Search Results
You are looking at 1 - 8 of 8 items for
- Author or Editor: R. Gall x
- Refine by Access: All Content x
Abstract
The sensitivity of the nocturnal minimum temperature over dry soil under clear calm conditions to changes in the distribution of temperature and humidity aloft is estimated. To make the estimates, the minimum temperature is calculated by means of a simple radiative-conductive model for thermal equilibrium at the surface. Changes due to systematic variations in the temperature and dewpoint depression of three tropospheric layers are computed. The results indicate that relatively small changes in temperature and humidity occurring aloft in the lower layers of the atmosphere produce changes of several degrees in the nocturnal minimum temperature at screen height.
Abstract
The sensitivity of the nocturnal minimum temperature over dry soil under clear calm conditions to changes in the distribution of temperature and humidity aloft is estimated. To make the estimates, the minimum temperature is calculated by means of a simple radiative-conductive model for thermal equilibrium at the surface. Changes due to systematic variations in the temperature and dewpoint depression of three tropospheric layers are computed. The results indicate that relatively small changes in temperature and humidity occurring aloft in the lower layers of the atmosphere produce changes of several degrees in the nocturnal minimum temperature at screen height.
Abstract
A numerical method is shown for solving problems of nonaxisymmetric perturbations in incompressible, inviscid, swirling flow for arbitrary undisturbed azimuthal and axial flows. Exclusive attention is given to the hydrodynamic stability at large azimuthal and axial wavenumbers for a flow with a zone of constant and coinciding vertical and azimuthal vorticities. An infinite sequence of unstable modes is found having very large amplitude within the verticity zone. Analysis of a 3-level model confirms the instability in the 46-level numerical model and shows the instability criterion to be krζ z + nζθ < 0, where k (>0) and n (≥0) are axial and azimuthal wavenumbers, respectively, ζ z (>0) and ζθ (≥0) are the corresponding vorticities, and r is radius. Maximum instability occurs approximately where k and n are related by 2krζ z + nζθ = 0, provided ζθ is small compared to ζ z . Growth rates, for a tornado vortex of typical dimensions and vorticities, are significant fractions of a second.
Abstract
A numerical method is shown for solving problems of nonaxisymmetric perturbations in incompressible, inviscid, swirling flow for arbitrary undisturbed azimuthal and axial flows. Exclusive attention is given to the hydrodynamic stability at large azimuthal and axial wavenumbers for a flow with a zone of constant and coinciding vertical and azimuthal vorticities. An infinite sequence of unstable modes is found having very large amplitude within the verticity zone. Analysis of a 3-level model confirms the instability in the 46-level numerical model and shows the instability criterion to be krζ z + nζθ < 0, where k (>0) and n (≥0) are axial and azimuthal wavenumbers, respectively, ζ z (>0) and ζθ (≥0) are the corresponding vorticities, and r is radius. Maximum instability occurs approximately where k and n are related by 2krζ z + nζθ = 0, provided ζθ is small compared to ζ z . Growth rates, for a tornado vortex of typical dimensions and vorticities, are significant fractions of a second.
Abstract
The barotropic instability of a tornado described by Hoecker is investigated. It is found that wavenumbers of the order of 1 to 5 are significantly unstable, with growth rates of the order of 0.1 to 0.5 s−1, yielding e-folding times small compared to tornado lifetimes. The instability is related to departures from the wind distribution of a combined Rankine vortex. Small departures can yield significant growth rates.
Also investigated is the barotropic instability of an idealized vortex with a core of constant angular velocity and constant vorticity surrounded by a belt with twice the core vorticity, extending out to the wind maximum, beyond which the vorticity vanishes. Growth rates of the order of 0.1 s−1 are found at wavenumbers between 3.6 and 4.0, with negligible growth rates outside this region.
It is suggested that the instability may lead ultimately to the “suction vortices” described by Fujita in connection with tornadoes and dust devils.
Abstract
The barotropic instability of a tornado described by Hoecker is investigated. It is found that wavenumbers of the order of 1 to 5 are significantly unstable, with growth rates of the order of 0.1 to 0.5 s−1, yielding e-folding times small compared to tornado lifetimes. The instability is related to departures from the wind distribution of a combined Rankine vortex. Small departures can yield significant growth rates.
Also investigated is the barotropic instability of an idealized vortex with a core of constant angular velocity and constant vorticity surrounded by a belt with twice the core vorticity, extending out to the wind maximum, beyond which the vorticity vanishes. Growth rates of the order of 0.1 s−1 are found at wavenumbers between 3.6 and 4.0, with negligible growth rates outside this region.
It is suggested that the instability may lead ultimately to the “suction vortices” described by Fujita in connection with tornadoes and dust devils.
Abstract
A four-level quasigeostrophic model of a baroclinic atmosphere is used to examine the instability of short (∼2000 km) baroclinic waves. It is determined that only a slight decrease in the low-level static stability or increase in the low-level wind shear relative to the stale stability and wind shear in the middle and upper troposphere can mean the difference between the maximum growth rate occurring at a wavelength of 4000 km (∼wavenumber 7) or 2000 km (∼wavenumber 15). Similar changes of static stability in the upper troposphere relative to the middle and lower troposphere have very little effect on the growth-rate spectrum.
This effect of vertical variations in the static stability and wind shear on the growth-rate spectrum is consistent with the structure of the short wavelengths. Wavelengths <3000 km are essentially confined below 500 mb, while wavelengths >4000 km extend through the depth of the troposphere. Therefore, changes in the static stability of the basic zonal flow near the earth's surface have a more profound effect on the short wavelengths than on the longer waves.
It is noted that the spurious short-wave neutrality shifts to shorter and shorter wavelengths as the number of model levels is increased. This shift is related to lowering of the level of maximum vertical velocity with decreasing wavelength until, at a sufficiently short wavelength, the difference form of ∂ω/∂p in the lowest layer fails to describe the derivative accurately.
Abstract
A four-level quasigeostrophic model of a baroclinic atmosphere is used to examine the instability of short (∼2000 km) baroclinic waves. It is determined that only a slight decrease in the low-level static stability or increase in the low-level wind shear relative to the stale stability and wind shear in the middle and upper troposphere can mean the difference between the maximum growth rate occurring at a wavelength of 4000 km (∼wavenumber 7) or 2000 km (∼wavenumber 15). Similar changes of static stability in the upper troposphere relative to the middle and lower troposphere have very little effect on the growth-rate spectrum.
This effect of vertical variations in the static stability and wind shear on the growth-rate spectrum is consistent with the structure of the short wavelengths. Wavelengths <3000 km are essentially confined below 500 mb, while wavelengths >4000 km extend through the depth of the troposphere. Therefore, changes in the static stability of the basic zonal flow near the earth's surface have a more profound effect on the short wavelengths than on the longer waves.
It is noted that the spurious short-wave neutrality shifts to shorter and shorter wavelengths as the number of model levels is increased. This shift is related to lowering of the level of maximum vertical velocity with decreasing wavelength until, at a sufficiently short wavelength, the difference form of ∂ω/∂p in the lowest layer fails to describe the derivative accurately.
Abstract
Since 1988, what appears to be an abnormal number of maximum temperature records has been set at the National Weather Service Office in Tucson, Arizona (TUS). We present several analyses that indicate that the current measurement system at TUS is indicating daytime temperatures that are 2 to 3 degrees too high. It appears that the instrument is not appropriately aspirated so that, during the day, temperature readings are significantly warmer than ambient air temperatures, while at night they are slightly cooler. The system at TUS is similar to one that has been installed at many National Weather Service sites around the country. We speculate on the impact this system may have on the climate record if the errors noted at Tucson are similar at the other sites.
Abstract
Since 1988, what appears to be an abnormal number of maximum temperature records has been set at the National Weather Service Office in Tucson, Arizona (TUS). We present several analyses that indicate that the current measurement system at TUS is indicating daytime temperatures that are 2 to 3 degrees too high. It appears that the instrument is not appropriately aspirated so that, during the day, temperature readings are significantly warmer than ambient air temperatures, while at night they are slightly cooler. The system at TUS is similar to one that has been installed at many National Weather Service sites around the country. We speculate on the impact this system may have on the climate record if the errors noted at Tucson are similar at the other sites.
Abstract
Gravity waves forced by nonhydrostatic and nongeostrophic processes within a frontal zone are discussed. In particular, stationary waves immediately above and below the surface front are considered.
The waves that appear above the front are horizontally stationary with respect to the front, but are vertically propagating. The vertical wavelength here is given by 2πυ/N, since the waves are nearly hydrostatic.
The horizontal wavelength of the waves above the front is determined by standing waves that set up below the Front. These waves corrugate the frontal surface, and these corrugations, in turn, determine the horizontal scale of the waves above the front.
The waves under the front are standing and are trapped between the earth's surface and the frontal zone which, due to its conditions of flow reversal and small Ri, is assumed to be a reflector of gravity waves. The horizontal scale of the standing waves is determined by their vertical wavelength and the slope of the frontal surface. These waves are shown to break, and additional stationary waves appear above each of the breaking zones.
We suggest that the waves described here might account for some of the banding seen in satellite images of frontal zones.
Abstract
Gravity waves forced by nonhydrostatic and nongeostrophic processes within a frontal zone are discussed. In particular, stationary waves immediately above and below the surface front are considered.
The waves that appear above the front are horizontally stationary with respect to the front, but are vertically propagating. The vertical wavelength here is given by 2πυ/N, since the waves are nearly hydrostatic.
The horizontal wavelength of the waves above the front is determined by standing waves that set up below the Front. These waves corrugate the frontal surface, and these corrugations, in turn, determine the horizontal scale of the waves above the front.
The waves under the front are standing and are trapped between the earth's surface and the frontal zone which, due to its conditions of flow reversal and small Ri, is assumed to be a reflector of gravity waves. The horizontal scale of the standing waves is determined by their vertical wavelength and the slope of the frontal surface. These waves are shown to break, and additional stationary waves appear above each of the breaking zones.
We suggest that the waves described here might account for some of the banding seen in satellite images of frontal zones.
Abstract
A series of numerical experiments of a surface front forced by stretching deformation using Clark's nonhydrostatic model at very high resolution is presented. These simulations are compared to those reported by Williams who used hydrostatic models at lower resolution. The main purpose was to determine whether this front would collapse (in the absence of friction) to a scale similar to that reported by Shapiro et al. (most of the temperature gradient contained in 200 m). The question is whether there is a natural physical process in the frontal dynamics which limits the frontal collapse in the absence of diffusion processes.
For this front we could not find a natural limiting process, although the mechanism discussed by Orlanski et al. appears to be operating. The minimum scale is determined by the vertical resolution. At the vertical and horizontal resolutions we tried, the vertical resolution determined the scale because the slope of the front is so shallow.
Some of the structure found by Cullen and Purser by extending the semigeostrophic models beyond the initial development of a discontinuity is apparent in our solutions.
Abstract
A series of numerical experiments of a surface front forced by stretching deformation using Clark's nonhydrostatic model at very high resolution is presented. These simulations are compared to those reported by Williams who used hydrostatic models at lower resolution. The main purpose was to determine whether this front would collapse (in the absence of friction) to a scale similar to that reported by Shapiro et al. (most of the temperature gradient contained in 200 m). The question is whether there is a natural physical process in the frontal dynamics which limits the frontal collapse in the absence of diffusion processes.
For this front we could not find a natural limiting process, although the mechanism discussed by Orlanski et al. appears to be operating. The minimum scale is determined by the vertical resolution. At the vertical and horizontal resolutions we tried, the vertical resolution determined the scale because the slope of the front is so shallow.
Some of the structure found by Cullen and Purser by extending the semigeostrophic models beyond the initial development of a discontinuity is apparent in our solutions.
Abstract
The Hurricane Weather Research and Forecasting Model (HWRF) is an operational model used to provide numerical guidance in support of tropical cyclone forecasting at the National Hurricane Center. HWRF is a complex multicomponent system, consisting of the Weather Research and Forecasting (WRF) atmospheric model coupled to the Princeton Ocean Model for Tropical Cyclones (POM-TC), a sophisticated initialization package including a data assimilation system and a set of postprocessing and vortex tracking tools. HWRF’s development is centralized at the Environmental Modeling Center of NOAA’s National Weather Service, but it incorporates contributions from a variety of scientists spread out over several governmental laboratories and academic institutions. This distributed development scenario poses significant challenges: a large number of scientists need to learn how to use the model, operational and research codes need to stay synchronized to avoid divergence, and promising new capabilities need to be tested for operational consideration. This article describes how the Developmental Testbed Center has engaged in the HWRF developmental cycle in the last three years and the services it provides to the community in using and developing HWRF.
Abstract
The Hurricane Weather Research and Forecasting Model (HWRF) is an operational model used to provide numerical guidance in support of tropical cyclone forecasting at the National Hurricane Center. HWRF is a complex multicomponent system, consisting of the Weather Research and Forecasting (WRF) atmospheric model coupled to the Princeton Ocean Model for Tropical Cyclones (POM-TC), a sophisticated initialization package including a data assimilation system and a set of postprocessing and vortex tracking tools. HWRF’s development is centralized at the Environmental Modeling Center of NOAA’s National Weather Service, but it incorporates contributions from a variety of scientists spread out over several governmental laboratories and academic institutions. This distributed development scenario poses significant challenges: a large number of scientists need to learn how to use the model, operational and research codes need to stay synchronized to avoid divergence, and promising new capabilities need to be tested for operational consideration. This article describes how the Developmental Testbed Center has engaged in the HWRF developmental cycle in the last three years and the services it provides to the community in using and developing HWRF.
