Search Results
You are looking at 1 - 10 of 17 items for
- Author or Editor: Michael Sprenger x
- Refine by Access: All Content x
Abstract
A novel approach is introduced to identify potential vorticity (PV) streamers and cutoffs as indicators of Rossby wave breaking near the extratropical tropopause and to compile climatologies of these features on different isentropic surfaces. The method is based on a contour searching algorithm that identifies the dynamical tropopause [2 potential vorticity units (PVU; PVU ≡ 1 × 10−6 K kg−1 m2 s−1) isoline] on isentropic surfaces. The contour is then analyzed to search for cutoffs and filament-like streamers. Whereas the identification of cutoffs is unambiguous, the one for streamers requires the specification of two parameters that determine the width and length of the contour feature to be classified as a streamer. This technique has been applied to the PV distribution in the Northern Hemisphere on isentropes from 295 to 360 K during the time period from 1979 to 1993 using the 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15).
The climatology reveals a pronounced zonal asymmetry in the occurrence of PV streamers and cutoffs. On all isentropes considered there are clear frequency maxima whose location changes with altitude. For instance, in winter and on the 300-K isentrope, stratospheric streamers and cutoffs occur most frequently near 50°–60°N over the western side of Canada and Siberia. On higher isentropes, the maxima are located farther south and at the downstream end of the storm-track regions. Considering continental areas, the Mediterranean appears as a region with particularly abundant PV features. As noted in previous studies, there is a significant seasonal cycle if considering the frequency of PV features on individual isentropes. It is shown that this is mainly due to the seasonal cycle in the location of the isentropes themselves. Comparing the streamer and cutoff frequencies during different seasons on isentropes that are comparably located in the zonal mean yields a fairly robust pattern with almost no seasonal cycle. This indicates on the one hand that care should be taken when considering the seasonal cycle of dynamical processes on isentropes and on the other hand that Rossby wave breaking occurs year-round with almost constant frequency. A quantitative statistical analysis of individual PV features reveals that stratospheric and tropospheric streamers often occur in pairs.
Abstract
A novel approach is introduced to identify potential vorticity (PV) streamers and cutoffs as indicators of Rossby wave breaking near the extratropical tropopause and to compile climatologies of these features on different isentropic surfaces. The method is based on a contour searching algorithm that identifies the dynamical tropopause [2 potential vorticity units (PVU; PVU ≡ 1 × 10−6 K kg−1 m2 s−1) isoline] on isentropic surfaces. The contour is then analyzed to search for cutoffs and filament-like streamers. Whereas the identification of cutoffs is unambiguous, the one for streamers requires the specification of two parameters that determine the width and length of the contour feature to be classified as a streamer. This technique has been applied to the PV distribution in the Northern Hemisphere on isentropes from 295 to 360 K during the time period from 1979 to 1993 using the 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15).
The climatology reveals a pronounced zonal asymmetry in the occurrence of PV streamers and cutoffs. On all isentropes considered there are clear frequency maxima whose location changes with altitude. For instance, in winter and on the 300-K isentrope, stratospheric streamers and cutoffs occur most frequently near 50°–60°N over the western side of Canada and Siberia. On higher isentropes, the maxima are located farther south and at the downstream end of the storm-track regions. Considering continental areas, the Mediterranean appears as a region with particularly abundant PV features. As noted in previous studies, there is a significant seasonal cycle if considering the frequency of PV features on individual isentropes. It is shown that this is mainly due to the seasonal cycle in the location of the isentropes themselves. Comparing the streamer and cutoff frequencies during different seasons on isentropes that are comparably located in the zonal mean yields a fairly robust pattern with almost no seasonal cycle. This indicates on the one hand that care should be taken when considering the seasonal cycle of dynamical processes on isentropes and on the other hand that Rossby wave breaking occurs year-round with almost constant frequency. A quantitative statistical analysis of individual PV features reveals that stratospheric and tropospheric streamers often occur in pairs.
Abstract
Two distinct dynamical processes near the dynamical tropopause (2-PVU surface) and their relation are discussed in this study: stratosphere–troposphere exchange (STE) and the formation of distinct potential vorticity (PV) structures in the form of stratospheric and tropospheric streamers and cutoffs on isentropic surfaces. Two previously compiled climatologies based upon the 15-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-15) dataset (from 1979 to 1993) are used to establish and quantify the link between STE and these PV structures.
An event-based analysis reveals a strong relation between the two processes. For instance, on isentropes below 320 K, 30%–50% of the stratospheric streamers are associated with downward STE. In the reverse perspective, between 60% and 80% of all STE events between 290 and 350 K are found in the vicinity of at least one PV structure. On different isentropes, the averaged downward (STT) and upward (TST) mass fluxes associated with PV structures are quantified.
As a novel quantity, the activity of a particular PV structure is measured as the STT/TST flux per unit length of its boundary on the considered isentropic level. The STT activity for stratospheric streamers and the TST activity of tropospheric streamers reach similar values of 3 × 109 kg km−1 h−1. Thereby, the flux is not uniformly distributed along a streamer’s boundary. STT (TST) is found preferentially on the upstream (downstream) side of stratospheric streamers, and vice versa for tropospheric streamers. This asymmetry is lost for cutoffs, for which an essentially uniform distribution results along the boundaries.
Finally, the link between STE and PV structures shows considerable geographical variability. Particularly striking is the fact that nearly all deep STT events (reaching levels below 700 hPa) over central Europe and the North American west coast are associated with a stratospheric streamer.
Abstract
Two distinct dynamical processes near the dynamical tropopause (2-PVU surface) and their relation are discussed in this study: stratosphere–troposphere exchange (STE) and the formation of distinct potential vorticity (PV) structures in the form of stratospheric and tropospheric streamers and cutoffs on isentropic surfaces. Two previously compiled climatologies based upon the 15-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-15) dataset (from 1979 to 1993) are used to establish and quantify the link between STE and these PV structures.
An event-based analysis reveals a strong relation between the two processes. For instance, on isentropes below 320 K, 30%–50% of the stratospheric streamers are associated with downward STE. In the reverse perspective, between 60% and 80% of all STE events between 290 and 350 K are found in the vicinity of at least one PV structure. On different isentropes, the averaged downward (STT) and upward (TST) mass fluxes associated with PV structures are quantified.
As a novel quantity, the activity of a particular PV structure is measured as the STT/TST flux per unit length of its boundary on the considered isentropic level. The STT activity for stratospheric streamers and the TST activity of tropospheric streamers reach similar values of 3 × 109 kg km−1 h−1. Thereby, the flux is not uniformly distributed along a streamer’s boundary. STT (TST) is found preferentially on the upstream (downstream) side of stratospheric streamers, and vice versa for tropospheric streamers. This asymmetry is lost for cutoffs, for which an essentially uniform distribution results along the boundaries.
Finally, the link between STE and PV structures shows considerable geographical variability. Particularly striking is the fact that nearly all deep STT events (reaching levels below 700 hPa) over central Europe and the North American west coast are associated with a stratospheric streamer.
Abstract
For nearly a century, the study of atmospheric dynamics in the midlatitudes has presented dichotomic perspectives on one of its focal points: the birth and life cycle of cyclones. In particular, the role of fronts has driven much of the historical discourse on cyclogenesis. In the 1910s–20s, the Bergen School of Meteorology postulated that cyclogenesis occurs on a preexisting front. This concept was later replaced by the baroclinic instability paradigm, which describes the development of a surface front as a consequence of the growing cyclone rather than its cause. However, there is ample observational evidence for cyclogenesis on well-marked fronts (frontal-wave cyclones) as well as for cyclogenesis in the absence of fronts in broader baroclinic zones. Thus, after a century of research on the link between extratropical cyclones and fronts, this study has the objective of climatologically quantifying their relationship. By combining identification schemes for cyclones and fronts, the fraction of cyclones with attendant fronts is quantified at all times during the cyclones’ life cycle. The storm-track regions over the North Atlantic are dominated by cyclones that form on preexisting fronts. Over the North Pacific, the result more strongly depends on the front definition. Cyclones that acquire their fronts during the life cycle dominate over the continents and in the Mediterranean. Further, cyclones that develop attendant fronts during their life cycle typically do so around the time they attain maximum intensity. At the time of cyclolysis, at least 40% of all cyclones are still associated with a front. The number of occluded fronts at lysis has not been considered.
Abstract
For nearly a century, the study of atmospheric dynamics in the midlatitudes has presented dichotomic perspectives on one of its focal points: the birth and life cycle of cyclones. In particular, the role of fronts has driven much of the historical discourse on cyclogenesis. In the 1910s–20s, the Bergen School of Meteorology postulated that cyclogenesis occurs on a preexisting front. This concept was later replaced by the baroclinic instability paradigm, which describes the development of a surface front as a consequence of the growing cyclone rather than its cause. However, there is ample observational evidence for cyclogenesis on well-marked fronts (frontal-wave cyclones) as well as for cyclogenesis in the absence of fronts in broader baroclinic zones. Thus, after a century of research on the link between extratropical cyclones and fronts, this study has the objective of climatologically quantifying their relationship. By combining identification schemes for cyclones and fronts, the fraction of cyclones with attendant fronts is quantified at all times during the cyclones’ life cycle. The storm-track regions over the North Atlantic are dominated by cyclones that form on preexisting fronts. Over the North Pacific, the result more strongly depends on the front definition. Cyclones that acquire their fronts during the life cycle dominate over the continents and in the Mediterranean. Further, cyclones that develop attendant fronts during their life cycle typically do so around the time they attain maximum intensity. At the time of cyclolysis, at least 40% of all cyclones are still associated with a front. The number of occluded fronts at lysis has not been considered.
Abstract
This study explores the possibilities of employing machine learning algorithms to predict foehn occurrence in Switzerland at a north Alpine (Altdorf) and south Alpine (Lugano) station from its synoptic fingerprint in reanalysis data and climate simulations. This allows for an investigation on a potential future shift in monthly foehn frequencies. First, inputs from various atmospheric fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERAI) were used to train an XGBoost model. Here, similar predictive performance to previous work was achieved, showing that foehn can accurately be diagnosed from the coarse synoptic situation. In the next step, the algorithm was generalized to predict foehn based on the Community Earth System Model (CESM) ensemble simulations of a present-day and warming future climate. The best generalization between ERAI and CESM was obtained by including the present-day data in the training procedure and simultaneously optimizing two objective functions, namely, the negative log loss and squared mean loss, on both datasets, respectively. It is demonstrated that the same synoptic fingerprint can be identified in CESM climate simulation data. Finally, predictions for present-day and future simulations were verified and compared for statistical significance. Our model is shown to produce valid output for most months, revealing that south foehn in Altdorf is expected to become more common during spring, while north foehn in Lugano is expected to become more common during summer.
Abstract
This study explores the possibilities of employing machine learning algorithms to predict foehn occurrence in Switzerland at a north Alpine (Altdorf) and south Alpine (Lugano) station from its synoptic fingerprint in reanalysis data and climate simulations. This allows for an investigation on a potential future shift in monthly foehn frequencies. First, inputs from various atmospheric fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERAI) were used to train an XGBoost model. Here, similar predictive performance to previous work was achieved, showing that foehn can accurately be diagnosed from the coarse synoptic situation. In the next step, the algorithm was generalized to predict foehn based on the Community Earth System Model (CESM) ensemble simulations of a present-day and warming future climate. The best generalization between ERAI and CESM was obtained by including the present-day data in the training procedure and simultaneously optimizing two objective functions, namely, the negative log loss and squared mean loss, on both datasets, respectively. It is demonstrated that the same synoptic fingerprint can be identified in CESM climate simulation data. Finally, predictions for present-day and future simulations were verified and compared for statistical significance. Our model is shown to produce valid output for most months, revealing that south foehn in Altdorf is expected to become more common during spring, while north foehn in Lugano is expected to become more common during summer.
Abstract
The south foehn is a characteristic downslope windstorm in the valleys of the northern Alps in Europe that demands reliable forecasts because of its substantial economic and societal impacts. Traditionally, a foehn is predicted based on pressure differences and tendencies across the Alpine ridge. Here, a new objective method for foehn prediction is proposed based on a machine learning algorithm (called AdaBoost, short for adaptive boosting). Three years (2000–02) of hourly simulations of the Consortium for Small-Scale Modeling’s (COSMO) numerical weather prediction (NWP) model and corresponding foehn wind observations are used to train the algorithm to distinguish between foehn and nonfoehn events. The predictors (133 in total) are subjectively extracted from the 7-km COSMO reanalysis dataset based on the main characteristics of foehn flows. The performance of the algorithm is then assessed with a validation dataset based on a contingency table that concisely summarizes the cooccurrence of observed and predicted (non)foehn events. The main performance measures are probability of detection (88.2%), probability of false detection (2.9%), missing rate (11.8%), correct alarm ratio (66.2%), false alarm ratio (33.8%), and missed alarm ratio (0.8%). To gain insight into the prediction model, the relevance of the single predictors is determined, resulting in a predominance of pressure differences across the Alpine ridge (i.e., similar to the traditional methods) and wind speeds at the foehn stations. The predominance of pressure-related predictors is further established in a sensitivity experiment where ~2500 predictors are objectively incorporated into the prediction model using the AdaBoost algorithm. The performance is very similar to the run with the subjectively determined predictors. Finally, some practical aspects of the new foehn index are discussed (e.g., the predictability of foehn events during the four seasons). The correct alarm rate is highest in winter (86.5%), followed by spring (79.6%), and then autumn (69.2%). The lowest rates are found in summer (51.2%).
Abstract
The south foehn is a characteristic downslope windstorm in the valleys of the northern Alps in Europe that demands reliable forecasts because of its substantial economic and societal impacts. Traditionally, a foehn is predicted based on pressure differences and tendencies across the Alpine ridge. Here, a new objective method for foehn prediction is proposed based on a machine learning algorithm (called AdaBoost, short for adaptive boosting). Three years (2000–02) of hourly simulations of the Consortium for Small-Scale Modeling’s (COSMO) numerical weather prediction (NWP) model and corresponding foehn wind observations are used to train the algorithm to distinguish between foehn and nonfoehn events. The predictors (133 in total) are subjectively extracted from the 7-km COSMO reanalysis dataset based on the main characteristics of foehn flows. The performance of the algorithm is then assessed with a validation dataset based on a contingency table that concisely summarizes the cooccurrence of observed and predicted (non)foehn events. The main performance measures are probability of detection (88.2%), probability of false detection (2.9%), missing rate (11.8%), correct alarm ratio (66.2%), false alarm ratio (33.8%), and missed alarm ratio (0.8%). To gain insight into the prediction model, the relevance of the single predictors is determined, resulting in a predominance of pressure differences across the Alpine ridge (i.e., similar to the traditional methods) and wind speeds at the foehn stations. The predominance of pressure-related predictors is further established in a sensitivity experiment where ~2500 predictors are objectively incorporated into the prediction model using the AdaBoost algorithm. The performance is very similar to the run with the subjectively determined predictors. Finally, some practical aspects of the new foehn index are discussed (e.g., the predictability of foehn events during the four seasons). The correct alarm rate is highest in winter (86.5%), followed by spring (79.6%), and then autumn (69.2%). The lowest rates are found in summer (51.2%).
Abstract
Complementary key elements of meteorological education are the provision of a thorough theoretical understanding of the physical laws governing atmospheric motions, and the hands-on investigation and visualization of specific weather systems. However, the latter task is technically challenging, because specific skills must be acquired for flexibly handling meteorological data. Some examples are superimposing satellite pictures and reanalysis fields, producing an isentropic potential vorticity (PV) map, and visualizing a vertical section across a flow feature of interest. Although learning these technical issues has its own merits, it can distract students from investigating the complexities of meteorology. This experience from teaching beginner classes in synoptic meteorology at ETH Zurich and the University of Mainz was the main motivation for developing the educational software tool IWAL, the Interactive Weather Analysis Laboratory. IWAL is designed as a web application for easy, fast, and interactive access to large meteorological datasets, which enables active and curiosity-driven learning. The main target users of IWAL are students with little or no experience in the handling and visualization of such data. The interactivity; the option to very easily reproduce complex visualizations; and advanced features, such as the interactive computation of trajectories, are also of interest to more experienced students and lecturers.
Abstract
Complementary key elements of meteorological education are the provision of a thorough theoretical understanding of the physical laws governing atmospheric motions, and the hands-on investigation and visualization of specific weather systems. However, the latter task is technically challenging, because specific skills must be acquired for flexibly handling meteorological data. Some examples are superimposing satellite pictures and reanalysis fields, producing an isentropic potential vorticity (PV) map, and visualizing a vertical section across a flow feature of interest. Although learning these technical issues has its own merits, it can distract students from investigating the complexities of meteorology. This experience from teaching beginner classes in synoptic meteorology at ETH Zurich and the University of Mainz was the main motivation for developing the educational software tool IWAL, the Interactive Weather Analysis Laboratory. IWAL is designed as a web application for easy, fast, and interactive access to large meteorological datasets, which enables active and curiosity-driven learning. The main target users of IWAL are students with little or no experience in the handling and visualization of such data. The interactivity; the option to very easily reproduce complex visualizations; and advanced features, such as the interactive computation of trajectories, are also of interest to more experienced students and lecturers.
Abstract
Extreme precipitation events along the Alpine south side (AS) are often forced by upper-level positive potential vorticity (PV) anomalies over western Europe. These so-called PV streamers go along with a dynamical forcing for upward motion, a reduction of the static stability in the troposphere (hence facilitating convection), and are associated with low-level winds that transport moisture toward the Alps.
A case of heavy precipitation is examined using the 40-yr ECMWF Re-Analysis data. Piecewise PV inversion (PPVI) and the limited-area Climate High Resolution Model (CHRM) are used to assess the influences of mesoscale parts of the streamer on the precipitation event. The impacts on the vertical stability are quantified by the convective available potential energy (CAPE) and an index of static stability. Very sensitive areas in terms of the stability are located beneath the southern tip of the streamer; smaller changes in the stability are observed in the Alpine region.
The moisture transport toward the Alps is sensitive to the amplitude of the streamer, which influences the amount of water that can be transported along its eastern flank.
The impacts of the topography on the flow are assessed by calculating an average inverse Froude number. Whether or not the air parcels are blocked by or lifted over the barrier (going along with suppressed and enhanced precipitation, respectively) depends on the vertical stability and the impinging wind velocity, two parameters that are inherently linked to the PV streamer and its substructure.
Abstract
Extreme precipitation events along the Alpine south side (AS) are often forced by upper-level positive potential vorticity (PV) anomalies over western Europe. These so-called PV streamers go along with a dynamical forcing for upward motion, a reduction of the static stability in the troposphere (hence facilitating convection), and are associated with low-level winds that transport moisture toward the Alps.
A case of heavy precipitation is examined using the 40-yr ECMWF Re-Analysis data. Piecewise PV inversion (PPVI) and the limited-area Climate High Resolution Model (CHRM) are used to assess the influences of mesoscale parts of the streamer on the precipitation event. The impacts on the vertical stability are quantified by the convective available potential energy (CAPE) and an index of static stability. Very sensitive areas in terms of the stability are located beneath the southern tip of the streamer; smaller changes in the stability are observed in the Alpine region.
The moisture transport toward the Alps is sensitive to the amplitude of the streamer, which influences the amount of water that can be transported along its eastern flank.
The impacts of the topography on the flow are assessed by calculating an average inverse Froude number. Whether or not the air parcels are blocked by or lifted over the barrier (going along with suppressed and enhanced precipitation, respectively) depends on the vertical stability and the impinging wind velocity, two parameters that are inherently linked to the PV streamer and its substructure.
Abstract
Inspection of the potential vorticity (PV) distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive PV anomalies. To examine the space–time characteristics of these anomalies, a combined “identification and tracking” tool is developed that can catalog each individual anomaly’s effective amplitude, location, overall spatial structure, and movement from genesis to lysis. A 10-yr winter climatology of such anomalies in the Northern Hemisphere is derived for the period 1991–2001 based upon the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The climatology indicates that the anomalies are frequently evident above high topography and in a quasi-annular band at about 70°N, are long lived (days to weeks), and that their effective amplitude is typically 2 PV units (PVU) higher than that of the ambient environment. In addition, the derived climatologies and associated composites pose questions regarding the origin of the anomalies, detail their life cycle, and shed light on their dynamics and role as long-lived precursors of surface cyclogenesis.
Abstract
Inspection of the potential vorticity (PV) distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive PV anomalies. To examine the space–time characteristics of these anomalies, a combined “identification and tracking” tool is developed that can catalog each individual anomaly’s effective amplitude, location, overall spatial structure, and movement from genesis to lysis. A 10-yr winter climatology of such anomalies in the Northern Hemisphere is derived for the period 1991–2001 based upon the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The climatology indicates that the anomalies are frequently evident above high topography and in a quasi-annular band at about 70°N, are long lived (days to weeks), and that their effective amplitude is typically 2 PV units (PVU) higher than that of the ambient environment. In addition, the derived climatologies and associated composites pose questions regarding the origin of the anomalies, detail their life cycle, and shed light on their dynamics and role as long-lived precursors of surface cyclogenesis.
Abstract
Tropical cyclones are among the most devastating natural phenomena that can cause severe damage when undergoing landfall. In the wake of the poorly forecast 2013 North Atlantic hurricane season, Rossby wave breaking on the 350-K isentropic surface has been linked to tropical cyclone activity measured by the accumulated cyclone energy (ACE). Here, ERA5 data and HURDAT2 tropical cyclone data are used to argue that the latitude of the 2 potential vorticity unit (PVU; 1 PVU = 10−6 K kg−1 m2 s−1) contour on the 360-K isentropic surface in the western North Atlantic is linked to changes in vertical wind shear and relative humidity during the month of September. A more equatorward position of the 2-PVU contour is shown to be linked to an increase in vertical wind shear and a reduction in relative humidity, as manifested in an increased ventilation index, in the tropical western North Atlantic during September. The more equatorward position is further linked to a reduction in the number of named storms, storm and hurricane days, hurricane lifetime, and number of tropical cyclones making landfall. Changes in genesis location are shown to be of importance for the changes in landfall frequency and hurricane lifetime. In summary, the 2-PVU contour latitude in the western North Atlantic can, therefore, potentially be used as a predictor in seasonal and subseasonal forecasting.
Significance Statement
Forecasts for the North Atlantic hurricane season are operationally produced. Their aim is to predict the number of tropical cyclones and their total energy throughout the season. This study proposes to include the tropopause latitude in these forecasts, as it is shown to be linked to vertical wind shear and midtropospheric relative humidity in the western tropical North Atlantic. The tropopause latitude is thereby linked to the number of tropical cyclones, their lifetime, and the total energy throughout the season. This link is particularly strong during September.
Abstract
Tropical cyclones are among the most devastating natural phenomena that can cause severe damage when undergoing landfall. In the wake of the poorly forecast 2013 North Atlantic hurricane season, Rossby wave breaking on the 350-K isentropic surface has been linked to tropical cyclone activity measured by the accumulated cyclone energy (ACE). Here, ERA5 data and HURDAT2 tropical cyclone data are used to argue that the latitude of the 2 potential vorticity unit (PVU; 1 PVU = 10−6 K kg−1 m2 s−1) contour on the 360-K isentropic surface in the western North Atlantic is linked to changes in vertical wind shear and relative humidity during the month of September. A more equatorward position of the 2-PVU contour is shown to be linked to an increase in vertical wind shear and a reduction in relative humidity, as manifested in an increased ventilation index, in the tropical western North Atlantic during September. The more equatorward position is further linked to a reduction in the number of named storms, storm and hurricane days, hurricane lifetime, and number of tropical cyclones making landfall. Changes in genesis location are shown to be of importance for the changes in landfall frequency and hurricane lifetime. In summary, the 2-PVU contour latitude in the western North Atlantic can, therefore, potentially be used as a predictor in seasonal and subseasonal forecasting.
Significance Statement
Forecasts for the North Atlantic hurricane season are operationally produced. Their aim is to predict the number of tropical cyclones and their total energy throughout the season. This study proposes to include the tropopause latitude in these forecasts, as it is shown to be linked to vertical wind shear and midtropospheric relative humidity in the western tropical North Atlantic. The tropopause latitude is thereby linked to the number of tropical cyclones, their lifetime, and the total energy throughout the season. This link is particularly strong during September.
Abstract
This study focuses on the rainfall-producing weather systems in the southern Murray-Darling Basin (MDB), Australia. These weather systems are divided into objects: cyclones, fronts, anticyclones, warm conveyor belt (WCB) inflows, WCB ascents, potential vorticity (PV) streamers, and cut-off lows. We investigate the changes in the frequency, amplitude, and relative position of these objects as the daily and seasonal rainfall change. Days on which the rainfall is heavy, especially in winter, are characterized by more PV streamers, cut-off lows, cyclones, fronts and WCBs in the region. In contrast, dry days are characterized by more anticyclones over southeastern Australia in winter and summer.
The effect of upper-level weather objects (PV streamers and cut-off lows) on lower-level objects, and their importance in producing rainfall, is quantified using the quasi-geostrophic ω-equation and separating the vertical motion into that induced by the upper and lower levels. On heavy rainfall days in winter, PV streamers and cut-off lows force strong upward motion in the lower troposphere, promoting cyclogenesis at lower levels, forcing ascent in the WCBs, and producing rain downstream of the southern MDB. Lower-level ascent forced by upper-level objects is important for the development of heavy rainfall in both seasons, although particularly in winter.
Rainfall is attributed to individual objects. PV streamers and WCBs contribute most to the winter and summer rainfall respectively. The difference in rainfall between anomalously wet and dry years can be explained in winter by the changes in the rainfall associated with PV streamers, whereas in summer it is mostly due to a reduction in the rainfall associated with WCBs.
Abstract
This study focuses on the rainfall-producing weather systems in the southern Murray-Darling Basin (MDB), Australia. These weather systems are divided into objects: cyclones, fronts, anticyclones, warm conveyor belt (WCB) inflows, WCB ascents, potential vorticity (PV) streamers, and cut-off lows. We investigate the changes in the frequency, amplitude, and relative position of these objects as the daily and seasonal rainfall change. Days on which the rainfall is heavy, especially in winter, are characterized by more PV streamers, cut-off lows, cyclones, fronts and WCBs in the region. In contrast, dry days are characterized by more anticyclones over southeastern Australia in winter and summer.
The effect of upper-level weather objects (PV streamers and cut-off lows) on lower-level objects, and their importance in producing rainfall, is quantified using the quasi-geostrophic ω-equation and separating the vertical motion into that induced by the upper and lower levels. On heavy rainfall days in winter, PV streamers and cut-off lows force strong upward motion in the lower troposphere, promoting cyclogenesis at lower levels, forcing ascent in the WCBs, and producing rain downstream of the southern MDB. Lower-level ascent forced by upper-level objects is important for the development of heavy rainfall in both seasons, although particularly in winter.
Rainfall is attributed to individual objects. PV streamers and WCBs contribute most to the winter and summer rainfall respectively. The difference in rainfall between anomalously wet and dry years can be explained in winter by the changes in the rainfall associated with PV streamers, whereas in summer it is mostly due to a reduction in the rainfall associated with WCBs.
