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- Author or Editor: Frederick Sanders x
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Abstract
A cold front developed in Arizona in a region of initially small temperature gradient, developing to great intensity, accompanied by damaging winds over New Mexico, and then losing strength over the high plains of Oklahoma and Texas. The entire development of frontogenesis and frontolysis occurred in no more than 24 h.
The initial growth of temperature contrast was attributable mainly to horizontal variation of surface heat flux during the morning, with little heating in a region of dense cloud cover and scattered showers in the west and with intense heating in a region of only thin high clouds to the east. The accompanying ageostrophic circulation then resulted in a collapse toward discontinuity. The frontal zone maintained an approximately steady state for a few hours in early afternoon. At this time the westerly component of surface wind just ahead of the zone was not as strong as the eastward motion of the zone. The passage of the zone was accompanied by a veering and strengthening of the surface wind so that westerly components were briefly larger than the frontal motion. The tendency of the convergent wind field to produce a frontogenesis was evidently balanced by small-scale mixing.
Subsequently the pressure trough and surface wind shift propagated eastward more rapidly than the frontal temperature contrast. The contrast quickly weakened as the mixing then was unopposed. Severe convection developed during the evening as the convergent surface wind shift came into contact with humid unstably stratified air.
Abstract
A cold front developed in Arizona in a region of initially small temperature gradient, developing to great intensity, accompanied by damaging winds over New Mexico, and then losing strength over the high plains of Oklahoma and Texas. The entire development of frontogenesis and frontolysis occurred in no more than 24 h.
The initial growth of temperature contrast was attributable mainly to horizontal variation of surface heat flux during the morning, with little heating in a region of dense cloud cover and scattered showers in the west and with intense heating in a region of only thin high clouds to the east. The accompanying ageostrophic circulation then resulted in a collapse toward discontinuity. The frontal zone maintained an approximately steady state for a few hours in early afternoon. At this time the westerly component of surface wind just ahead of the zone was not as strong as the eastward motion of the zone. The passage of the zone was accompanied by a veering and strengthening of the surface wind so that westerly components were briefly larger than the frontal motion. The tendency of the convergent wind field to produce a frontogenesis was evidently balanced by small-scale mixing.
Subsequently the pressure trough and surface wind shift propagated eastward more rapidly than the frontal temperature contrast. The contrast quickly weakened as the mixing then was unopposed. Severe convection developed during the evening as the convergent surface wind shift came into contact with humid unstably stratified air.
Abstract
A heavy rainstorm over Kentucky, producing extensive flooding, was concentrated in a narrow band oriented nearly zonally just south of the Ohio River. Analysis of routine surface observations showed that an intense quasi-stationary surface front formed during the 24-h period of heaviest rainfall. This front was parallel to the rainband and was some distance to the south of it. Horizontal temperature gradients reached more than 20°F over 110 km. Analysis of sea level pressure showed that geostrophic deformation was present in a small region ahead of each of two small centers of low pressure that migrated eastward along the front. Vertical cross sections normal to the front showed that conditional upright and symmetric stabilities were small or negative in the frontal updraft. It was inferred from this that the frontal updraft was unusually intense and narrow, qualitatively consistent with the intensity of the rainband.
Abstract
A heavy rainstorm over Kentucky, producing extensive flooding, was concentrated in a narrow band oriented nearly zonally just south of the Ohio River. Analysis of routine surface observations showed that an intense quasi-stationary surface front formed during the 24-h period of heaviest rainfall. This front was parallel to the rainband and was some distance to the south of it. Horizontal temperature gradients reached more than 20°F over 110 km. Analysis of sea level pressure showed that geostrophic deformation was present in a small region ahead of each of two small centers of low pressure that migrated eastward along the front. Vertical cross sections normal to the front showed that conditional upright and symmetric stabilities were small or negative in the frontal updraft. It was inferred from this that the frontal updraft was unusually intense and narrow, qualitatively consistent with the intensity of the rainband.
Abstract
Present surface frontal analyses suffer from the defect that frontal positions are typically not collocated with zones of intense temperature contrast. Further, individuals typically do not agree as to the existence, type, and location of fronts.
The author argues that the lack of a surface temperature analysis is mainly responsible for these flaws, and it is proposed that such analysis, preferably of potential temperature in regions of variable terrain elevation, become part of routine procedure. Such an analysis will reveal nonfrontal baroclinic zones of considerable intensity. Most cold fronts, except the strongest ones, are denoted as baroclinic troughs, propagating eastward in the prevailing westerly flow. It is argued that when a meridional cold front exists in the presence of even a small meridional temperature gradient, the wind shift should propagate away from the intense surface temperature gradient, which then weakens. An explanation is provided, based on quasigeostrophic theory. It follows that fronts are short-lived phenomena.
Abstract
Present surface frontal analyses suffer from the defect that frontal positions are typically not collocated with zones of intense temperature contrast. Further, individuals typically do not agree as to the existence, type, and location of fronts.
The author argues that the lack of a surface temperature analysis is mainly responsible for these flaws, and it is proposed that such analysis, preferably of potential temperature in regions of variable terrain elevation, become part of routine procedure. Such an analysis will reveal nonfrontal baroclinic zones of considerable intensity. Most cold fronts, except the strongest ones, are denoted as baroclinic troughs, propagating eastward in the prevailing westerly flow. It is argued that when a meridional cold front exists in the presence of even a small meridional temperature gradient, the wind shift should propagate away from the intense surface temperature gradient, which then weakens. An explanation is provided, based on quasigeostrophic theory. It follows that fronts are short-lived phenomena.
Abstract
When the temperature of a parcel lifted adiabatically from the surface to a given pressure level (SLT) is subtracted from the ambient temperature at that level, the result is the surface index (SLI) for that level. This quantity, for the 500-mb pressure level, has been found useful in the very-short-range prediction of deep and often severe convection. We examine the thermodynamics of the lifting process, finding that the SLTs are sensitive mainly to surface dewpoint for very warm humid air and to surface temperature for cold air.
Nomograms are presented for determination of SLT's for 850, 700 and 500 mb, given surface temperature and dewpoint and a nominal surface pressure of 950 mb. An approximate correction is given for departures from this pressure. These are intended for use with the routine aviation surface observations. An additional nomogram is provided for use over the sea.
An example of application is shown, based on hourly surface observations and on ambient temperatures interpolated between standard rawinsonde analysis times. Severe convection is observed where large parcel buoyancy is indicated by SLI at 500 mb, with no negative buoyancy at 700 or 850 mb. Apparently unreliable measurements of dewpoint in warm humid air constitute a significant analysis problem, and reliable large-scale predictions of ambient temperature are a requirement for optimum real-time use.
Abstract
When the temperature of a parcel lifted adiabatically from the surface to a given pressure level (SLT) is subtracted from the ambient temperature at that level, the result is the surface index (SLI) for that level. This quantity, for the 500-mb pressure level, has been found useful in the very-short-range prediction of deep and often severe convection. We examine the thermodynamics of the lifting process, finding that the SLTs are sensitive mainly to surface dewpoint for very warm humid air and to surface temperature for cold air.
Nomograms are presented for determination of SLT's for 850, 700 and 500 mb, given surface temperature and dewpoint and a nominal surface pressure of 950 mb. An approximate correction is given for departures from this pressure. These are intended for use with the routine aviation surface observations. An additional nomogram is provided for use over the sea.
An example of application is shown, based on hourly surface observations and on ambient temperatures interpolated between standard rawinsonde analysis times. Severe convection is observed where large parcel buoyancy is indicated by SLI at 500 mb, with no negative buoyancy at 700 or 850 mb. Apparently unreliable measurements of dewpoint in warm humid air constitute a significant analysis problem, and reliable large-scale predictions of ambient temperature are a requirement for optimum real-time use.
Abstract
For assessment of the sufficiency of the approach of an upper-level vorticity maximum as a predictor of explosive surface cyclogenesis in the western Atlantic, a study was made of all 500-mb maxima which crossed the east coast of North America between 5 October 1985 and 4 April 1986. Of 96 such events, 38 produced bombs. This overall likelihood of 40% was greatest (50%) during the period from 19 December through 1 February, when crossings were most frequent.
In the 26 cases when the strength of the vorticity maximum was at least 22 × 10−5 s−1, bomb likelihood was also 50%. When the crossing occurred at or equatorward of latitude 41°N, bombs occurred in 17 of 33 instances (52%). The likelihood rose to 47% when the speed of the upper maximum exceeded 30 kts (15.5 m s−1). The best discrimination was found when this speed was multiplied by a measure of the strength of the vorticity gradients upstream and downstream from the center. On this basis, 16% of the 25 smallest values of this product produced bombs whereas 68% of the 25 largest values did so. Thus it appears that the intensity of the baroclinic forcing influences the probability of an explosive response. The effect of tropospheric stability, static or symmetric, was not examined.
Abstract
For assessment of the sufficiency of the approach of an upper-level vorticity maximum as a predictor of explosive surface cyclogenesis in the western Atlantic, a study was made of all 500-mb maxima which crossed the east coast of North America between 5 October 1985 and 4 April 1986. Of 96 such events, 38 produced bombs. This overall likelihood of 40% was greatest (50%) during the period from 19 December through 1 February, when crossings were most frequent.
In the 26 cases when the strength of the vorticity maximum was at least 22 × 10−5 s−1, bomb likelihood was also 50%. When the crossing occurred at or equatorward of latitude 41°N, bombs occurred in 17 of 33 instances (52%). The likelihood rose to 47% when the speed of the upper maximum exceeded 30 kts (15.5 m s−1). The best discrimination was found when this speed was multiplied by a measure of the strength of the vorticity gradients upstream and downstream from the center. On this basis, 16% of the 25 smallest values of this product produced bombs whereas 68% of the 25 largest values did so. Thus it appears that the intensity of the baroclinic forcing influences the probability of an explosive response. The effect of tropospheric stability, static or symmetric, was not examined.
Abstract
The skill of the Nested-Grid Model (NGM) and the global spectral model (GLBL) at the National Meterological Center in the prediction of explosive cyclogenesis was evaluated for the period 1 September 1986–30 April 1987. Manual analyses covering the eastern North Pacific, North America and the North Atlantic eastward to 20°W were used as ground truth. The criterion for a bomb event in the analyses of the forcasts was a deepening of the center of at least 24 mb at 60°N, normalized geostrophically at other latitudes, in a period of 24 h, beginning at 0000 or 1200 UTC.
Both models displayed skill out to 48 h for the NGM and 60 h for the GLBL. The NGM performed notably better in the innermost fine-grid area than in the surrounding area of overlap with a more coarse grid. For the GLBL in the Atlantic and North America, similar skill was seen through 36 h; skill was very small in the Pacific region. 12-h deepening beginning 12 h after initialization was compared with analyzed deepening for both models. Correlations ranged from 0.72 for the NGM in the inner grid over the Atlantic and North America to 0.03 in the Pacific. The GLBL values were intermediate, again better in the Atlantic than in the Pacific. All samples showed an average shortfall of predicted deepening from 12–24 h after initialization, ranging from 1 mb for the inner NGM grid to 7 mb for the overlap area, with the GLBL intermediate; again, it was much better in the Atlantic than in the Pacific.
Growth of skill over the past few years is attributable to improved analyses, better model resolution and better treatment of bounndary-layer fluxes. Initial data limitations are now the most important factor, both in models and in verifying analyses. These results alter the nature of the problem of research on explosive cyclogenesis from one of discovering a missing ingredient to one of improving the performance and extending the range of predictability.
Abstract
The skill of the Nested-Grid Model (NGM) and the global spectral model (GLBL) at the National Meterological Center in the prediction of explosive cyclogenesis was evaluated for the period 1 September 1986–30 April 1987. Manual analyses covering the eastern North Pacific, North America and the North Atlantic eastward to 20°W were used as ground truth. The criterion for a bomb event in the analyses of the forcasts was a deepening of the center of at least 24 mb at 60°N, normalized geostrophically at other latitudes, in a period of 24 h, beginning at 0000 or 1200 UTC.
Both models displayed skill out to 48 h for the NGM and 60 h for the GLBL. The NGM performed notably better in the innermost fine-grid area than in the surrounding area of overlap with a more coarse grid. For the GLBL in the Atlantic and North America, similar skill was seen through 36 h; skill was very small in the Pacific region. 12-h deepening beginning 12 h after initialization was compared with analyzed deepening for both models. Correlations ranged from 0.72 for the NGM in the inner grid over the Atlantic and North America to 0.03 in the Pacific. The GLBL values were intermediate, again better in the Atlantic than in the Pacific. All samples showed an average shortfall of predicted deepening from 12–24 h after initialization, ranging from 1 mb for the inner NGM grid to 7 mb for the overlap area, with the GLBL intermediate; again, it was much better in the Atlantic than in the Pacific.
Growth of skill over the past few years is attributable to improved analyses, better model resolution and better treatment of bounndary-layer fluxes. Initial data limitations are now the most important factor, both in models and in verifying analyses. These results alter the nature of the problem of research on explosive cyclogenesis from one of discovering a missing ingredient to one of improving the performance and extending the range of predictability.
Abstract
The subjective process of probability forecasting is analyzed. It is found to contain a sorling aspect, in which the forecaster distributes all instances into an ordered set of categories of likelihood of occurrence, and a laboling aspect, in which the forecaster assigns an anticipated relative frequency, or probability, of occurrence for each category. These two aspects are identified with the concepts of sharpness and validity, which have been introduced by other writers. The verification score proposed by Brier is shown to consist of the sum of measures of these two qualities. A satisfactory measure of synoptic skill is obtained by applying the Brier score to the synoptic probability forecast and to a control forecast of the climatological probability, and by expressing the difference as a percentage of the control score.
In an analysis of a large number of short-range probability forecasts made by instructors and students in the synoptic laboratory of the Massachusetts Institute of Technology it is found that even inexperienced forecasters are capable of displaying validity and skill except when dealing with events which occur very rarely or nearly always. Skill for average or net conditions over 24-hr periods is found to he roughly twice the skill in forecasts for a particular instant and is found to vary with the directness with which the weather element can be inferred from prognostic charts. The average of the judgment of two or more forecasters with comparable experience is found to be a more skillful statement than the forecast of the most skilled individual.
Abstract
The subjective process of probability forecasting is analyzed. It is found to contain a sorling aspect, in which the forecaster distributes all instances into an ordered set of categories of likelihood of occurrence, and a laboling aspect, in which the forecaster assigns an anticipated relative frequency, or probability, of occurrence for each category. These two aspects are identified with the concepts of sharpness and validity, which have been introduced by other writers. The verification score proposed by Brier is shown to consist of the sum of measures of these two qualities. A satisfactory measure of synoptic skill is obtained by applying the Brier score to the synoptic probability forecast and to a control forecast of the climatological probability, and by expressing the difference as a percentage of the control score.
In an analysis of a large number of short-range probability forecasts made by instructors and students in the synoptic laboratory of the Massachusetts Institute of Technology it is found that even inexperienced forecasters are capable of displaying validity and skill except when dealing with events which occur very rarely or nearly always. Skill for average or net conditions over 24-hr periods is found to he roughly twice the skill in forecasts for a particular instant and is found to vary with the directness with which the weather element can be inferred from prognostic charts. The average of the judgment of two or more forecasters with comparable experience is found to be a more skillful statement than the forecast of the most skilled individual.
Abstract
Abstract
Abstract
Quasi-geostrophic diagnosis of the fields of vertical motion and streamfunction tendency, based on wind analysis, was undertaken for the monsoon depression of 5–8 July 1979, a uniquely well-observed case during Summer MONEX. Wind analysis, in terms of streamfunction and velocity potential, was performed for data averaged in three layers of equal pressure depth front 1000 to 100 mb. The thermal stratification near the depression above 850 mb was approximately moist-adiabatic. The wind analyses showed that the basic zonal currents, westerly in the lower troposphere and easterly in the middle levels, weakened as the depression formed. Central vorticity increased rapidly on 5–6 July to a magnitude three times the Coriolis parameter. The center tilted toward the southwest with elevation.
The quasi-geostrophic omega-equation was solved at the interfaces between layers, for a range of stabilities from the full dry-adiabatic value to 1% of it. Ascent west of the center for the near-neutral stability was sufficient to account for the storm-scale average observed precipitation. Divergences for each layer were combined with vorticity advections to calculate the quasi-geostrophic streamfunction tendencies. The observed slow westward motion of the system was reasonably well accounted for, but the temporally increasing meridional slope of the system was somewhat exaggerated. The strong increase in central vorticity during development could not be accounted for by divergence arising from advections of temperature and vorticity.
Abstract
Quasi-geostrophic diagnosis of the fields of vertical motion and streamfunction tendency, based on wind analysis, was undertaken for the monsoon depression of 5–8 July 1979, a uniquely well-observed case during Summer MONEX. Wind analysis, in terms of streamfunction and velocity potential, was performed for data averaged in three layers of equal pressure depth front 1000 to 100 mb. The thermal stratification near the depression above 850 mb was approximately moist-adiabatic. The wind analyses showed that the basic zonal currents, westerly in the lower troposphere and easterly in the middle levels, weakened as the depression formed. Central vorticity increased rapidly on 5–6 July to a magnitude three times the Coriolis parameter. The center tilted toward the southwest with elevation.
The quasi-geostrophic omega-equation was solved at the interfaces between layers, for a range of stabilities from the full dry-adiabatic value to 1% of it. Ascent west of the center for the near-neutral stability was sufficient to account for the storm-scale average observed precipitation. Divergences for each layer were combined with vorticity advections to calculate the quasi-geostrophic streamfunction tendencies. The observed slow westward motion of the system was reasonably well accounted for, but the temporally increasing meridional slope of the system was somewhat exaggerated. The strong increase in central vorticity during development could not be accounted for by divergence arising from advections of temperature and vorticity.
