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- Author or Editor: Markus Furger x
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Abstract
Elevated moisture layers in the lower free troposphere (2000–6000 m MSL) in the lee of the Alps were investigated. Specific humidity was analyzed within a Lagrangian concept for fair-weather days during a 12-yr period at the windward and the leeward sides of the Alps for the sounding sites of Payerne, Switzerland, and Milan, Italy. During daytime fair-weather conditions (different criteria), specific humidity increased significantly in air masses that advected from Payerne to Milan in a layer ranging from ∼2500 to 4000 m MSL. The maximum relative increase of specific humidity in this layer was ∼0.3, meaning that ∼30% of the air in this layer originated from the Alpine atmospheric boundary layer. On average, ∼30% of the mass of the Alpine boundary layer was vented to altitudes higher than 2500 m MSL per hour during the daytime. The total precipitable water within a layer reaching from 2500 to 3500 m MSL increased by ∼1.3 mm. Similar elevated layers were observed for different selection methods of fair-weather days, and climatologically for the whole of June, July, and August. Average observations of the relative increase and boundary layer export rate agree with results from the local case studies. Daytime thermally driven flow systems seem to be the main source of additional water vapor in the observed elevated layers over the Alps. Subsequently, horizontal advection toward flat terrain where the average ABL top was well below the elevated layer bottom results in the export of ABL air to the free troposphere (mountain venting). Mountain venting was enhanced in situations with larger global radiation, lower atmospheric stability, and additional moist convection as was detected by lightning activity.
Abstract
Elevated moisture layers in the lower free troposphere (2000–6000 m MSL) in the lee of the Alps were investigated. Specific humidity was analyzed within a Lagrangian concept for fair-weather days during a 12-yr period at the windward and the leeward sides of the Alps for the sounding sites of Payerne, Switzerland, and Milan, Italy. During daytime fair-weather conditions (different criteria), specific humidity increased significantly in air masses that advected from Payerne to Milan in a layer ranging from ∼2500 to 4000 m MSL. The maximum relative increase of specific humidity in this layer was ∼0.3, meaning that ∼30% of the air in this layer originated from the Alpine atmospheric boundary layer. On average, ∼30% of the mass of the Alpine boundary layer was vented to altitudes higher than 2500 m MSL per hour during the daytime. The total precipitable water within a layer reaching from 2500 to 3500 m MSL increased by ∼1.3 mm. Similar elevated layers were observed for different selection methods of fair-weather days, and climatologically for the whole of June, July, and August. Average observations of the relative increase and boundary layer export rate agree with results from the local case studies. Daytime thermally driven flow systems seem to be the main source of additional water vapor in the observed elevated layers over the Alps. Subsequently, horizontal advection toward flat terrain where the average ABL top was well below the elevated layer bottom results in the export of ABL air to the free troposphere (mountain venting). Mountain venting was enhanced in situations with larger global radiation, lower atmospheric stability, and additional moist convection as was detected by lightning activity.
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.
Abstract
Several large-aperture scintillometers were built at the Paul Scherrer Institute with the aim to measure wind over complex terrain. A prototype instrument was tested over flat ground, and the performance of six analyzing techniques was evaluated by comparing them with conventional anemometers. Next, a set of five improved scintillometers was used in an experiment over complex terrain. This experiment represents a unique opportunity for evaluating scintillometer performance by comparing their results to sodar, aircraft, and ground station measurements. The results complement and partly contradict the observations previously published; the so-called peak technique is the most reliable and frequency techniques fail to provide faithful results in many cases. The measurements demonstrate that scintillometry is useful and reliable for wind and turbulence measurements over complex terrain.
Abstract
Several large-aperture scintillometers were built at the Paul Scherrer Institute with the aim to measure wind over complex terrain. A prototype instrument was tested over flat ground, and the performance of six analyzing techniques was evaluated by comparing them with conventional anemometers. Next, a set of five improved scintillometers was used in an experiment over complex terrain. This experiment represents a unique opportunity for evaluating scintillometer performance by comparing their results to sodar, aircraft, and ground station measurements. The results complement and partly contradict the observations previously published; the so-called peak technique is the most reliable and frequency techniques fail to provide faithful results in many cases. The measurements demonstrate that scintillometry is useful and reliable for wind and turbulence measurements over complex terrain.
Abstract
Measurements of the horizontal and vertical wind component by a crosswind scintillometer during foehn, the chinooklike downslope windstorm in the Alps, are presented. Because of the sparsity of vertical velocity measurements in the immediate vicinity, the scintillometer calibration is checked mainly with horizontal wind measurements. Then it is assumed that the calibration is the same for both components. The concept was tested during the Mesoscale Alpine Programme field campaign in the autumn of 1999, during which two scintillometers were deployed. Strong, long-lasting, quasi-stationary downward motions on the order of 5 m s−1 and horizontal wind speeds of over 30 m s−1 were detected during strong foehn phases within the valley. Aircraft measurements of various transects near the light paths are compared with two crosswind evaluation techniques. One of them, the slope method, tends to overestimate the actual wind speed by about 20%, whereas the peak technique gives values that are about 10% too low for high wind speeds. The peak method also fails to measure meaningful vertical crosswind speeds. The scintillometer data of one particular foehn storm are compared with nearby Doppler lidar data. The agreement of the horizontal measurements is reasonable. Discrepancies are attributed to topographic and dynamic effects that cause significant spatial inhomogeneities in the wind field. The applicability of continuous scintillometer vertical crosswind measurements in mountainous terrain is demonstrated.
Abstract
Measurements of the horizontal and vertical wind component by a crosswind scintillometer during foehn, the chinooklike downslope windstorm in the Alps, are presented. Because of the sparsity of vertical velocity measurements in the immediate vicinity, the scintillometer calibration is checked mainly with horizontal wind measurements. Then it is assumed that the calibration is the same for both components. The concept was tested during the Mesoscale Alpine Programme field campaign in the autumn of 1999, during which two scintillometers were deployed. Strong, long-lasting, quasi-stationary downward motions on the order of 5 m s−1 and horizontal wind speeds of over 30 m s−1 were detected during strong foehn phases within the valley. Aircraft measurements of various transects near the light paths are compared with two crosswind evaluation techniques. One of them, the slope method, tends to overestimate the actual wind speed by about 20%, whereas the peak technique gives values that are about 10% too low for high wind speeds. The peak method also fails to measure meaningful vertical crosswind speeds. The scintillometer data of one particular foehn storm are compared with nearby Doppler lidar data. The agreement of the horizontal measurements is reasonable. Discrepancies are attributed to topographic and dynamic effects that cause significant spatial inhomogeneities in the wind field. The applicability of continuous scintillometer vertical crosswind measurements in mountainous terrain is demonstrated.
