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N. Andrew Crook

Abstract

The sensitivity of moist convection to a number of low-level thermodynamic parameters is examined with a high-resolution, nonhydrostatic numerical model. The parameters examined are the surface temperature dropoff (defined as the difference between the potential temperature measured at the surface and that in the well-mixed boundary layer), the surface moisture dropoff (defined similarly for moisture), the boundary layer moisture dropoff (defined as the vertical decrease in moisture within the boundary layer), and the depth of the moisture. The typical variability in these parameters is estimated from two field experiments in northeastern Colorado. Sensitivity is then defined relative to this typical observational variability.

Two convection initiation cases from northeastern Colorado are examined. In both cases, convection initiation is found to be most sensitive to the surface temperature dropoff and the surface moisture dropoff. It is found that variations in boundary layer temperature and moisture that are within typical observational variability (1°C and 1 g kg−1, respectively) can make the difference between no initiation and intense convection. For cases in which convection is well developed, the storm's strength is more sensitive to the typical observational variability in moisture than in temperature. However. at the convection/no convection boundary, the storm's strength is more sensitive to the surface temperature dropoff than to the surface moisture dropoff (both in terms of equivalent moist static energy and, for the cases studied, in terms of typical observational variability). It is shown that this is due to the greater sensitivity of the negative area (or convective inhibition) to temperature variations than to moisture variations. The implications of these results for the predictability of convection initiation are then briefly discussed.

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N. Andrew Crook

Abstract

Linear and nonlinear models are used to examine the development of island thunderstorms, in particular the Hector convective system that forms over the Tiwi Islands just north of Australia. The linear model is used to examine the flow response to an isolated, elliptical, heat source. It is found that the low-level convergence is maximized when the flow is weak and along the major axis of the heat source. A dry version of the nonlinear model verifies the trends predicted by the linear model except at very low flow speeds where the convergence is bounded in the nonlinear model but increases indefinitely in the linear model.

Deep convection develops over the heat source when a moisture profile with positive convective available potential energy (CAPE) is added to the nonlinear model. The sensitivity of the convective strength (defined by the accumulated rainfall and total condensate) to wind speed and direction, surface fluxes, and low-level moisture is then examined. It is shown that the strength increases as the wind speed decreases and as the wind direction turns toward the major axis of the island, in agreement with the prediction of increased low-level convergence from the linear and nonlinear dry models. Sensitivity experiments indicate that the convective strength increases as both the heat and moisture fluxes increase. The strength is more sensitive to the heat flux since this drives the large-scale convergence and sea breezes that generate convection. As the low-level moisture in the upstream sounding increases, the accumulated rainfall over the islands increases monotonically; however, the total condensate reaches a maximum at a CAPE of around 1500 J kg−1 and then decreases thereafter. It is shown that the low-level moisture is an important predictor of the form of convective development. Finally, simulations with a single coastline are performed to show that one of the reasons the Hector convective system is so strong is that it develops over an island where the land–sea circulation from all coastlines can contribute.

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N. Andrew Crook

Abstract

A numerical model is used to examine the effects of ambient stratification on the behavior of an atmospheric undular bore. It is shown that stratification reduces the amplitude of the disturbance at low levels by allowing energy to propagate upward. This reduction of amplitude can be inhibited by specifying winds opposing the wave motion in the middle and upper troposphere. Mean observations in the region where the Morning Glory is prevalent support this result.

The effects of moisture are also examined. If condensation does not occur, moisture increases the disturbance amplitude by reducing, through virtual temperature effects, the stability of the atmosphere. However, if condensation occurs, the wave amplitude is decreased compared with a dry atmosphere with the same effective stability. Finally, it is shown that cloud formation increases the wavelength of the disturbance.

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N. Andrew Crook

Abstract

The characteristics of internal gravity waves propagating on a layer of high stratification near the ground with a deeper, weakly stratified layer above are examined with the aid of a nonhydrostatic numerical model. Simulations are performed of a density current propagating into an environment with a typically observed thermodynamic structure and with no shear. These simulations indicate that the amplitude of the disturbance that forms ahead of the density current is limited considerably by the upward propagation of energy in the upper layer.

To explain the large amplitude of observed gravity waves there must exist some additional mechanism, besides the weak stratification in the upper layer, to trap energy at low levels. A thorough examination of several observed gravity wave events suggested three commonly occurring mechanisms. The first mechanism, explored in a previous paper, occurs when winds in the upper layer oppose the wave motion. This reduces the Scorer parameter l 2 = N 2/(Uc)2U″/(Uc) in the upper layer and causes waves to evanesce in that region. The second mechanism, which also depends on a reduction in the Scorer parameter, occurs when a jet exists in the lower layer that opposes the wave motion. It is shown that the curvature in the velocity profile above this jet can produce a layer of negative Scorer parameter. Numerical simulations indicate that a considerable amount of energy can be trapped below this region of curvature.

The third mechanism involves an inversion at a certain height above the lower stable layer. In this system the Scorer parameter is actually increased, however for certain inversion hieghts energy can be reflected off the inversion and lead to an enhancement of the wave amplitude at the ground.

Observations of low-level internal gravity waves are then examined in an attempt to determine the relative importance of the three trapping mechanisms in the real atmosphere. This examination suggests that the low-level opposing flow is the most prevalent mechanism for trapping energy at low levels.

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N. Andrew Crook

Abstract

The motion of a surface cold front in an environment that is unstable to moist convection is studied with the aid ofboth hydrostatic and nonhydrostatic two-dimensional models. Simulations with the hydrostatic model essentially extend the work reported by Ross and Orlanski. It is shown that when deep convection occurs, Coriolis turning of the flow into the convective line creates a poleward low-level jet ahead of the front. It is also shown that after the generation and decay of the first convective element, another line develops on the order of a day later. It is found that the intensity of this line increases significantly ira north-south gradient of moisture is specified.

The periodicity in convective activity at the front is explained in terms of an inertial gravity oscillation in the low-level convergence. The first convective system, which decays when the subeloud layer is dried out by the convection, forces a geostrophic imbalance in the surface front and the surrounding environment. In returning to geostrophic balance after the decay of the first system, the front and surrounding environment tend to oscillate as inertial gravity waves propagate away from the region of imbalance. It is shown that the convergence at low levels ahead of the front oscillates with a period of approximately 12 hours and that ascent returns to the frontal zone 6 hours after the decay of the first system.. The second system then develops when this low-level convergence destabilizes the atmosphere. The discrepancy between the inertial gravity wave period and the period between convective line generation obtained in the hydrostatic model (20-27 h) is explained by the crude representationof moist convection in the hydrostatic, filtered model. A nonhydrostatic model with horizontal resolution of 2.8 km is used to study the same system and a periodicity in convective activity is again found, this time with the second system commencing some 7 hours after the decay of the first.

Finally, observations of cold fronts in the Midwest of the United States are analyzed to explore the importance of convective oscillations in the motion of surface cold fronts.

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Juanzhen Sun
and
N. Andrew Crook

Abstract

The adjoint technique for retrieval of the thermodynamic fields is compared with the traditional technique of Gal-Chen and Hane. The comparison is performed using both Doppler radar observations and simulated data. The real dataset is a gust-front case observed during the Phoenix II experiment. The simulated data are from a numerical experiment of a collapsing cold pool. In the simulated data study, we assume that observations of the horizontal velocity are available, either from a dual-Doppler synthesis or from a single-Doppler retrieval. Tests are performed on data that have been degraded in various ways to replicate real data. These tests include the sensitivity to the temporal sampling frequency, random error, spatially correlated error, and divergent/rotational error in the forcing terms. In most of the cases examined, it is found that the adjoint method is able to retrieve the buoyancy field more accurately than the traditional technique.

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Donna F. Tucker
and
N. Andrew Crook

Abstract

A mesoscale convective system (MCS) that formed just to the east of Denver is investigated with a nonhydrostatic numerical model to determine which processes were important in its initiation. The MCS developed from outflow from previous convective activity in the Rocky Mountains to the west. Model results indicate that this outflow was necessary for the development of the MCS even though a convergence line was already present in the area where the MCS developed. A simulation with a 3-km grid spacing more fully resolves the convective activity in the mountains but the development of the MCS can be simulated with a 6.67-km grid. Cloud effects on solar radiation and ice sedimentation both influence the strength of the outflow from the mountain convection but only the ice sedimentation makes a significant impact on the development of the MCS after its initiation.

The frequent convective activity in the Rocky Mountains during the warm season provides outflow that would make MCS generation favorable in this region. Thus, there is a close connection between mountain convective activity and MCS generation. The implications of such a connection are discussed and possible directions of future research are indicated.

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N. Andrew Crook
and
Donna F. Tucker

Abstract

The flow past heated topography is examined with both linear and nonlinear models. It is first shown that the forcing of an obstacle with horizontally homogenous surface heating can be approximated by the forcing of an obstacle with surface heating isolated over the obstacle. The small-amplitude flow past an obstacle with isolated heating is then examined with a linear model. Under the linear approximation, the flow response to heated topography is simply the addition of the separate responses to thermal and orographic forcing. These separate responses are first considered individually and then the combined response is examined. Nondimensional parameters are developed that measure the relative importance of thermal and orographic forcing. Nonaxisymmetric forcing is then considered by examining the flow along and across a heated elliptically shaped obstacle. It is shown that the low-level lifting is maximized when the flow is along the major axis of the obstacle.

The linear solutions are then tested in a nonlinear anelastic model. The response to a heat source and orography are first examined separately. Good agreement is found between nonlinear and linear models for the individual responses to thermal and orographic forcing. The case of uniformly heated flow past an obstacle is then examined. In these simulations, the thermal response is isolated by subtracting the orographic-only response from the full thermal–orographic response. The numerical simulations are able to capture the main features of the thermal response. Finally, numerical simulations of the flow along and across an elliptically shaped heated obstacle are examined, where it is verified that the lifting is maximized when the flow is along the major axis of the obstacle.

These results are extended in Part II of this study to examine the moist convective response to flow over both idealized terrain and the complex terrain of the Rocky Mountains of the United States.

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Donna F. Tucker
and
N. Andrew Crook

Abstract

Previous studies have shown that thunderstorms in the Rocky Mountain region have preferred areas in which to form. There has been some indication that these areas depend on the midtropospheric wind direction. A nonhydrostatic model with a terrain-following horizontal grid is employed to investigate the initiation of precipitating convection over heated topography. Horizontally homogeneous meteorological conditions with no directional shear in the vertical wind profile are used.

The numerical simulations indicate that precipitating convection was more likely to be generated downwind of ridges than upwind of them. Initiation of these storms was more likely downwind of ridges with their long axis parallel to the wind direction than downwind of ridges with their long axis perpendicular to the wind direction. In Part I of this study it was shown that heating-induced convergence is larger downwind of a ridge with its longer axis parallel to the wind direction. For the orographic configuration of the Rocky Mountains, total precipitation is maximized for southerly and northwesterly winds. Slower wind speeds are more likely and faster wind speeds are less likely to produce convective storms. Soundings with larger instability are more likely to produce convection. The soundings with a greater temperature lapse rate produce more initiation locations, and soundings with greater moisture produce greater amounts of precipitation.

Even though a number of assumptions were made for this study, the authors believe the results explain a significant amount of the observed variability in the initiation locations of precipitating convection in the Rocky Mountains during the summer. Because of the theoretical basis for this work, detailed in Part I of this study, the authors believe it should explain convective initiation in other mountainous areas that are subject to strong solar heating.

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Juanzhen Sun
and
N. Andrew Crook

Abstract

The purpose of the research reported in this paper is to develop a variational data analysis system that can be used to assimilate data from one or more Doppler radars. In the first part of this two-part study, the technique used in this analysis system is described and tested using data from a simulated warm rain convective storm. The analysis system applies the 4D variational data assimilation technique to a cloud-scale model with a warm rain parameterization scheme. The 3D wind, thermodynamical, and microphysical fields are determined by minimizing a cost function, defined by the difference between both radar observed radial velocities and reflectivities (or rainwater mixing ratio) and their model predictions. The adjoint of the numerical model is used to provide the sensitivity of the cost function with respect to the control variables.

Experiments using data from a simulated convective storm demonstrated that the variational analysis system is able to retrieve the detailed structure of wind, thermodynamics, and microphysics using either dual-Doppler or single-Doppler information. However, less accurate velocity fields are obtained when single-Doppler data were used. In both cases, retrieving the temperature field is more difficult than the retrieval of the other fields. Results also show that assimilating the rainwater mixing ratio obtained from the reflectivity data results in a better performance of the retrieval procedure than directly assimilating the reflectivity. It is also found that the system is robust to variations in the Zq r relation, but the microphysical retrieval is quite sensitive to parameters in the warm rain scheme. The technique is robust to random errors in radial velocity and calibration errors in reflectivity.

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