Search Results
You are looking at 1 - 10 of 202 items for
- Author or Editor: J. Brown x
- Refine by Access: All Content x
Abstract
The hydrodynamic instability characteristics of planetary zonal flows are investigated through use of a quasi-geostrophic numerical model of high spatial resolution. An initial-value technique is employed to obtain solutions of the linear problem.
Certain zonal flows containing both vertical and lateral shears, which are representative of those observed in the earth's atmosphere, are found to be unstable with respect to the large-scale quasi-geostrophic disturbances. Westerly currents, each characterized by a latitudinally symmetric jet containing absolute vorticity extrema at various latitudes, amplify perturbations of some scales through a dominating baroclinic mechanism, and amplify perturbations of other scales through a dominating barotropic mechanism. For these flows, the unstable perturbations of relatively short zonal wavelength convert zonal available potential energy into perturbation energy and simultaneously strengthen the zonal kinetic energy of the basic flow. On the other hand, the unstable perturbations of relatively long zonal wavelength reduce both the zonal kinetic and available potential energies of the basic flow, with the former reduction dominating. For certain flows, these combinations produce two distinct wavelengths of maximum instability. Flows which are similar but contain no vanishing meridional gradient of absolute vorticity are found to produce baroclinically unstable perturbations with a tendency toward barotropic damping.
Abstract
The hydrodynamic instability characteristics of planetary zonal flows are investigated through use of a quasi-geostrophic numerical model of high spatial resolution. An initial-value technique is employed to obtain solutions of the linear problem.
Certain zonal flows containing both vertical and lateral shears, which are representative of those observed in the earth's atmosphere, are found to be unstable with respect to the large-scale quasi-geostrophic disturbances. Westerly currents, each characterized by a latitudinally symmetric jet containing absolute vorticity extrema at various latitudes, amplify perturbations of some scales through a dominating baroclinic mechanism, and amplify perturbations of other scales through a dominating barotropic mechanism. For these flows, the unstable perturbations of relatively short zonal wavelength convert zonal available potential energy into perturbation energy and simultaneously strengthen the zonal kinetic energy of the basic flow. On the other hand, the unstable perturbations of relatively long zonal wavelength reduce both the zonal kinetic and available potential energies of the basic flow, with the former reduction dominating. For certain flows, these combinations produce two distinct wavelengths of maximum instability. Flows which are similar but contain no vanishing meridional gradient of absolute vorticity are found to produce baroclinically unstable perturbations with a tendency toward barotropic damping.
Abstract
The numerical model of Part I is used in certain experiments designed to reveal special instability effects caused by the vertical walls which specify the lateral boundary conditions at the northern and southern boundaries of the atmosphere. In these examples the walls suppress instability of the barotropically dominated perturbations and have little influence on the westward progressions of the unstable waves.
Small-scale eddy momentum and heat diffusion processes are simulated in an example of a basic westerly wind field containing absolute vorticity extrema. The inclusion of these mechanisms is found to inhibit instabilities of all zonal wavelengths, with major effects noted for short shallow waves. The significant modifying influence is attributed to large effects of drag at the ground.
The behavior of an unstable wave interacting with the zonal current is obtained through nonlinear numerical calculations. The equilibrium state approached in the presence of a time-independent diabatic heating differential oscillates about a steady state. The several energy conversion rates vary in time in such a way as to minimize the time-rates-of-change of the different types of energies. The equilibrium energy levels appear to be governed by the required baroclinic process, and the resulting period of the oscillating regime is dictated by the barotropic mechanism.
Abstract
The numerical model of Part I is used in certain experiments designed to reveal special instability effects caused by the vertical walls which specify the lateral boundary conditions at the northern and southern boundaries of the atmosphere. In these examples the walls suppress instability of the barotropically dominated perturbations and have little influence on the westward progressions of the unstable waves.
Small-scale eddy momentum and heat diffusion processes are simulated in an example of a basic westerly wind field containing absolute vorticity extrema. The inclusion of these mechanisms is found to inhibit instabilities of all zonal wavelengths, with major effects noted for short shallow waves. The significant modifying influence is attributed to large effects of drag at the ground.
The behavior of an unstable wave interacting with the zonal current is obtained through nonlinear numerical calculations. The equilibrium state approached in the presence of a time-independent diabatic heating differential oscillates about a steady state. The several energy conversion rates vary in time in such a way as to minimize the time-rates-of-change of the different types of energies. The equilibrium energy levels appear to be governed by the required baroclinic process, and the resulting period of the oscillating regime is dictated by the barotropic mechanism.
Abstract
A multiple linear regression equation is developed which relates the yield of winter wheat to “effective” precipitation during certain months of a crop year. Antecedent precipitation and other weather factors are investigated to determine their effect on yields.
Abstract
A multiple linear regression equation is developed which relates the yield of winter wheat to “effective” precipitation during certain months of a crop year. Antecedent precipitation and other weather factors are investigated to determine their effect on yields.
Abstract
In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.
Abstract
In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.
Abstract
The uncertainty in the projection of future drought occurrence was explored for four different drought indices using two model ensembles. The first ensemble expresses uncertainty in the parameter space of the third Hadley Centre climate model, and the second is a multimodel ensemble that additionally expresses structural uncertainty in the climate modeling process. The standardized precipitation index (SPI), the precipitation and potential evaporation anomaly (PPEA), the Palmer drought severity index (PDSI), and the soil moisture anomaly (SMA) were derived for both a single CO2 (1×CO2) and a double CO2 (2×CO2) climate. The change in moderate drought, defined by the 20th percentile of the relevant 1×CO2 distribution, was calculated. SPI, based solely on precipitation, shows little change in the proportion of the land surface in drought. All the other indices, which include a measure of the atmospheric demand for moisture, show a significant increase with an additional 5%–45% of the land surface in drought. There are large uncertainties in regional changes in drought. Regions where the precipitation decreases show a reproducible increase in drought across ensemble members and indices. In other regions the sign and magnitude of the change in drought is dependent on index definition and ensemble member, suggesting that the selection of appropriate drought indices is important for impact studies.
Abstract
The uncertainty in the projection of future drought occurrence was explored for four different drought indices using two model ensembles. The first ensemble expresses uncertainty in the parameter space of the third Hadley Centre climate model, and the second is a multimodel ensemble that additionally expresses structural uncertainty in the climate modeling process. The standardized precipitation index (SPI), the precipitation and potential evaporation anomaly (PPEA), the Palmer drought severity index (PDSI), and the soil moisture anomaly (SMA) were derived for both a single CO2 (1×CO2) and a double CO2 (2×CO2) climate. The change in moderate drought, defined by the 20th percentile of the relevant 1×CO2 distribution, was calculated. SPI, based solely on precipitation, shows little change in the proportion of the land surface in drought. All the other indices, which include a measure of the atmospheric demand for moisture, show a significant increase with an additional 5%–45% of the land surface in drought. There are large uncertainties in regional changes in drought. Regions where the precipitation decreases show a reproducible increase in drought across ensemble members and indices. In other regions the sign and magnitude of the change in drought is dependent on index definition and ensemble member, suggesting that the selection of appropriate drought indices is important for impact studies.
Abstract
Data from eight water vapor soundings made with the dew-point hygrometer instrument at McMurdo Sound, Antarctica, are presented. These data are compared with various north temperate latitude data. The soundings are too sparse to present a valid time cross section, but structure (layering) is evident on individual soundings. The mean Antarctic summer mixing ratio profile shows decreasing moisture to 4×10−6 gm/gm at the tropopause, then increasing to 1×10−4 gm/gm at 30 km. The Antarctic stratosphere appears to be as moist as the mid-latitude stratosphere, but both sets of data are influenced by an unknown amount of sample contamination.
Abstract
Data from eight water vapor soundings made with the dew-point hygrometer instrument at McMurdo Sound, Antarctica, are presented. These data are compared with various north temperate latitude data. The soundings are too sparse to present a valid time cross section, but structure (layering) is evident on individual soundings. The mean Antarctic summer mixing ratio profile shows decreasing moisture to 4×10−6 gm/gm at the tropopause, then increasing to 1×10−4 gm/gm at 30 km. The Antarctic stratosphere appears to be as moist as the mid-latitude stratosphere, but both sets of data are influenced by an unknown amount of sample contamination.
Abstract
The “anomaly correlation” between a predicted time series and a verifying series, both of which have had the same climatic history subtracted, combines their “direct” correlation with their two climate correlations and with the variance of the historical series. The direct correlation can be freed of climatic effects to become the “partial” correlation, which therefore seems a better parameter for judging predictive skill.
Abstract
The “anomaly correlation” between a predicted time series and a verifying series, both of which have had the same climatic history subtracted, combines their “direct” correlation with their two climate correlations and with the variance of the historical series. The direct correlation can be freed of climatic effects to become the “partial” correlation, which therefore seems a better parameter for judging predictive skill.
Abstract
Summertime cloud-to-ground lightning strikes are responsible for the majority of wildfire ignitions across vast sections of the seasonally dry western United States. In this study, a strong connection between active phases of the Madden–Julian oscillation (MJO) and regional summertime lightning activity was found across the interior western United States. This intraseasonal mode of lightning activity emanates northward from the desert Southwest across the Great Basin and into the northern Rocky Mountains. The MJO is shown to provide favorable conditions for the northward propagation of widespread lightning activity through the amplification of the upper-level ridge over the western United States and the development of midtropospheric instability. Given the relative predictability of the MJO with long lead times, results allude to the potential for intraseasonal predictability of lightning activity and proactive fire management planning.
Abstract
Summertime cloud-to-ground lightning strikes are responsible for the majority of wildfire ignitions across vast sections of the seasonally dry western United States. In this study, a strong connection between active phases of the Madden–Julian oscillation (MJO) and regional summertime lightning activity was found across the interior western United States. This intraseasonal mode of lightning activity emanates northward from the desert Southwest across the Great Basin and into the northern Rocky Mountains. The MJO is shown to provide favorable conditions for the northward propagation of widespread lightning activity through the amplification of the upper-level ridge over the western United States and the development of midtropospheric instability. Given the relative predictability of the MJO with long lead times, results allude to the potential for intraseasonal predictability of lightning activity and proactive fire management planning.
Abstract
This paper reports on results of idealized numerical simulations testing the influence of low-level humidity, and thus lifting condensation level (LCL), on the morphology and evolution of low-level rotation in supercell thunderstorms. Previous studies have shown that the LCL can influence outflow buoyancy, which can in turn affect generation and stretching of near-surface vertical vorticity. A less explored hypothesis is tested: that the LCL affects the relative positioning of near-surface circulation and the overlying mesocyclone, thus influencing the dynamic lifting and intensification of near-surface vertical vorticity. To test this hypothesis, a set of three base-state thermodynamic profiles with varying LCLs are implemented and compared over a variety of low-level wind profiles. The thermodynamic properties of the simulations are sensitive to variations in the LCL, with higher LCLs contributing to more negatively buoyant cold pools. These outflow characteristics allow for a more forward propagation of near-surface circulation relative to the midlevel mesocyclone. When the mid- and low-level mesocyclones become aligned with appreciable near-surface circulation, favorable dynamic updraft forcing is able to stretch and intensify this rotation. The strength of the vertical vorticity generated ultimately depends on other interrelated factors, including the amount of near-surface circulation generated within the cold pool and the buoyancy of storm outflow. However, these simulations suggest that mesocyclone alignment with near-surface circulation is modulated by the ambient LCL, and is a necessary condition for the strengthening of near-surface vertical vorticity. This alignment is also sensitive to the low-level wind profile, meaning that the LCL most favorable for the formation of intense vorticity may change based on ambient low-level shear properties.
Abstract
This paper reports on results of idealized numerical simulations testing the influence of low-level humidity, and thus lifting condensation level (LCL), on the morphology and evolution of low-level rotation in supercell thunderstorms. Previous studies have shown that the LCL can influence outflow buoyancy, which can in turn affect generation and stretching of near-surface vertical vorticity. A less explored hypothesis is tested: that the LCL affects the relative positioning of near-surface circulation and the overlying mesocyclone, thus influencing the dynamic lifting and intensification of near-surface vertical vorticity. To test this hypothesis, a set of three base-state thermodynamic profiles with varying LCLs are implemented and compared over a variety of low-level wind profiles. The thermodynamic properties of the simulations are sensitive to variations in the LCL, with higher LCLs contributing to more negatively buoyant cold pools. These outflow characteristics allow for a more forward propagation of near-surface circulation relative to the midlevel mesocyclone. When the mid- and low-level mesocyclones become aligned with appreciable near-surface circulation, favorable dynamic updraft forcing is able to stretch and intensify this rotation. The strength of the vertical vorticity generated ultimately depends on other interrelated factors, including the amount of near-surface circulation generated within the cold pool and the buoyancy of storm outflow. However, these simulations suggest that mesocyclone alignment with near-surface circulation is modulated by the ambient LCL, and is a necessary condition for the strengthening of near-surface vertical vorticity. This alignment is also sensitive to the low-level wind profile, meaning that the LCL most favorable for the formation of intense vorticity may change based on ambient low-level shear properties.
Abstract
For the Salt Lake Valley and adjacent western slopes of the Wasatch Mountains 25-, 50-, and 100-year snow loads were determined, using the theory of extreme probability. Maximum seasonal snow depths were used to estimate the maximum water equivalent at stations where the latter was not measured. Snow loads, in turn, were computed from maximum water equivalents. A high correlation between elevation and maximum snow load was found.
Abstract
For the Salt Lake Valley and adjacent western slopes of the Wasatch Mountains 25-, 50-, and 100-year snow loads were determined, using the theory of extreme probability. Maximum seasonal snow depths were used to estimate the maximum water equivalent at stations where the latter was not measured. Snow loads, in turn, were computed from maximum water equivalents. A high correlation between elevation and maximum snow load was found.
