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- Author or Editor: Markus Furger x
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Abstract
Uncertainties in the evaluation of the atmospheric heat budget, in which the turbulent heat flux divergence term is calculated as a residual, are investigated for a triangular array of 915-MHz wind profilers—radio acoustic sounding systems (RASS) using a surface-integral method. A scaling analysis of the residual error heat budget equation reveals the basic characteristics and magnitudes of the uncertainties. These values are verified with a Monte Carlo simulation technique for synthetic datasets in which the triangle size is of the order of 30 km (meso-γ scale). The uncertainties depend on measurement errors, atmospheric stability, mean wind speed, triangle size, and averaging time. In addition, we estimate the effects of baroclinity and mean wind divergence on the accuracy of the calculation of the heat budget.
Idealized, barotropic, and divergence-free conditions are studied to investigate the influence of various instrument accuracies on profiles of the turbulent virtual potential temperature flux divergence term. Results show that this term can be computed as a residual of the other terms with an uncertainty that varies from approximately 0.4 to 1.6 K h−1 for typical ranges of mean wind speed and stability, given current accuracies for 1-h averages of wind profiler—RASS. Uncertainties of the remaining terms in the equation are smaller. Although the uncertainties found are of about the same magnitude as typical maximum daytime boundary layer turbulent sensible heat flux divergences, 1.2 K h−1, it is found that under favorable conditions meaningful turbulent heat flux divergences can be obtained. The computations, however, become very uncertain under conditions of strong baroclinity or wind divergence.
Abstract
Uncertainties in the evaluation of the atmospheric heat budget, in which the turbulent heat flux divergence term is calculated as a residual, are investigated for a triangular array of 915-MHz wind profilers—radio acoustic sounding systems (RASS) using a surface-integral method. A scaling analysis of the residual error heat budget equation reveals the basic characteristics and magnitudes of the uncertainties. These values are verified with a Monte Carlo simulation technique for synthetic datasets in which the triangle size is of the order of 30 km (meso-γ scale). The uncertainties depend on measurement errors, atmospheric stability, mean wind speed, triangle size, and averaging time. In addition, we estimate the effects of baroclinity and mean wind divergence on the accuracy of the calculation of the heat budget.
Idealized, barotropic, and divergence-free conditions are studied to investigate the influence of various instrument accuracies on profiles of the turbulent virtual potential temperature flux divergence term. Results show that this term can be computed as a residual of the other terms with an uncertainty that varies from approximately 0.4 to 1.6 K h−1 for typical ranges of mean wind speed and stability, given current accuracies for 1-h averages of wind profiler—RASS. Uncertainties of the remaining terms in the equation are smaller. Although the uncertainties found are of about the same magnitude as typical maximum daytime boundary layer turbulent sensible heat flux divergences, 1.2 K h−1, it is found that under favorable conditions meaningful turbulent heat flux divergences can be obtained. The computations, however, become very uncertain under conditions of strong baroclinity or wind divergence.
Abstract
A severe bow-echo storm over northern Switzerland is investigated. Wind damage occurred along a track 15 km long and some 100 m wide. Damage data, meteorological data from a ground micronet, and Doppler radar data are analyzed. Volume-scan radar data in the direction of the approaching storm are available every 2.5 min.
The storm reached a weak-evolution mode when the damage occurred. Updraft impulses followed each other in time steps of typically 5 min. The damage track can be attributed to a strong radar-observed vortex of 2–7-km diameter. The vortex developed at a shear line that was formed by the downdraft outflow of an earlier thunderstorm cell. Most of the damage was collocated with the strongest Doppler winds but some of the damage occurred beneath the strongest signature of azimuthal shear. A weak tornado was observed in that shear region.
The two extremes in Doppler velocity, associated with the vortex and referred to as inflow and outflow velocities, are analyzed separately. Early strengthening of the vortex at 2–4-km altitude was due to an acceleration of inflow velocity, caused by the rising updraft impulses. Subsequent strengthening at low layers (0–2 km) could be related to acceleration of both the inflow and outflow velocities. At this stage, the diameter of the vortex decreased from about 7 to less than 2 km. The low-level intensification of the vortex is attributed to vortex stretching. Later on, the vortex and inflow velocity at low layers weakened but the outflow velocity remained strong.
Abstract
A severe bow-echo storm over northern Switzerland is investigated. Wind damage occurred along a track 15 km long and some 100 m wide. Damage data, meteorological data from a ground micronet, and Doppler radar data are analyzed. Volume-scan radar data in the direction of the approaching storm are available every 2.5 min.
The storm reached a weak-evolution mode when the damage occurred. Updraft impulses followed each other in time steps of typically 5 min. The damage track can be attributed to a strong radar-observed vortex of 2–7-km diameter. The vortex developed at a shear line that was formed by the downdraft outflow of an earlier thunderstorm cell. Most of the damage was collocated with the strongest Doppler winds but some of the damage occurred beneath the strongest signature of azimuthal shear. A weak tornado was observed in that shear region.
The two extremes in Doppler velocity, associated with the vortex and referred to as inflow and outflow velocities, are analyzed separately. Early strengthening of the vortex at 2–4-km altitude was due to an acceleration of inflow velocity, caused by the rising updraft impulses. Subsequent strengthening at low layers (0–2 km) could be related to acceleration of both the inflow and outflow velocities. At this stage, the diameter of the vortex decreased from about 7 to less than 2 km. The low-level intensification of the vortex is attributed to vortex stretching. Later on, the vortex and inflow velocity at low layers weakened but the outflow velocity remained strong.
