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A transitory pattern of waves associated with an Alberta hailstorm is described. Aerial photographs are compared with radar and sounding data. The waves appear to have been gravity waves in a layer of marked stability overlying the boundary layer.
A transitory pattern of waves associated with an Alberta hailstorm is described. Aerial photographs are compared with radar and sounding data. The waves appear to have been gravity waves in a layer of marked stability overlying the boundary layer.
Abstract
Two simple equations are used for calculating the shapes of the axes of updrafts in convective storms, based on consideration of changes of horizontal momentum in rising air. Momentum change parameters are obtained empirically by a comparison with observations of two storms. The results indicate that both mixing and aerodynamic drag are important influences on updrafts.
Abstract
Two simple equations are used for calculating the shapes of the axes of updrafts in convective storms, based on consideration of changes of horizontal momentum in rising air. Momentum change parameters are obtained empirically by a comparison with observations of two storms. The results indicate that both mixing and aerodynamic drag are important influences on updrafts.
Abstract
Statistics of echoes observed on day 261 of GATE, using the radar aboard the Canadian ship Quadra, are described. This is a case in which detailed observations have been obtained from many sensors during the growth of a cloud cluster.
Dividing the radar field of view into many “bins,” it was found that rainfall rates and echo top heights were log-normally distributed UP to a certain limit, with a few additional high values. After identifying echo cores, bounded by a reflectivity threshold 24 dB above 1 mm6 m−3, their area were found to be distributed in a similar manner.
Upper limits of log-normality before (and during) the growth of the cluster were about 5 (and 30) mm h−1 for rainfall rate, 7 (and 7) km for echo top height (at the 29 dB level of intensity) and 100 (and 400) km2 for area (within contours of intensity 24 dBZ). To echoes organized on a length scale of about 100 km for more, one attributes maximum rainfall rates reaching roughly 50 mm h−1, peak heights reaching 14 km, and maximum areas of elongated cores reaching 5000 km2.
Abstract
Statistics of echoes observed on day 261 of GATE, using the radar aboard the Canadian ship Quadra, are described. This is a case in which detailed observations have been obtained from many sensors during the growth of a cloud cluster.
Dividing the radar field of view into many “bins,” it was found that rainfall rates and echo top heights were log-normally distributed UP to a certain limit, with a few additional high values. After identifying echo cores, bounded by a reflectivity threshold 24 dB above 1 mm6 m−3, their area were found to be distributed in a similar manner.
Upper limits of log-normality before (and during) the growth of the cluster were about 5 (and 30) mm h−1 for rainfall rate, 7 (and 7) km for echo top height (at the 29 dB level of intensity) and 100 (and 400) km2 for area (within contours of intensity 24 dBZ). To echoes organized on a length scale of about 100 km for more, one attributes maximum rainfall rates reaching roughly 50 mm h−1, peak heights reaching 14 km, and maximum areas of elongated cores reaching 5000 km2.
Abstract
The one-dimensional propagation of a spectrum of gravity waves through a realistic middle atmosphere is investigated, separating as far as possible the propagation-invariant aspects from the more empirical wave-breaking and other nonlinear aspects. The latter are parameterized by a simple broadband spectral saturation criterion, but the conceptual framework allows for other wave-breaking parameterizations. An upward propagating initial or “launch” spectrum is prescribed in the lower stratosphere. The propagation aspects are handled with careful attention to the mappings and their Jacobians between spectral spaces.
Results for several test cases produce realistic behavior, including cases where some of the waves are back-reflected, as in the summer stratosphere, with much of the spectrum propagating conservatively through substantial altitude ranges. Any launch spectrum can be used in the computational scheme; for definiteness attention is concentrated on the model spectrum of Fritts and VanZandt, but sensitivity tests are also carried out in which the shape and total energy are varied. Other sensitivity tests include varying the steepness of the saturation criterion. The shapes and magnitudes of the computed profiles of wave-induced force, as a function of altitude, are sensitive to some of these changes, especially to the asymptotic shape of the launch spectrum at the smallest values of vertical wavenumber m, about which there is little direct observational evidence. However, the maxima and minima of the profiles are located at similar altitudes in each case. Besides pointing, to ways of improving gravity wave parameterization schemes for general circulation models, the results may help to tighten observational constraints on spectra for small m.
Abstract
The one-dimensional propagation of a spectrum of gravity waves through a realistic middle atmosphere is investigated, separating as far as possible the propagation-invariant aspects from the more empirical wave-breaking and other nonlinear aspects. The latter are parameterized by a simple broadband spectral saturation criterion, but the conceptual framework allows for other wave-breaking parameterizations. An upward propagating initial or “launch” spectrum is prescribed in the lower stratosphere. The propagation aspects are handled with careful attention to the mappings and their Jacobians between spectral spaces.
Results for several test cases produce realistic behavior, including cases where some of the waves are back-reflected, as in the summer stratosphere, with much of the spectrum propagating conservatively through substantial altitude ranges. Any launch spectrum can be used in the computational scheme; for definiteness attention is concentrated on the model spectrum of Fritts and VanZandt, but sensitivity tests are also carried out in which the shape and total energy are varied. Other sensitivity tests include varying the steepness of the saturation criterion. The shapes and magnitudes of the computed profiles of wave-induced force, as a function of altitude, are sensitive to some of these changes, especially to the asymptotic shape of the launch spectrum at the smallest values of vertical wavenumber m, about which there is little direct observational evidence. However, the maxima and minima of the profiles are located at similar altitudes in each case. Besides pointing, to ways of improving gravity wave parameterization schemes for general circulation models, the results may help to tighten observational constraints on spectra for small m.
Abstract
This paper describes a new computationally efficient, ultrasimple nonorographic spectral gravity wave parameterization model. Its predictions compare favorably, though not perfectly, with a model of gravity wave propagation and breaking that computes the evolution with altitude of a full, frequency- and wavenumber-dependent gravity wave spectrum. The ultrasimple model depends on making the midfrequency (hydrostatic, nonrotating) approximation to the dispersion relation, as in Hines’ parameterization. This allows the full frequency–wavenumber spectrum of pseudomomentum flux to be integrated with respect to frequency, and thus reduced to a spectrum that depends on vertical wavenumber m and azimuthal direction ϕ only. The ultrasimple model treats the m dependence as consisting of up to three analytically integrable segments, or “parts.” This allows the total pseudomomentum flux to be evaluated by using analytical expressions for the areas under the parts rather than by performing numerical quadratures. The result is a much greater computational efficiency.
The model performs significantly better than an earlier model that treated the m dependence as consisting of up to two parts. Numerical experiments show that similar models with more than three parts using the midfrequency approximation yield little further improvement. The limiting factor is the midfrequency approximation and not the number of parts.
Abstract
This paper describes a new computationally efficient, ultrasimple nonorographic spectral gravity wave parameterization model. Its predictions compare favorably, though not perfectly, with a model of gravity wave propagation and breaking that computes the evolution with altitude of a full, frequency- and wavenumber-dependent gravity wave spectrum. The ultrasimple model depends on making the midfrequency (hydrostatic, nonrotating) approximation to the dispersion relation, as in Hines’ parameterization. This allows the full frequency–wavenumber spectrum of pseudomomentum flux to be integrated with respect to frequency, and thus reduced to a spectrum that depends on vertical wavenumber m and azimuthal direction ϕ only. The ultrasimple model treats the m dependence as consisting of up to three analytically integrable segments, or “parts.” This allows the total pseudomomentum flux to be evaluated by using analytical expressions for the areas under the parts rather than by performing numerical quadratures. The result is a much greater computational efficiency.
The model performs significantly better than an earlier model that treated the m dependence as consisting of up to two parts. Numerical experiments show that similar models with more than three parts using the midfrequency approximation yield little further improvement. The limiting factor is the midfrequency approximation and not the number of parts.
Abstract
Coupled ocean–atmospheric general circulation models indicate that warming of up to 3°C may occur over the next century in waters immediately to the north of the Amery Ice Shelf. The impact of this warming on the ocean cavity under the Amery Ice Shelf and the mass exchange at the interface between the ocean cavity and the ice shelf is investigated using a three-dimensional ocean model. Warming of between 0.25° and 3.0°C is applied along the ice front in a series of model runs, rather than in a single transient run. Changes in salinity are also considered for larger amounts of warming. The model results show that the circulation in the ocean cavity changes as warming increases, particularly in the gyres that dominate the horizontal circulation. The changes in the heat flux from the warming increase the melt rates from the base of the Amery Ice Shelf, from the present-day mean melt rate and net mass loss estimates of 0.28 m yr−1 and 14.2 Gt yr−1, respectively, by approximately 0.55 m yr−1°C−1 and 28.4 Gt yr−1°C−1. The maximum melt rates increase much more strongly, by around 10 m yr−1°C−1. These increased rates of melting suggest substantial modification of the ice shelf would occur in a warmer climate, particularly near the grounding line, and thus indicate that warming of the oceans around Antarctica has the potential for significant impact on the Antarctic ice sheet.
Abstract
Coupled ocean–atmospheric general circulation models indicate that warming of up to 3°C may occur over the next century in waters immediately to the north of the Amery Ice Shelf. The impact of this warming on the ocean cavity under the Amery Ice Shelf and the mass exchange at the interface between the ocean cavity and the ice shelf is investigated using a three-dimensional ocean model. Warming of between 0.25° and 3.0°C is applied along the ice front in a series of model runs, rather than in a single transient run. Changes in salinity are also considered for larger amounts of warming. The model results show that the circulation in the ocean cavity changes as warming increases, particularly in the gyres that dominate the horizontal circulation. The changes in the heat flux from the warming increase the melt rates from the base of the Amery Ice Shelf, from the present-day mean melt rate and net mass loss estimates of 0.28 m yr−1 and 14.2 Gt yr−1, respectively, by approximately 0.55 m yr−1°C−1 and 28.4 Gt yr−1°C−1. The maximum melt rates increase much more strongly, by around 10 m yr−1°C−1. These increased rates of melting suggest substantial modification of the ice shelf would occur in a warmer climate, particularly near the grounding line, and thus indicate that warming of the oceans around Antarctica has the potential for significant impact on the Antarctic ice sheet.
Abstract
Data from a wide variety of measurement platforms are integrated to analyze a GATE cloud cluster and its environment. Lower tropospheric mass and moisture fluxes are computed on several scales using rawinsonde, aircraft, radar and satellite data from 18 September 1974. Lower tropospheric mass and moisture budget analyses are combined with GATE tethered-balloon measurements of cloud properties at cloud base to diagnose active cloud and turbulent fluxes in three areas of different scale ranging from 2.3-34.8 × 1010 m 2 (1.8-28.2° latitude)2.
The significant convection was concentrated in the region of strongest and deepest low-level convergence and highest conditional instability. Observed layer clouds were well correlated with levels of synoptic-scale divergence. Despite large differences in low-level convergence and echo-area coverage, all three regions exhibited similar vertical eddy moisture fluxes at cloud base. Increased area coverage by precipitating clouds was found to be associated with increased subsidence and reduced turbulent moisture flux in the air outside of these clouds.
Abstract
Data from a wide variety of measurement platforms are integrated to analyze a GATE cloud cluster and its environment. Lower tropospheric mass and moisture fluxes are computed on several scales using rawinsonde, aircraft, radar and satellite data from 18 September 1974. Lower tropospheric mass and moisture budget analyses are combined with GATE tethered-balloon measurements of cloud properties at cloud base to diagnose active cloud and turbulent fluxes in three areas of different scale ranging from 2.3-34.8 × 1010 m 2 (1.8-28.2° latitude)2.
The significant convection was concentrated in the region of strongest and deepest low-level convergence and highest conditional instability. Observed layer clouds were well correlated with levels of synoptic-scale divergence. Despite large differences in low-level convergence and echo-area coverage, all three regions exhibited similar vertical eddy moisture fluxes at cloud base. Increased area coverage by precipitating clouds was found to be associated with increased subsidence and reduced turbulent moisture flux in the air outside of these clouds.
Abstract
The impact of a parameterized spectrum of gravity waves on the simulation of the stratosphere in the Met Office Unified Model (UM) is investigated. In the extratropical mesosphere, the gravity wave forcing acts against the mean zonal wind and it dominates over the resolved wave forcing. In the extratropical stratosphere, the gravity wave forcing gives a small acceleration in the direction of the mean zonal wind. Both summer and winter stratospheric jets have improved maximum strength and tilt with height when the parameterized gravity wave forcing is included, although the southern winter jet is still more vertically aligned than in observational analyses. The timing of the seasonal breakdown of the southern winter vortex is also improved by the addition of gravity wave forcing. In the Tropics, the most obvious impact is that the model reproduces the quasi-biennial oscillation (QBO) with a realistic mean and range of periods. It also reproduces most of the observed asymmetries between the easterly and westerly phases of the oscillation. The sensitivity of this modeled QBO to horizontal diffusion parameters is investigated and it is shown that diffusion set to damp out grid-length disturbances can also attenuate the QBO due to its long period, particularly in the narrower westerly phase.
Abstract
The impact of a parameterized spectrum of gravity waves on the simulation of the stratosphere in the Met Office Unified Model (UM) is investigated. In the extratropical mesosphere, the gravity wave forcing acts against the mean zonal wind and it dominates over the resolved wave forcing. In the extratropical stratosphere, the gravity wave forcing gives a small acceleration in the direction of the mean zonal wind. Both summer and winter stratospheric jets have improved maximum strength and tilt with height when the parameterized gravity wave forcing is included, although the southern winter jet is still more vertically aligned than in observational analyses. The timing of the seasonal breakdown of the southern winter vortex is also improved by the addition of gravity wave forcing. In the Tropics, the most obvious impact is that the model reproduces the quasi-biennial oscillation (QBO) with a realistic mean and range of periods. It also reproduces most of the observed asymmetries between the easterly and westerly phases of the oscillation. The sensitivity of this modeled QBO to horizontal diffusion parameters is investigated and it is shown that diffusion set to damp out grid-length disturbances can also attenuate the QBO due to its long period, particularly in the narrower westerly phase.
Abstract
We describe a procedure for the initialization of the divergent wind component in a mesoscale, numerical weather prediction model and evaluate it in terms of its ability to provide an improved short-range precipitation forecast. The divergent wind component was obtained from a vertical velocity field that was diagnosed using an omega equation. The diabatic term of the omega equation was dominant in regions of observed precipitation. Five precipitation forecasts were performed for the same 12 h period but each was initialized in a different manner. One procedure combined the diagnosed divergent component with the observed nondivergent wind. This total wind was used in a divergent balance equation to obtain the temperature field. The precipitation forecast based on these initial conditions was compared with those started from nondivergent, balanced initial conditions as well as from unbalanced data that contained the observed divergent component. The use of the diagnosed divergence significantly improved the precipitation forecast during the first three hours compared to the nondivergent forecasts. The forecast based on the unbalanced data with the observed divergence showed no improvement. A large fraction of the initial, diagnosed divergence was retained by the model because areas of observed precipitation were saturated in the initial moisture analysis causing the initial upward vertical motions to be sustained by latent heating. After 6 h of integration, the precipitation rates for all the forecasts were approximately the same.
Abstract
We describe a procedure for the initialization of the divergent wind component in a mesoscale, numerical weather prediction model and evaluate it in terms of its ability to provide an improved short-range precipitation forecast. The divergent wind component was obtained from a vertical velocity field that was diagnosed using an omega equation. The diabatic term of the omega equation was dominant in regions of observed precipitation. Five precipitation forecasts were performed for the same 12 h period but each was initialized in a different manner. One procedure combined the diagnosed divergent component with the observed nondivergent wind. This total wind was used in a divergent balance equation to obtain the temperature field. The precipitation forecast based on these initial conditions was compared with those started from nondivergent, balanced initial conditions as well as from unbalanced data that contained the observed divergent component. The use of the diagnosed divergence significantly improved the precipitation forecast during the first three hours compared to the nondivergent forecasts. The forecast based on the unbalanced data with the observed divergence showed no improvement. A large fraction of the initial, diagnosed divergence was retained by the model because areas of observed precipitation were saturated in the initial moisture analysis causing the initial upward vertical motions to be sustained by latent heating. After 6 h of integration, the precipitation rates for all the forecasts were approximately the same.
