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Abstract
The potential for a stably stratified air mass upstream of the Sierra Nevada (California) to descend as foehn into the nearly 3-km-deep Owens Valley was studied for the 2 March 2006 case with observations from sondes, weather stations, and two aircraft flights. While upstream conditions remained almost unchanged throughout the day, strong diurnal heating on the downstream side warmed the valley air mass sufficiently to permit flow through the passes to descend to the valley floor only in the late afternoon. Potential temperatures of air crossing the crest were too warm to descend past a virtual floor formed by the strong potential temperature step at the top of the valley air mass, the height of which changed throughout the day primarily due to diurnal heating in the valley. The descending stably stratified flow and its rebound with vertical velocities as high as 8 m s−1 were shaped by the underlying topography and the virtual valley floor.
Abstract
The potential for a stably stratified air mass upstream of the Sierra Nevada (California) to descend as foehn into the nearly 3-km-deep Owens Valley was studied for the 2 March 2006 case with observations from sondes, weather stations, and two aircraft flights. While upstream conditions remained almost unchanged throughout the day, strong diurnal heating on the downstream side warmed the valley air mass sufficiently to permit flow through the passes to descend to the valley floor only in the late afternoon. Potential temperatures of air crossing the crest were too warm to descend past a virtual floor formed by the strong potential temperature step at the top of the valley air mass, the height of which changed throughout the day primarily due to diurnal heating in the valley. The descending stably stratified flow and its rebound with vertical velocities as high as 8 m s−1 were shaped by the underlying topography and the virtual valley floor.
Abstract
Foehn winds often depend on topographical features of a scale that is not sufficiently resolved in numerical models. Consequently, a successful foehn forecast has crucially depended on the experience of bench forecasters. This study provides a method for an objective, probabilistic forecast of foehn occurrence and strength, based on an operational global model (ECMWF). Because model topography differs from real topography, forecasted wind is not a reliable indicator of a foehn. Instead, using the larger-scale fingerprint of foehn from cross-barrier pressure differences and the descent of isentropes is more successful. These foehn predictors were tested over a period of 3 yr for the subgrid-scale Wipp Valley in the central Alps, which is instrumented sufficiently for objectively diagnosing the occurrence and strength of a foehn. The joint probability from pressure differences and isentropic descent is better at diagnosing a foehn from model analyses than from the distributions of the individual parameters. The larger the pressure difference and the isentropic descent, the higher the foehn probability. As wind speed and pressure gradient are directly connected by the Bernoulli equation, the cross-barrier pressure difference in the model proved to be a suitable predictor for the strength of the foehn. Despite being a small-scale weather phenomenon, the skill of a objective foehn forecast out to 3 days degrades little compared to the analysis. Afterward, the predictability decreases progressively.
Abstract
Foehn winds often depend on topographical features of a scale that is not sufficiently resolved in numerical models. Consequently, a successful foehn forecast has crucially depended on the experience of bench forecasters. This study provides a method for an objective, probabilistic forecast of foehn occurrence and strength, based on an operational global model (ECMWF). Because model topography differs from real topography, forecasted wind is not a reliable indicator of a foehn. Instead, using the larger-scale fingerprint of foehn from cross-barrier pressure differences and the descent of isentropes is more successful. These foehn predictors were tested over a period of 3 yr for the subgrid-scale Wipp Valley in the central Alps, which is instrumented sufficiently for objectively diagnosing the occurrence and strength of a foehn. The joint probability from pressure differences and isentropic descent is better at diagnosing a foehn from model analyses than from the distributions of the individual parameters. The larger the pressure difference and the isentropic descent, the higher the foehn probability. As wind speed and pressure gradient are directly connected by the Bernoulli equation, the cross-barrier pressure difference in the model proved to be a suitable predictor for the strength of the foehn. Despite being a small-scale weather phenomenon, the skill of a objective foehn forecast out to 3 days degrades little compared to the analysis. Afterward, the predictability decreases progressively.
Abstract
A combination of real and virtual topography is shown to be crucial to describe the essentials of stratified flow over mountain ranges and leeside valleys. On 14 March 2006 [Intensive Observation Period 4 of the Terrain-Induced Rotor Experiment (T-REX)], a nearly neutral cloud-filled layer, capped by a strong density step, overflowed the Sierra Nevada and separated from the lee slope upon encountering a cooler valley air mass. The flow in this lowest layer was asymmetric across and hydraulically controlled at the crest with subcritical flow upstream and supercritical flow downstream. The density step at the top of this flowing layer formed a virtual topography, which descended 1.9 km and determined the horizontal scale and shape of the flow response aloft reaching into the stratosphere. A comparison shows that the 11 January 1972 Boulder, Colorado, windstorm case was similar: hydraulically controlled at the crest with the same strength and descent of the virtual topography. In the 18 February 1970 Boulder case, however, the layer beneath the stronger virtual topography was subcritical everywhere with a symmetric dip across the Continental Divide of only 0.5 km. In all three cases, the response and strength of the flow aloft depend on the virtual topography. The layer up to the next strong density step at or near the tropopause was hydraulically supercritical for the 18 February case, subcritical for the T-REX case, and critically controlled for the 11 January case, for which a weak density step and isolating layer aloft made possible the strong response aloft for which it is famous.
Abstract
A combination of real and virtual topography is shown to be crucial to describe the essentials of stratified flow over mountain ranges and leeside valleys. On 14 March 2006 [Intensive Observation Period 4 of the Terrain-Induced Rotor Experiment (T-REX)], a nearly neutral cloud-filled layer, capped by a strong density step, overflowed the Sierra Nevada and separated from the lee slope upon encountering a cooler valley air mass. The flow in this lowest layer was asymmetric across and hydraulically controlled at the crest with subcritical flow upstream and supercritical flow downstream. The density step at the top of this flowing layer formed a virtual topography, which descended 1.9 km and determined the horizontal scale and shape of the flow response aloft reaching into the stratosphere. A comparison shows that the 11 January 1972 Boulder, Colorado, windstorm case was similar: hydraulically controlled at the crest with the same strength and descent of the virtual topography. In the 18 February 1970 Boulder case, however, the layer beneath the stronger virtual topography was subcritical everywhere with a symmetric dip across the Continental Divide of only 0.5 km. In all three cases, the response and strength of the flow aloft depend on the virtual topography. The layer up to the next strong density step at or near the tropopause was hydraulically supercritical for the 18 February case, subcritical for the T-REX case, and critically controlled for the 11 January case, for which a weak density step and isolating layer aloft made possible the strong response aloft for which it is famous.
Abstract
Cross-barrier density differences and westerly flow established a descending stratified flow across the Sierra Nevada (United States) on 9–10 April 2006. Downslope flow and an internal hydraulic jump occurred only when the potential temperature of the westerly descending flow was at least as cold as the existing upvalley-flowing valley air mass. The onset was observed in sequences of visible satellite images and with weather stations. The University of Wyoming King Air flew through the stratified flow and imaged the structure of the internal hydraulic jump with its cloud radar. Shear-layer instabilities, which first developed near the jump face, grew and paired downstream, mixing the internal hydraulic jump layer. A single wave response to the downslope flow and internal hydraulic jump was observed aloft, but only after the downslope flow had become established.
Abstract
Cross-barrier density differences and westerly flow established a descending stratified flow across the Sierra Nevada (United States) on 9–10 April 2006. Downslope flow and an internal hydraulic jump occurred only when the potential temperature of the westerly descending flow was at least as cold as the existing upvalley-flowing valley air mass. The onset was observed in sequences of visible satellite images and with weather stations. The University of Wyoming King Air flew through the stratified flow and imaged the structure of the internal hydraulic jump with its cloud radar. Shear-layer instabilities, which first developed near the jump face, grew and paired downstream, mixing the internal hydraulic jump layer. A single wave response to the downslope flow and internal hydraulic jump was observed aloft, but only after the downslope flow had become established.
Abstract
Three methods to make probabilistic weather forecasts by using analogs are presented and tested. The basic idea of these methods is that finding similar NWP model forecasts to the current one in an archive of past forecasts and taking the corresponding analyses as prediction should remove all systematic errors of the model. Furthermore, this statistical postprocessing can convert NWP forecasts to forecasts for point locations and easily turn deterministic forecasts into probabilistic ones. These methods are tested in the idealized Lorenz96 system and compared to a benchmark bracket formed by ensemble relative frequencies from direct model output and logistic regression. The analog methods excel at longer lead times.
Abstract
Three methods to make probabilistic weather forecasts by using analogs are presented and tested. The basic idea of these methods is that finding similar NWP model forecasts to the current one in an archive of past forecasts and taking the corresponding analyses as prediction should remove all systematic errors of the model. Furthermore, this statistical postprocessing can convert NWP forecasts to forecasts for point locations and easily turn deterministic forecasts into probabilistic ones. These methods are tested in the idealized Lorenz96 system and compared to a benchmark bracket formed by ensemble relative frequencies from direct model output and logistic regression. The analog methods excel at longer lead times.
Abstract
The evolution of low-level flow upstream of the Continental Divide (Rocky Mountains) and the Wasatch Range from being unable to surmount the mountain range, to becoming unblocked and blocked again is studied observationally. During two months in the winter of 1991/92, a transect of three wind profilers measured the wind field every few minutes with unprecedented temporal detail.
The average state of that region during winter is blocked. A total of 47 blocked events were observed. A blocked flow event lasted on the average one and a half days, but the duration varied widely from a few hours to eight days controlled by the synoptic situation. The transition between the two states happened rapidly on the order of 1 h with a minimum of 20 min and a maximum of 4 h. The depth of the blocked layer during one blocking episode fluctuated considerably but reached on the average one-half to two-thirds of the barrier depth (depending on the location).
Previous research of idealized equilibrium situations focused on changes of the cross-barrier wind speed and stability as determining variables to build a mesoscale high over the barrier. Since their values were in the blocked range, other mechanisms had to trigger the transitions to an unblocked state.
A conceptual model proposes synoptic and radiative forcing to drive the blocking evolution. When the mountain-induced mesoscale high blocks the low-level flow, an opposing synoptic cross-barrier pressure gradient can negate the mesoscale high. Therefore unblocking happens most frequently when the trough axis of a short wave is immediately upstream of the harder, but synoptic pressure gradients caused by contrasts in vorticity and differential temperature advection are sometimes also strong enough. The flow returns to its blocked state when the ridge behind the trough approaches the barrier so that the synoptic cross-barrier pressure gradient reinforces the mesoscale high.
For a lower barrier or stronger solar insulation, a well-mixed boundary layer can grow almost to the height of the barrier by afternoon and reconnect the blocked layer with the higher cross-barrier winds above the mountain. After sunset the thermal forcing changes sign as the radiative cooling stabilizes the lower atmosphere again and the transition back to the blocked state occurs.
Abstract
The evolution of low-level flow upstream of the Continental Divide (Rocky Mountains) and the Wasatch Range from being unable to surmount the mountain range, to becoming unblocked and blocked again is studied observationally. During two months in the winter of 1991/92, a transect of three wind profilers measured the wind field every few minutes with unprecedented temporal detail.
The average state of that region during winter is blocked. A total of 47 blocked events were observed. A blocked flow event lasted on the average one and a half days, but the duration varied widely from a few hours to eight days controlled by the synoptic situation. The transition between the two states happened rapidly on the order of 1 h with a minimum of 20 min and a maximum of 4 h. The depth of the blocked layer during one blocking episode fluctuated considerably but reached on the average one-half to two-thirds of the barrier depth (depending on the location).
Previous research of idealized equilibrium situations focused on changes of the cross-barrier wind speed and stability as determining variables to build a mesoscale high over the barrier. Since their values were in the blocked range, other mechanisms had to trigger the transitions to an unblocked state.
A conceptual model proposes synoptic and radiative forcing to drive the blocking evolution. When the mountain-induced mesoscale high blocks the low-level flow, an opposing synoptic cross-barrier pressure gradient can negate the mesoscale high. Therefore unblocking happens most frequently when the trough axis of a short wave is immediately upstream of the harder, but synoptic pressure gradients caused by contrasts in vorticity and differential temperature advection are sometimes also strong enough. The flow returns to its blocked state when the ridge behind the trough approaches the barrier so that the synoptic cross-barrier pressure gradient reinforces the mesoscale high.
For a lower barrier or stronger solar insulation, a well-mixed boundary layer can grow almost to the height of the barrier by afternoon and reconnect the blocked layer with the higher cross-barrier winds above the mountain. After sunset the thermal forcing changes sign as the radiative cooling stabilizes the lower atmosphere again and the transition back to the blocked state occurs.
Abstract
Diagnosing foehn winds from weather station data downwind of topographic obstacles requires distinguishing them from other downslope winds, particularly nocturnal ones driven by radiative cooling. An automatic classification scheme to obtain reproducible results that include information about the (un)certainty of the diagnosis is presented. A statistical mixture model separates foehn and no-foehn winds in a measured time series of wind. In addition to wind speed and direction, it accommodates other physically meaningful classifiers such as the (potential) temperature difference to an upwind station (e.g., near the crest) or relative humidity. The algorithm was tested for Wipp Valley in the central Alps against human expert classification and a previous objective method (), which the new method outperforms. Climatologically, using only wind information gives nearly identical foehn frequencies as when using additional covariables. A data record length of at least one year is required for satisfactory results. The suitability of mixture models for objective classification of foehn at other locations will have to be tested in further studies.
Abstract
Diagnosing foehn winds from weather station data downwind of topographic obstacles requires distinguishing them from other downslope winds, particularly nocturnal ones driven by radiative cooling. An automatic classification scheme to obtain reproducible results that include information about the (un)certainty of the diagnosis is presented. A statistical mixture model separates foehn and no-foehn winds in a measured time series of wind. In addition to wind speed and direction, it accommodates other physically meaningful classifiers such as the (potential) temperature difference to an upwind station (e.g., near the crest) or relative humidity. The algorithm was tested for Wipp Valley in the central Alps against human expert classification and a previous objective method (), which the new method outperforms. Climatologically, using only wind information gives nearly identical foehn frequencies as when using additional covariables. A data record length of at least one year is required for satisfactory results. The suitability of mixture models for objective classification of foehn at other locations will have to be tested in further studies.
Abstract
An instrument package to measure temperature, pressure, humidity, and position was designed to be quickly deployable on any automobile to be used for the study of gap and other orographically influenced flows. Differential GPS (global positioning system) measurements together with a distance counter gave the submeter accuracy of vertical position that was needed for observation of changes in the horizontal pressure field, which is an integral measure of the flow field aloft. A slantwise pressure reduction method was tailored for this application and verified with data from radio soundings. The automobile platform was successfully used during the field phase of the Mesoscale Alpine Programme (MAP) to classify flow states and observe hydraulic jumps in gap flows and to extend aircraft measurements to the ground.
Abstract
An instrument package to measure temperature, pressure, humidity, and position was designed to be quickly deployable on any automobile to be used for the study of gap and other orographically influenced flows. Differential GPS (global positioning system) measurements together with a distance counter gave the submeter accuracy of vertical position that was needed for observation of changes in the horizontal pressure field, which is an integral measure of the flow field aloft. A slantwise pressure reduction method was tailored for this application and verified with data from radio soundings. The automobile platform was successfully used during the field phase of the Mesoscale Alpine Programme (MAP) to classify flow states and observe hydraulic jumps in gap flows and to extend aircraft measurements to the ground.
Abstract
A case study of a south foehn windstorm observed across the Brenner Pass in the Wipp Valley near the Austrian–Italian border is presented based on a detailed comparison and verification of high-resolution numerical simulations with observations. The event of 24 through 25 October 1999 was part of the Intensive Observing Period 10 of the Mesoscale Alpine Programme (MAP). The simulations were performed with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The observations were collected with a ground-based scanning Doppler lidar, an airborne aerosol backscatter lidar, a Doppler sodar, several weather stations, and two radiosounding systems. The study provides a synoptic-scale and mesoscale overview of the event and focuses on a comparison of simulated and observed fields for a 9-h period on 24 October 1999. The quantitative agreement between the numerical results and the observations is discussed in terms of root-mean-square error (rmse) and mean error (ME). Rmse values are high during the early stage of the event (∼7 m s−1), have a transient peak for about 1 h at 1400 UTC, and are minimal at the fully developed foehn stage near 1500 UTC (∼5 m s−1). The discrepancies at the beginning are likely to be related to deficiencies in the model profile on the upstream side of the pass, exhibiting a too low inversion and a too shallow southerly flow. The transient error peak at 1400 UTC is related to a mismatch in the timing of the enhancement of the upper-level winds. Moreover, evidence is found for an overestimation of the mass flux through the lower Brenner gap, which is the narrowest and deepest part of the incision in the main Alpine crest, and a subsequent underestimation of the flow descent into the Wipp Valley on the leeward side of the Brenner Pass. Considering mass continuity, the latter effect is probably a result of the former. Nevertheless, the model captures most of the striking foehn features: Simulated isentropes and aerosol backscatter measurements consistently indicate regions of flow descent, across-valley asymmetries, and hydraulic jump–like features. The across-valley asymmetry of the foehn strength near the Wipp Valley exit is particularly well reproduced by the model. The primary reason for the stronger winds on the eastern sidewall is the asymmetry in the position of the mountain ridges protruding into the valley together with the westward bending of the valley axis.
Abstract
A case study of a south foehn windstorm observed across the Brenner Pass in the Wipp Valley near the Austrian–Italian border is presented based on a detailed comparison and verification of high-resolution numerical simulations with observations. The event of 24 through 25 October 1999 was part of the Intensive Observing Period 10 of the Mesoscale Alpine Programme (MAP). The simulations were performed with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The observations were collected with a ground-based scanning Doppler lidar, an airborne aerosol backscatter lidar, a Doppler sodar, several weather stations, and two radiosounding systems. The study provides a synoptic-scale and mesoscale overview of the event and focuses on a comparison of simulated and observed fields for a 9-h period on 24 October 1999. The quantitative agreement between the numerical results and the observations is discussed in terms of root-mean-square error (rmse) and mean error (ME). Rmse values are high during the early stage of the event (∼7 m s−1), have a transient peak for about 1 h at 1400 UTC, and are minimal at the fully developed foehn stage near 1500 UTC (∼5 m s−1). The discrepancies at the beginning are likely to be related to deficiencies in the model profile on the upstream side of the pass, exhibiting a too low inversion and a too shallow southerly flow. The transient error peak at 1400 UTC is related to a mismatch in the timing of the enhancement of the upper-level winds. Moreover, evidence is found for an overestimation of the mass flux through the lower Brenner gap, which is the narrowest and deepest part of the incision in the main Alpine crest, and a subsequent underestimation of the flow descent into the Wipp Valley on the leeward side of the Brenner Pass. Considering mass continuity, the latter effect is probably a result of the former. Nevertheless, the model captures most of the striking foehn features: Simulated isentropes and aerosol backscatter measurements consistently indicate regions of flow descent, across-valley asymmetries, and hydraulic jump–like features. The across-valley asymmetry of the foehn strength near the Wipp Valley exit is particularly well reproduced by the model. The primary reason for the stronger winds on the eastern sidewall is the asymmetry in the position of the mountain ridges protruding into the valley together with the westward bending of the valley axis.
