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- Author or Editor: C. A. Doswell III x
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Abstract
A set of mesoscale numerical simulations using the Pennsylvania State University–National Center for Atmospheric Research model is used to investigate two cases of extreme precipitation over eastern Spain. Both cases (3–4 November 1987 and 20 October 1982) were characterized by quasi-stationary mesoscale convective systems that developed over the Valencia region and lasted more than 30 and 12 h, respectively. Rainfall totals in 24 h exceeded 800 mm on 3–4 November and 400 mm on 20 October at some localities of that region. The first event occurred within a weak and very stagnant synoptic pattern under a persistent easterly/northeasterly low-level jet stream impinging on the Valencian orography. In contrast, the second case involved a westward-moving surface low driven by an upper-level jet streak, which evolved along the northern edge of an upper-level cutoff low over North Africa. In both cases, the mesoscale model forecast spatial details of the precipitation field reasonably accurately, as well as capturing its long duration, but underestimated the storm total precipitation. Model output fields suggest that the development of a surface mesolow by latent heat release, as well as lee cyclogenesis induced by the Atlas Mountains, could have played an important role in both events by providing low-level convergence and enhanced upslope winds. Thus, a factor separation technique is used to explore this issue. For the event of 3–4 November 1987, latent heat is decisive for explaining the precipitation maximum over central Valencia, and the Atlas orography induces rainfall enhancement over the same zone. For the event of 20 October 1982, the latent heat release is again shown to be important, whereas the Atlas Mountains orography factor appears to inhibit rainfall. This is the first time that it has been documented that the Atlas-induced modulation of the surface pressure field is not a positive factor for heavy precipitations over eastern Spain. This exceptional case may be due to a negative interaction between the Atlas Mountains and the upper-level dynamics and frontal forcing for this event.
Abstract
A set of mesoscale numerical simulations using the Pennsylvania State University–National Center for Atmospheric Research model is used to investigate two cases of extreme precipitation over eastern Spain. Both cases (3–4 November 1987 and 20 October 1982) were characterized by quasi-stationary mesoscale convective systems that developed over the Valencia region and lasted more than 30 and 12 h, respectively. Rainfall totals in 24 h exceeded 800 mm on 3–4 November and 400 mm on 20 October at some localities of that region. The first event occurred within a weak and very stagnant synoptic pattern under a persistent easterly/northeasterly low-level jet stream impinging on the Valencian orography. In contrast, the second case involved a westward-moving surface low driven by an upper-level jet streak, which evolved along the northern edge of an upper-level cutoff low over North Africa. In both cases, the mesoscale model forecast spatial details of the precipitation field reasonably accurately, as well as capturing its long duration, but underestimated the storm total precipitation. Model output fields suggest that the development of a surface mesolow by latent heat release, as well as lee cyclogenesis induced by the Atlas Mountains, could have played an important role in both events by providing low-level convergence and enhanced upslope winds. Thus, a factor separation technique is used to explore this issue. For the event of 3–4 November 1987, latent heat is decisive for explaining the precipitation maximum over central Valencia, and the Atlas orography induces rainfall enhancement over the same zone. For the event of 20 October 1982, the latent heat release is again shown to be important, whereas the Atlas Mountains orography factor appears to inhibit rainfall. This is the first time that it has been documented that the Atlas-induced modulation of the surface pressure field is not a positive factor for heavy precipitations over eastern Spain. This exceptional case may be due to a negative interaction between the Atlas Mountains and the upper-level dynamics and frontal forcing for this event.
Abstract
A flow where the pressure gradient, Coriolis and viscous forces are balanced is examined. It is found that such a flow is a reasonable approximation to the “steady state” flow in the vicinity of the contact layer. Kinematic effects implicit in the adjustment of an arbitrary flow to this balanced state are examined and can be used to explain several features of the nocturnal low-level jet. A method to use this balanced flow operationally to infer Ekman layer features is developed and several cases are examined.
Abstract
A flow where the pressure gradient, Coriolis and viscous forces are balanced is examined. It is found that such a flow is a reasonable approximation to the “steady state” flow in the vicinity of the contact layer. Kinematic effects implicit in the adjustment of an arbitrary flow to this balanced state are examined and can be used to explain several features of the nocturnal low-level jet. A method to use this balanced flow operationally to infer Ekman layer features is developed and several cases are examined.
Abstract
The life cycle and interactions of a series of convective systems that developed in northeastern Spain during the afternoon of 7 August 1996 are examined based on remote sensing products, surface observations, and numerical simulations. Most of the convection was organized in two mesoscale convective systems (MCSs) and a line of storms attached to the Pyrenees Mountains. One of these storms produced rainfalls in excess of 200 mm in 3 h and severe flash floods in the Biescas area. The end of the convection in the Biescas area occurred after merger of the storm with one of the MCSs that approached from the southwest. A high-resolution (4-km grid length) simulation of the event reproduces the observed timing and interactions of the convective systems, as well as the general rainfall pattern. The highly localized rainfall core at Biescas is well located, although the peak rainfall amount is underestimated. The success of the model at triggering the convection at the proper locations and time results from a reasonably accurate prediction of mesoscale features of the low-level flow pattern, such as a thermal mesolow in the Ebro valley, deformation zone, upslope wind systems, and the pushing of a cold front in the upper portion of the valley. After the onset of the initial convection, the role of the convectively induced cold pools and outflows for the propagation of the convective systems is shown to be very important. In particular, the MCS interacting with the Biescas convection was driven by strong mesoscale ascent established between the convectively induced outflows, the upvalley southeasterly winds sustained by the mesolow, and the downvalley northwesterly winds associated with the cold front. After the interaction, the convection in Biescas ceased because of the interruption of the southerly upslope flow caused by the convective cold pools. Finally, the explosive character of convection after noon and its initial focusing in uplands and slopes suggest that diurnal forcing could have played a decisive role. This idea is validated by means of simulations.
Abstract
The life cycle and interactions of a series of convective systems that developed in northeastern Spain during the afternoon of 7 August 1996 are examined based on remote sensing products, surface observations, and numerical simulations. Most of the convection was organized in two mesoscale convective systems (MCSs) and a line of storms attached to the Pyrenees Mountains. One of these storms produced rainfalls in excess of 200 mm in 3 h and severe flash floods in the Biescas area. The end of the convection in the Biescas area occurred after merger of the storm with one of the MCSs that approached from the southwest. A high-resolution (4-km grid length) simulation of the event reproduces the observed timing and interactions of the convective systems, as well as the general rainfall pattern. The highly localized rainfall core at Biescas is well located, although the peak rainfall amount is underestimated. The success of the model at triggering the convection at the proper locations and time results from a reasonably accurate prediction of mesoscale features of the low-level flow pattern, such as a thermal mesolow in the Ebro valley, deformation zone, upslope wind systems, and the pushing of a cold front in the upper portion of the valley. After the onset of the initial convection, the role of the convectively induced cold pools and outflows for the propagation of the convective systems is shown to be very important. In particular, the MCS interacting with the Biescas convection was driven by strong mesoscale ascent established between the convectively induced outflows, the upvalley southeasterly winds sustained by the mesolow, and the downvalley northwesterly winds associated with the cold front. After the interaction, the convection in Biescas ceased because of the interruption of the southerly upslope flow caused by the convective cold pools. Finally, the explosive character of convection after noon and its initial focusing in uplands and slopes suggest that diurnal forcing could have played a decisive role. This idea is validated by means of simulations.
Abstract
Wind fields derived from a network of three VHF Doppler radars are used to calculate the mean kinematic properties of the wind field over Colorado and an area-averaged geostrophic and ageostrophic wind. A numerical technique that is equivalent to the line integral method is used to compute the kinematic quantities. Details of this technique, termed the linear vector point function method (LVPF) are discussed. The behavior of the vorticity, divergence, deformation, geostrophic wind and ageostrophic wind are examined for two case studies when the synoptic scale weather patterns over Colorado are dominated by moderately intense upper-level troughs and jet streams. We find that the computed quantities of divergence, absolute vorticity, deformation, geostrophic and ageostrophic wind are modified by the passage of the weather systems in a manner consistent with our present understanding of upper-level dynamics. In addition, temporal variations in the kinematic properties, geostrophic wind and ageostrophic wind are revealed that are beyond the resolution of the existing rawinsonde network.
Abstract
Wind fields derived from a network of three VHF Doppler radars are used to calculate the mean kinematic properties of the wind field over Colorado and an area-averaged geostrophic and ageostrophic wind. A numerical technique that is equivalent to the line integral method is used to compute the kinematic quantities. Details of this technique, termed the linear vector point function method (LVPF) are discussed. The behavior of the vorticity, divergence, deformation, geostrophic wind and ageostrophic wind are examined for two case studies when the synoptic scale weather patterns over Colorado are dominated by moderately intense upper-level troughs and jet streams. We find that the computed quantities of divergence, absolute vorticity, deformation, geostrophic and ageostrophic wind are modified by the passage of the weather systems in a manner consistent with our present understanding of upper-level dynamics. In addition, temporal variations in the kinematic properties, geostrophic wind and ageostrophic wind are revealed that are beyond the resolution of the existing rawinsonde network.
Abstract
The notion of an “outbreak” of severe weather has been used for decades, but has never been formally defined. There are many different criteria by which outbreaks can be defined based on severe weather occurrence data, and there is not likely to be any compelling logic to choose any single criterion as ideal for all purposes. Therefore, a method has been developed that uses multiple variables and allows for considerable flexibility. The technique can be adapted easily to any project that needs to establish a ranking of weather events. The intended use involves isolating the most important tornado outbreak days, as well as important outbreak days of primarily nontornadic severe convective weather, during a period when the number of reports has been growing rapidly from nonmeteorological factors. The method is illustrated for both tornadic and primarily nontornadic severe weather event day cases. The impact of the secular trends in the data has been reduced by a simple detrending scheme. The effect of detrending is less important for the tornado outbreak cases and is illustrated by comparing rankings with and without detrending. It is shown that the resulting rankings are relatively resistant to secular trends in the data, as intended, and not strongly sensitive to the choices made in applying the method. The rankings are also consistent with subjective judgments of the relative importance of historical tornado outbreak cases.
Abstract
The notion of an “outbreak” of severe weather has been used for decades, but has never been formally defined. There are many different criteria by which outbreaks can be defined based on severe weather occurrence data, and there is not likely to be any compelling logic to choose any single criterion as ideal for all purposes. Therefore, a method has been developed that uses multiple variables and allows for considerable flexibility. The technique can be adapted easily to any project that needs to establish a ranking of weather events. The intended use involves isolating the most important tornado outbreak days, as well as important outbreak days of primarily nontornadic severe convective weather, during a period when the number of reports has been growing rapidly from nonmeteorological factors. The method is illustrated for both tornadic and primarily nontornadic severe weather event day cases. The impact of the secular trends in the data has been reduced by a simple detrending scheme. The effect of detrending is less important for the tornado outbreak cases and is illustrated by comparing rankings with and without detrending. It is shown that the resulting rankings are relatively resistant to secular trends in the data, as intended, and not strongly sensitive to the choices made in applying the method. The rankings are also consistent with subjective judgments of the relative importance of historical tornado outbreak cases.
Abstract
Careful screening of the National Severe Storms Forecast Center's tornado log eliminated almost 20% of the reports as doubtful, leaving 17 659 tornadoes during 27 years, 1950–76 (654 annually). Newspaper accounts and other local information provided intensifies (Fujita wale) for all but 2346 tornadoes and path lengths for all but 2011 tornadoes. There were 14 409 tornadoes for which both intensity and path length estimates were made. Of these, 61.7% were weak (<112 mph), 36% strong (113–206 mph), and only 2.3% violent (207–318 mph). However, the 340 violent tornadoes caused 68% of the 3070 fatalities attributed to tornadoes for which force estimates could he made (113.7 annually). Most violent tornadoes came in swarms except in southeastern United States, where no day had mart than one. Some 61% of the violent tornadoes had intermediate paths (3.2–31 mi), while 73% of weak and strong tornadoes had short paths. Violent tornadoes occurred at all times of day and night, while weak and strong tornadoes showed diurnal trends. May and June accounted for 40.8% of all tornadoes. Other aspects of tornado climatology are shown in tables, diagrams and maps of average annual incidence normalized to 10 000 mi2 area per year.
Abstract
Careful screening of the National Severe Storms Forecast Center's tornado log eliminated almost 20% of the reports as doubtful, leaving 17 659 tornadoes during 27 years, 1950–76 (654 annually). Newspaper accounts and other local information provided intensifies (Fujita wale) for all but 2346 tornadoes and path lengths for all but 2011 tornadoes. There were 14 409 tornadoes for which both intensity and path length estimates were made. Of these, 61.7% were weak (<112 mph), 36% strong (113–206 mph), and only 2.3% violent (207–318 mph). However, the 340 violent tornadoes caused 68% of the 3070 fatalities attributed to tornadoes for which force estimates could he made (113.7 annually). Most violent tornadoes came in swarms except in southeastern United States, where no day had mart than one. Some 61% of the violent tornadoes had intermediate paths (3.2–31 mi), while 73% of weak and strong tornadoes had short paths. Violent tornadoes occurred at all times of day and night, while weak and strong tornadoes showed diurnal trends. May and June accounted for 40.8% of all tornadoes. Other aspects of tornado climatology are shown in tables, diagrams and maps of average annual incidence normalized to 10 000 mi2 area per year.
