Melville E. Nicholls

A comprehensive meteorological modeling system—RAMS

SummaryThis paper presents a range of applications of the Regional Atmospheric Modeling System (RAMS), a comprehensive mesoscale meterological modeling system. Applications discussed in this paper include large eddy simulations (LES) and simulations of thunderstorms, cumulus fields, mesoscale convective systems, mid-latitude cirrus clouds, winter storms, mechanically- and thermally-forced mesoscale systems, and mesoscale atmospheric disperision. A summary of current RAMS options is also presented. Improvements to RAMS currently underway include refinements to the cloud radiation, cloud microphysics, cumulus, and surface soil/vegetative parameterization schemes, the parallelization of the code, development of a more versatile visualization capability, and research into meso-α-scale cumulus parameterization.

Precision requirements for space-based XCO 2 data

Precision requirements are determined for space-based column-averaged CO_2 dry air mole fraction (X_(CO)_2) data. These requirements result from an assessment of spatial and temporal gradients in (X_(CO)_2) the relationship between (X_(CO)_2) precision and surface CO_2 flux uncertainties inferred from inversions of the (X_(CO)_2) data, and the effects of (X_(CO)_2) biases on the fidelity of CO_2 flux inversions. Observational system simulation experiments and synthesis inversion modeling demonstrate that the Orbiting Carbon Observatory mission design and sampling strategy provide the means to achieve these (X_(CO)_2) data precision requirements.

Thermally forced gravity waves in an atmosphere at rest

Abstract The transient linear response of a quiescent, two-dimensional, nonrotating atmosphere to prescribed heat sources and sinks is investigated. Analytical solutions of the hydrostatic Boussinesq equations are obtained for a rigid lid and for a semi-infinite region. For the rigid lid solution, vertically trapped gravity waves propagate away from the source with a speed that depends on the Brunt–Vaisala frequency and the vertical wavenumber of the heating. The amplitude of the disturbance field in the region of the forcing approaches a constant value. Two modes are of particular interest: 1) a deep fast-moving mode which is responsible for subsidence warming through the depth of the troposphere; 2) a slower moving mode which corresponds to midlevel inflow and lower- and upper-level outflows. A solution is also obtained for a semi-infinite region. Although gravity wave energy can now propagate upward, the structure of the low-level fields still shows many similarities with the rigid lid solution. An ana...

A Two-Dimensional Numerical Investigation of the interaction between Sea Breezes and Deep Convection over the Florida Peninsula

Abstract Deep convection initiated by sea breezes over the Florida peninsula is simulated using a two-dimensional nonhydrostatic model. Reasonable agreement is obtained between model results and observations for the three types of undisturbed days classified by Blanchard and Lopez. The convergence of the east and west coast sea breezes is the primary control on the timing and location of rapid convective development, and this is mainly determined by the low-level winds. The simulated convection is spatially concentrated and does not produce an extensive stratiform region. Sensitivity tests are performed for a variety of wind and thermodynamic profiles, and for different soil moisture contents. During the early stages of these simulations, small convective cells develop in between the sea-breeze fronts. As the outer cells at the sea-breeze fronts deepen these smaller cells are suppressed. Typically, during the midafternoon a new cell explosively develops in between the sea-breeze fronts and the outer cells...

A Composite Analysis of the Boundary Layer Accompanying a Tropical Squall Line

Abstract Rawinsonde data from the GARP Atlantic Tropical Experiment (GATE) have been used to prepare a composite of the boundary layer associated with a tropical squall line on 12 September 1974. Soundings from fifteen GATE ships have been composited with respect to the center of the squall radar echo, as determined by Gamache and Houze (1982), for a 9-hour period during which the squall system was approximately in steady-state moving to the southwest at 13.5 m s−1 Particular emphasis is given to the modified boundary layer or wake that trails the leading edge of the squall system. Immediately behind the ∼200 km wide squall line to a distance of ∼100 km exists a very cool, stable layer of air several hundred meters deep just above the ocean surface. This region is followed by a Shallow mixed layer ∼100 m deep which gradually increases to ∼500 m (a depth characteristic of undisturbed regions) at distances behind the line ranging from 500 km on the northwest side to ∼300 km on the southeast side. Surface te...

The Sensitivity of Two-Dimensional Simulations of Tropical Squall Lines to Environmental Profiles

Abstract Two dimensional experiments are carried out to determine the effect of various wind and thermodynamic structure on squall line characteristics. Two ideas concerning the effect of shear are found useful in explaining many of the outcomes of the numerical experiments. First, in two dimensions, shear in the absence of vorticity sources and sinks is detrimental to convection (Kuo, Asai). Second, there is a specific value of low-level slim interacting with a cold pool which produces deep uplift and hence strong forcing of convection (Rotunno et al.). Results suggest that moist midlevel air tends to be favorable for squall lines. Increasing the total buoyancy or altering the distribution of buoyancy with height, such that it is increased at low levels, produces stronger systems with updrafts more tilted from the vertical. The formula by von Karm´n for the speed of a gravity current gives qualitative agreement with the speed of most of the simulated systems. However, at least two additional factors need...

A Comparison of the Results of a Two-Dimensional Numerical Simulation of a Tropical Squall Line with Observations

Abstract A comparison of the results of a two-dimensional numerical simulation of a tropical squall line with observations shows good agreement in most respects. The two-dimensional model should provide a valuable research tool for understanding the physical processes that are taking place and for discovering the effects of different wind and thermodynamic environments on squall-line characteristics. In the simulation high pressure develops ahead of the system relative to that behind at most levels. The momentum flux is mainly confined to below 6 km and in the region of strong convection. The squall line is multicellular and significant perturbations are associated with convection embedded in the stratiform region. Considerable horizontal accelerations occur at large distances from the region of strong convection, away from the system aloft and towards it at low levels. The effect of excluding the ice phase in the simulation is shown to lead to cloud tops some 3 km lower than when it is incorporated. Alth...

Large eddy simulation of microburst winds flowing around a building

A large eddy simulation of a microburst producing thunderstorm is carried out. The thunderstorm is initiated when a thermal within a developing mixed layer reaches the lifting condensation level and strong latent heating occurs. A microburst is subsequently produced as condensate from the thunderstorm falls beneath the melting level. In this study, the viability of using two-way interactive multiple nested grids to investigate the interaction of the outflow from the microburst with a much smaller scale architectural structure is investigated. The relationship of the fluctuating winds around the building to the microburst structure is described.

Applications of the RAMS Numerical Model to Dispersion over Urban Areas

This paper presents an overview of the Regional Atmospheric Modeling System (RAMS) and examples of applications to the dispersion of pollutants in urban areas. Applications to be discussed include dispersion of pollutants from industrial sources near Lake Michigan, the effects of vegetation on pollutant dispersion in Athens, the depletion of ozone by thunderstorm activity over Atlanta, and small scale simulations of airflow around buildings.

Thermally Induced Compression Waves and Gravity Waves Generated by Convective Storms

Abstract A three-dimensional, fully compressible cloud model is used to simulate a convective storm in order to investigate the properties of compression waves and gravity waves induced by latent heat release. Time series of the low-level pressure perturbations caused by the propagating waves are examined at various distances from the storm. A compression wave that is close to hydrostatic balance and can be considered to be a Lamb wave, which propagates in the horizontal plane, emerges from the storm. This latter property gives the wave a distinctly two-dimensional character that is clarified by comparison with a linear model of a two-dimensional thermally induced compression wave. This has implications for its shape and results in a decay rate with distance propagated from the source of 1/(distance)1/2. The period of the Lamb wave is determined primarily by the time it takes for the storm to develop and decay. The fast-moving Lamb wave is trailed by slower-moving thermally induced gravity waves. It is fo...