able to identify actively entraining convective plumes in a dry, shear-free convective PBL with a weak capping inversion, and to follow the evolution of these plumes in time. The accompanying animation shows the temporal and spatial evolution of potential temperature contours and velocity vectors in a partial 2D yz plane taken from our 3D simulations. In the simulations, the entrainment mechanism is dominated by large-eddy impingement and engulfment processes. The bulk Richardson number, based on the strength of the inversion and the convective velocity scale for the present simulation, is about 20. The fine-nested grid spacing [(dx,dy,dz) = (33,33,10) m] is depicted in the upper right-hand corner of the animation.
The STERAO (Stratosphere-Troposphere Experiments: Radiation, Aerosols, and Ozone) Deep Convection Experiment conducted in northeastern Colorado in the summer of 1996 by James Dye as well as other researchers from the Atmospheric Chemistry Division (ACD), the Atmospheric Technology Division (ATD), the NOAA Aeronomy Laboratory, Colorado State University (CSU), University of North Dakota (UND), and several other institutions. The experiment combined extensive chemical, air motion, and electrical measurements of thunderstorms. This unique combination of measurements was used to examine the effects of deep convection, including transport and NOx production by lightning, on the chemical composition and vertical distribution of constituents in the troposphere and tropopause regions. Data used as forecasting aids were obtained from: (1) airborne instruments on the NOAA (National Oceanic and Atmospheric Administration) P3 and UND Citation aircraft; (2) the CSU/CHILL (University of Chicago/Illinois State Water Survey Radar) multiparameter Doppler radar; (3) the French ONERA (Office National d'Etudes et de Recherches Aerospatiales) lightning interferometer (for 3D mapping of lightning discharges); (4) surface detection of both cloud-to-ground and intra-cloud lightning; (5) NCAR mobile class soundings; and (6) output from the MM5 (NCAR/Pennsylvania State University Mesoscale Model, Version 5) model, maintained by NCAR's MMM Division, and the University of Maryland/Goddard (Space Flight Center) Cumulus Ensemble model. Results from the project will be the focus of much analysis in the next few years.
Radar measurements made during the Small Cumulus Microphysics Study led by Charles Knight in Florida in 1995 show the existence of adiabatic cores in newly formed clouds up to about 1 km above cloud base, where the maximum reflectivity factor becomes too large for just cloud droplets, indicating coalescence growth. Comparisons by Knight, William Cooper, and Jean-Louis Brenguier [MMM/ATD (Atmospheric Technology Division, NCAR) affiliate scientist from Meteo France, Toulouse] of growth rates expected for the combined effects of condensation and coalescence were in reasonable agreement with the observations of droplet size distributions during the early stages of development in these adiabatic regions. In the later stages of development in mixed regions the agreement with observations was not as good as in the ascent of unmixed parcels.
A study of the relationship of TOGA COARE (Tropical Ocean and Global Atmosphere Program Coupled Ocean-Atmosphere Response Experiment) convective structure to the environment has been completed by Margaret LeMone and Edward Zipser (Texas A&M University), with the help of Michael Dey (former MMM student assistant) and several of Zipser's A&M students. The relationship of the convective structure to the vertical shear of the horizontal wind was found to be similar to that for GATE [GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment] and AMTEX (Air Mass Transformation Experiment). COARE convective soundings are deeper, and hence have higher CaPE (Convection and Precipitation/Electrification Experiment) than those for GATE, but CaPE evaluated to 500 mb is about the same.
The thermodynamic profile for the TOGA COARE 22 February 1993 squall line was used with two different wind hodographs (9 Feb, 22 Feb) to demonstrate the simulation of two distinct types of convective bands from a single cell. William Skamarock and Stanley Trier performed this demonstration using an adaptive grid model developed by Skamarock and Louis Wicker (Texas A&M University).
A 3D, non-hydrostatic, time-dependent Navier-Stokes solver has been implemented by William Anderson and Piotr Smolarkiewicz. The solver uses both HPF (High Performance Fortran) and MSG (message passing) for parallelization, and has Eulerian and semi-Lagrangian algorithms. The performances of the two parallelization methods on a Cray T3D have been compared for simulations of low Froude number flow past a mountain. The semi-Lagrangian model admits large time steps, so its overall performance is superior to the Eulerian model despite its inherent irregular communication patterns. It is to be used to simulate cloud systems in cold air outbreaks, a new project to be coordinated by the Clouds in Climate Program (CCP).
Cloud-resolving modeling by Xiaoqing Wu (long-term visitor), Wojciech Grabowski, and Mitchell Moncrieff faithfully reproduces the evolution among various types of GATE convective cloud systems in three spatial dimensions. Even shallow shear-parallel lines which are not well resolved are reproduced, demonstrating that large-scale variables (e.g., shear and forcing) are the key quantities. Also, for the first time, a squall line was shown to evolve spontaneously from a field of clouds. This is good news for deterministic parameterizations of organized cloud systems for GCMs (General Circulation Models).
Studies of flow over Lantau Island at the site of a new airport in Hong Kong have pioneered simulations of mechanical turbulence. High resolution simulations showed how, in topographically distorted flow, mean shears became dynamically unstable and broke down into transient eddies. A new visualization and analysis technique developed by Terry Clark shows the structure of separation bubbles in flow over the mountainous island.
A coupled atmosphere-fire numerical model developed by Terry Clark revealed the causes of common wildfire shapes and behavior and demonstrated how rotating columns near the fire front can rapidly intensify into a dangerous blowup. In agreement with observations, short-line fires bowed into a conical shape, while longer fire lines bent into multiple conical shapes due to feedback between the hot convective plumes and near-surface convergence at the fire front. Other experiments showed that when negative ambient wind shear existed, a pair of rotating updrafts at the fire front can touch down within the fire and break up the fire line.
Mesoscale convective vortices (MCVs) play an important role in the evolution of mesoscale convective systems as well as in the triggering of new convection. Through the use of idealized non-hydrostatic numerical cloud model simulations, Morris Weisman and Christopher Davis (joint appointment with RAP) have identified the mechanisms by which these MCVs are produced, and their dependence on the strength of the ambient vertical windshear. These results emphasize that divergent, convective motion must be included in numerical or theoretical models that are designed to study or forecast the generation of such features.
The interaction of fronts with steep coastal orography can promote a variety of cool-season weather events such as high winds associated with barrier jets and heavy orographically enhanced rainfall. Scott Braun (Advanced Studies Program (ASP) post-doctoral visitor), Richard Rotunno, and Joseph Klemp have completed a numerical study of these interactions, and found that while the front is strongly modified by the orography, the important interaction is the modification of the mountain flow by the stability perturbations of the front, rather than the development of across-frontal circulations induced by the mountain. Superposition of the barrier jet with frontal jets determines the winds in the coastal zone (e.g., a southerly, prefrontal jet can combine with the barrier jet to produce very strong winds prior to frontal passage).
An analysis of diurnally forced, anticyclonic circulations over northeastern Colorado was conducted by Christopher Davis with the aid of the WISP94 (Winter Icing and Storm Project 94) data set. Davis's results suggest that anticyclonic vorticity generation occurs within the lowest kilometer AGL (Above Ground Level) when that layer is nearly vertically mixed in both potential temperature and velocity. The mixed-layer mechanism is also favored as it naturally explains the diurnal tendency of the circulations, and may therefore explain the observed late-day snowfall maximum along the Front Range of Colorado.
Version 2 of the MM5 model was released to the user community in the summer of 1996. Developed primarily by Jimy Dudhia, Wei Wang, Daniel Hansen, and Sue Chen (former MMM associate scientist), this model contains several new options for physical parameterization. These include the Betts-Miller, Kain-Fritsch, and Fritsch-Chappell cumulus parameterization schemes; the Burk-Thompson second order, level 3 PBL scheme; and the Goddard-3 ice cloud microphysics scheme.