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Abstract
Input data from the AYE-SESAME I experiment are utilized to describe the effects of random errors in rawinsonde data on the computation of ageostrophic winds Computer-generated random errors for wind direction and speed and temperature are introduced into the station soundings at 25 mb intervals from which isentropic data sets are created. The ageostrophic wind and its components are computed for both the 312 K and 324 K isentropic surfaces. The total form of the ageostrophic wind components consists of the local wind tendency and inertial-advective contributions. The geostrophic momentum form of the ageostrophic wind components consists of the isallobaric and inertial-geostrophic-advective contributions. All winds are computed for 2000 GMT 10 April 1979 except the isallobaric and the local wind tendency, which are calculated for the 1700–2000 GMT time period. The ageostrophic wind components were quite strong during this time.
Quantitative analyses utilizing various statistical parameters and qualitative analyses from a visual comparison of fields are discussed. Results show that the geostrophic momentum forms of the ageostrophic wind components are generally less sensitive to rawinsonde error than their total form counterparts. This is most likely due to the fact that the mass field is observed more accurately by a rawinsonde than the momentum field. Divergence fields generated from the various ageostrophic wind components are presented as they are more sensitive to rawinsonde error than the various ageostrophic wind components themselves. These divergence fields revel that the isallobaric and inertial-geostrophic-advective divergences are less affected by rawinsonde random errors then the divergence of the local wind tendency or inertial-advective winds. Finally, this study indicates that the ageostrophic wind can be reliably diagnosed from the rawinsonde data gathered during the AYE-SESAME I experiment in 1979.
Abstract
Input data from the AYE-SESAME I experiment are utilized to describe the effects of random errors in rawinsonde data on the computation of ageostrophic winds Computer-generated random errors for wind direction and speed and temperature are introduced into the station soundings at 25 mb intervals from which isentropic data sets are created. The ageostrophic wind and its components are computed for both the 312 K and 324 K isentropic surfaces. The total form of the ageostrophic wind components consists of the local wind tendency and inertial-advective contributions. The geostrophic momentum form of the ageostrophic wind components consists of the isallobaric and inertial-geostrophic-advective contributions. All winds are computed for 2000 GMT 10 April 1979 except the isallobaric and the local wind tendency, which are calculated for the 1700–2000 GMT time period. The ageostrophic wind components were quite strong during this time.
Quantitative analyses utilizing various statistical parameters and qualitative analyses from a visual comparison of fields are discussed. Results show that the geostrophic momentum forms of the ageostrophic wind components are generally less sensitive to rawinsonde error than their total form counterparts. This is most likely due to the fact that the mass field is observed more accurately by a rawinsonde than the momentum field. Divergence fields generated from the various ageostrophic wind components are presented as they are more sensitive to rawinsonde error than the various ageostrophic wind components themselves. These divergence fields revel that the isallobaric and inertial-geostrophic-advective divergences are less affected by rawinsonde random errors then the divergence of the local wind tendency or inertial-advective winds. Finally, this study indicates that the ageostrophic wind can be reliably diagnosed from the rawinsonde data gathered during the AYE-SESAME I experiment in 1979.
Fields of divergence, vertical motion, stability, and surface pressure tendency are examined at 3 h intervals for the first regional scale AVE-SESAME '79 (Atmospheric Variability Experiment—Severe Environmental Storms and Mesoscale Experiment) day. Two areas of severe storms formed during the period from 1200 GMT 10 April through 1200 GMT 11 April. The Red River Valley outbreak began during the afternoon of 10 April, while a second area formed in southwestern Texas during the early evening hours. Results show the rapid changes in environmental conditions associated with these two storm areas.
The propagation of an upper level jet streak into the region was a major factor in producing the Red River Valley outbreak. This streak was associated with the formation of a strong low-level jet and a small-scale surface pressure perturbation. The sudden development of a strong upper tropospheric wind maximum over Oklahoma and Kansas corresponded with major changes in kinematic parameters at that level. Instability over the Red River Valley was released by strong upward motion producing intense convection.
Similar features were responsible for the storms in southwestern Texas. Although this area was quite unstable, forcing mechanisms appear somewhat weaker than in the earlier outbreak.
Fields of divergence, vertical motion, stability, and surface pressure tendency are examined at 3 h intervals for the first regional scale AVE-SESAME '79 (Atmospheric Variability Experiment—Severe Environmental Storms and Mesoscale Experiment) day. Two areas of severe storms formed during the period from 1200 GMT 10 April through 1200 GMT 11 April. The Red River Valley outbreak began during the afternoon of 10 April, while a second area formed in southwestern Texas during the early evening hours. Results show the rapid changes in environmental conditions associated with these two storm areas.
The propagation of an upper level jet streak into the region was a major factor in producing the Red River Valley outbreak. This streak was associated with the formation of a strong low-level jet and a small-scale surface pressure perturbation. The sudden development of a strong upper tropospheric wind maximum over Oklahoma and Kansas corresponded with major changes in kinematic parameters at that level. Instability over the Red River Valley was released by strong upward motion producing intense convection.
Similar features were responsible for the storms in southwestern Texas. Although this area was quite unstable, forcing mechanisms appear somewhat weaker than in the earlier outbreak.
Abstract
A diagnostic study of a continental occluding extratropical cyclone (ETC) during 1–2 November 1992 is presented using initializations from the Mesoscale Atmospheric Prediction System (MAPS), a hybrid sigma–isentropic coordinate model. Whereas recent studies have concentrated on maritime ETCs and have used numerical model simulations, this study employs diagnostic, observational data and model initializations to develop an occlusion model. In addition, isentropic parcel trajectories from a diabatic trajectory model are examined to trace the origin and termination of air parcels associated with the development of the occluded front. The chosen storm was a moderately deepening (i.e., typical) ETC over a data-rich continental region. This storm developed over the central United States, where commercial aircraft and a network of wind profilers provided copious asynoptic data aloft, which was ingested by the MAPS. Analyses of this well-defined occluded cyclone tend to verify that the advancing cold front overtakes the retreating warm front, though it does not “ride up” the warm front, and that warm-sector parcels are lifted upward in the vicinity of the occluded front, thereby confirming that some of the parcels aloft over the surface occluded front do originate near the surface prior to occlusion. Discussion is also provided on the nature of the occluded front as a true frontal boundary.
Abstract
A diagnostic study of a continental occluding extratropical cyclone (ETC) during 1–2 November 1992 is presented using initializations from the Mesoscale Atmospheric Prediction System (MAPS), a hybrid sigma–isentropic coordinate model. Whereas recent studies have concentrated on maritime ETCs and have used numerical model simulations, this study employs diagnostic, observational data and model initializations to develop an occlusion model. In addition, isentropic parcel trajectories from a diabatic trajectory model are examined to trace the origin and termination of air parcels associated with the development of the occluded front. The chosen storm was a moderately deepening (i.e., typical) ETC over a data-rich continental region. This storm developed over the central United States, where commercial aircraft and a network of wind profilers provided copious asynoptic data aloft, which was ingested by the MAPS. Analyses of this well-defined occluded cyclone tend to verify that the advancing cold front overtakes the retreating warm front, though it does not “ride up” the warm front, and that warm-sector parcels are lifted upward in the vicinity of the occluded front, thereby confirming that some of the parcels aloft over the surface occluded front do originate near the surface prior to occlusion. Discussion is also provided on the nature of the occluded front as a true frontal boundary.
Abstract
Terms of the balance equation were calculated at 300 mb to diagnose unbalanced flow in the upper troposphere both prior to and during a period of strong convection which took place during the AVE-SESAME I period. The sum of the balance equation terms displayed large imbalances(>50×10−10s−2)over the Red River Valley as early as 2100 UTC. This region grew in magnitude, expanding over Oklahoma during the next 3–6 hours. The vorticity and Laplacian terms in the balance equation dominated this imbalance.
An examination of ageostrophic, geostrophic and actual 300 mb winds at 2100 UTC revealed that the ageostrophic winds over Oklahoma were directed towards lower geopotential heights, indicating that the flow was neither in geostrophic balance nor merely responding to curved flow as described by the gradient wind equation. Such imbalance led to substantial increases in divergence and midlevel upward vertical motion.
The remaining terms of the divergence equation were computed and summed. These terms partially compensated for the strong divergence tendencies created from the balance equation. In this way, the divergence and vertical motion terms of the divergence equation checked the growth of divergence in upper levels.
Finally, an error assessment was conducted on the terms of the divergence equation. Although balance equation terms are susceptible to substantial error due to random errors in wind and height data, the patterns of these terms are more reliable, thereby permitting the conclusions of this case study.
Abstract
Terms of the balance equation were calculated at 300 mb to diagnose unbalanced flow in the upper troposphere both prior to and during a period of strong convection which took place during the AVE-SESAME I period. The sum of the balance equation terms displayed large imbalances(>50×10−10s−2)over the Red River Valley as early as 2100 UTC. This region grew in magnitude, expanding over Oklahoma during the next 3–6 hours. The vorticity and Laplacian terms in the balance equation dominated this imbalance.
An examination of ageostrophic, geostrophic and actual 300 mb winds at 2100 UTC revealed that the ageostrophic winds over Oklahoma were directed towards lower geopotential heights, indicating that the flow was neither in geostrophic balance nor merely responding to curved flow as described by the gradient wind equation. Such imbalance led to substantial increases in divergence and midlevel upward vertical motion.
The remaining terms of the divergence equation were computed and summed. These terms partially compensated for the strong divergence tendencies created from the balance equation. In this way, the divergence and vertical motion terms of the divergence equation checked the growth of divergence in upper levels.
Finally, an error assessment was conducted on the terms of the divergence equation. Although balance equation terms are susceptible to substantial error due to random errors in wind and height data, the patterns of these terms are more reliable, thereby permitting the conclusions of this case study.
Abstract
A mesoscale convective system (MCS) developed during the morning hours of 6 June 1993 and moved across northern and central Missouri, resulting in a narrow swath of excessive rainfall (>150 mm). The MCS developed well north of a surface warm front above a cool, stable boundary layer and moved east–southeast across the state. Although some features of the synoptic environment agree with the frontal flash flood composite model, predicting the elevated thunderstorms that composed the MCS posed a unique forecasting challenge. This paper first describes the diagnostic parameters of the prestorm environment that would have been helpful to predict the initiation of the MCS and the resultant locally excessive precipitation. Attention is then drawn to the MCS itself via IR satellite and WSR-88D imagery. Finally, the similarities and differences of this episode to previous studies of flash flooding and elevated thunderstorms are noted, and a summary of key parameters useful in the anticipation of this type of convection and associated heavy rainfall are offered.
Abstract
A mesoscale convective system (MCS) developed during the morning hours of 6 June 1993 and moved across northern and central Missouri, resulting in a narrow swath of excessive rainfall (>150 mm). The MCS developed well north of a surface warm front above a cool, stable boundary layer and moved east–southeast across the state. Although some features of the synoptic environment agree with the frontal flash flood composite model, predicting the elevated thunderstorms that composed the MCS posed a unique forecasting challenge. This paper first describes the diagnostic parameters of the prestorm environment that would have been helpful to predict the initiation of the MCS and the resultant locally excessive precipitation. Attention is then drawn to the MCS itself via IR satellite and WSR-88D imagery. Finally, the similarities and differences of this episode to previous studies of flash flooding and elevated thunderstorms are noted, and a summary of key parameters useful in the anticipation of this type of convection and associated heavy rainfall are offered.
Abstract
A diagnostic/prognostic sounding analysis package is presented to aid operational forecasters. First, a diagnostic sounding analysis is shown which computes standard thermodynamic parameters while including a scheme to estimate the maximum hail diameter based on the positive areas above the convective condensation level (CCL) and the level of free convection (LFC). The hail algorithm basically uses the Anthes’ one-dimensional cloud model to estimate vertical motion which then is used to compute the hail diameter. A melting scheme is also presented to account for the inciting of hailstones in deep, warm wet-bulb temperature layers. Compared to the traditional Fawbush-Miller approach, the Pino-Moore hail algorithm showed improvement, significant at the 0.5% level when tested during actual hail events.
A forecast-sounding algorithm based on a technique described by McGinley is used to modify the morning sounding to account for insulation. For soundings without inversions mixing is also permitted to change the boundary layer dew point profile in accordance with results provided by Schaefer. Above the boundary layer a small percentage of the geostrophic thermal advection is used to estimate changes in the temperature aloft. Interaction with the forecaster is emphasized during each step of the forecast-sounding algorithm. The forecast sounding can then be used in the stability analysis program to generate a more realistic afternoon sounding with which to estimate the maximum hail diameter.
Abstract
A diagnostic/prognostic sounding analysis package is presented to aid operational forecasters. First, a diagnostic sounding analysis is shown which computes standard thermodynamic parameters while including a scheme to estimate the maximum hail diameter based on the positive areas above the convective condensation level (CCL) and the level of free convection (LFC). The hail algorithm basically uses the Anthes’ one-dimensional cloud model to estimate vertical motion which then is used to compute the hail diameter. A melting scheme is also presented to account for the inciting of hailstones in deep, warm wet-bulb temperature layers. Compared to the traditional Fawbush-Miller approach, the Pino-Moore hail algorithm showed improvement, significant at the 0.5% level when tested during actual hail events.
A forecast-sounding algorithm based on a technique described by McGinley is used to modify the morning sounding to account for insulation. For soundings without inversions mixing is also permitted to change the boundary layer dew point profile in accordance with results provided by Schaefer. Above the boundary layer a small percentage of the geostrophic thermal advection is used to estimate changes in the temperature aloft. Interaction with the forecaster is emphasized during each step of the forecast-sounding algorithm. The forecast sounding can then be used in the stability analysis program to generate a more realistic afternoon sounding with which to estimate the maximum hail diameter.
Abstract
On 30-31 January 1982 a modest low pressure system moved through the lower Mississippi Valley into western Tennessee. During this 24-hour period rain changed to snow over central Missouri and Illinois, increasing in intensity over the last 12 hours, In addition, embedded convection took place over east-central Missouri- Illinois including the St. Louis metropolitan area. Overall snow totals were over 25 cm within a narrow band along the edge of the precipitation shield. Frontogenetical forcing together with conditional symmetric instability are discussed as possible physical explanations for the intense precipitation which was not well forecast.
It is shown that moderate-strong ascent was part of a thermally direct ageostrophic circulation created by frontogenetical forcing. Frontogenesis is shown both at the surface and aloft to increase in the 12 hour period prior to the heavy precipitation. It is strongest in the low levels and slopes to the west-northwest with height along an advancing cold frontal zone. Both deformation and divergence components of the frontogenetical function equation play key roles in the total frontogenesis. Quasi-geostrophic frontogenesis is also shown to be quite strong, especially at low levels. Q-vector forcing of vertical motion increased with the quasi-geostrophic frontogenesis and helped create a direct thermal circulation of warm air rising in south-central Missouri-Illinois and cool air sinking in northern portions of those states normal to the axis of maximum frontogenetical forcing.
Conditional symmetric instability was also diagnosed in the region of the updraft of the direct thermal circulation. This “slantwise convection”, diagnosed where surfaces of constant geostrophic angular momentum slope less than surfaces of equivalent potential temperature, is believed to have both increased the intensity of the updraft and decreased the scale length of the phenomena. The conditional symmetric instability may have helped to create pulselike eruptions of elevated cloud tops seen in satellite imagery and often noted with East Coast cyclogenesis by several researchers.
Abstract
On 30-31 January 1982 a modest low pressure system moved through the lower Mississippi Valley into western Tennessee. During this 24-hour period rain changed to snow over central Missouri and Illinois, increasing in intensity over the last 12 hours, In addition, embedded convection took place over east-central Missouri- Illinois including the St. Louis metropolitan area. Overall snow totals were over 25 cm within a narrow band along the edge of the precipitation shield. Frontogenetical forcing together with conditional symmetric instability are discussed as possible physical explanations for the intense precipitation which was not well forecast.
It is shown that moderate-strong ascent was part of a thermally direct ageostrophic circulation created by frontogenetical forcing. Frontogenesis is shown both at the surface and aloft to increase in the 12 hour period prior to the heavy precipitation. It is strongest in the low levels and slopes to the west-northwest with height along an advancing cold frontal zone. Both deformation and divergence components of the frontogenetical function equation play key roles in the total frontogenesis. Quasi-geostrophic frontogenesis is also shown to be quite strong, especially at low levels. Q-vector forcing of vertical motion increased with the quasi-geostrophic frontogenesis and helped create a direct thermal circulation of warm air rising in south-central Missouri-Illinois and cool air sinking in northern portions of those states normal to the axis of maximum frontogenetical forcing.
Conditional symmetric instability was also diagnosed in the region of the updraft of the direct thermal circulation. This “slantwise convection”, diagnosed where surfaces of constant geostrophic angular momentum slope less than surfaces of equivalent potential temperature, is believed to have both increased the intensity of the updraft and decreased the scale length of the phenomena. The conditional symmetric instability may have helped to create pulselike eruptions of elevated cloud tops seen in satellite imagery and often noted with East Coast cyclogenesis by several researchers.
Abstract
Diagnostic techniques are presented that aid in determining whether a cold front is of the anafront (upslope flow) or katafront (downslope flow) type, as well as in measuring the intensity of the vertical motions in the vicinity of the frontal zone. Anafronts are characterized by postfrontal cloudiness and precipitation, while katafronts typically have precipitation in a band along or ahead of the cold front.
The techniques described include: 1) examination of the vertical profile of front-normal and front-parallel winds, 2) construction of an isentropic cross-section normal to the front including front-normal wind components, 3) computation of vertical motion using either the Bellamy triangle approach or adiabatic vertical motion on isentropic surfaces, and 4) analysis of cold air soundings for inversions, relative humidity profiles, and the degree of backing of winds in the vertical.
Case studies are then shown in which we apply the above techniques and prove their efficacy in classifying, anafronts and katafronts. Each technique is then discussed in terms of how useful it might be to an operational forecaster who typically has limited time for diagnosis.
Abstract
Diagnostic techniques are presented that aid in determining whether a cold front is of the anafront (upslope flow) or katafront (downslope flow) type, as well as in measuring the intensity of the vertical motions in the vicinity of the frontal zone. Anafronts are characterized by postfrontal cloudiness and precipitation, while katafronts typically have precipitation in a band along or ahead of the cold front.
The techniques described include: 1) examination of the vertical profile of front-normal and front-parallel winds, 2) construction of an isentropic cross-section normal to the front including front-normal wind components, 3) computation of vertical motion using either the Bellamy triangle approach or adiabatic vertical motion on isentropic surfaces, and 4) analysis of cold air soundings for inversions, relative humidity profiles, and the degree of backing of winds in the vertical.
Case studies are then shown in which we apply the above techniques and prove their efficacy in classifying, anafronts and katafronts. Each technique is then discussed in terms of how useful it might be to an operational forecaster who typically has limited time for diagnosis.
Abstract
Conditional symmetric instability (CSI) is an important property of the atmosphere when diagnosing and predicting mesoscale bands of moderate to heavy precipitation within winter cyclones. Within regions of CSI, slantwise convection can increase snow totals over narrow regions. Typically, CSI is evaluated in a cross-sectional plane chosen normal to the middle-tropospheric thermal wind using Mg , the absolute geostrophic momentum, and θ e , the equivalent potential temperature. Regions where Mg , surfaces slope less than θ e , surfaces are subject to CSI. We describe an objective measure of CSI, called the equivalent potential vorticity (EPV), that makes evaluating CSI quick and effective. Cross sections of Mg , versus θ e , and EPV are compared for two cases to demonstrate the effectiveness of using EPV cross sections to diagnose CSI. The distinction between slantwise convection and upright convection is also demonstrated by these case studies.
Abstract
Conditional symmetric instability (CSI) is an important property of the atmosphere when diagnosing and predicting mesoscale bands of moderate to heavy precipitation within winter cyclones. Within regions of CSI, slantwise convection can increase snow totals over narrow regions. Typically, CSI is evaluated in a cross-sectional plane chosen normal to the middle-tropospheric thermal wind using Mg , the absolute geostrophic momentum, and θ e , the equivalent potential temperature. Regions where Mg , surfaces slope less than θ e , surfaces are subject to CSI. We describe an objective measure of CSI, called the equivalent potential vorticity (EPV), that makes evaluating CSI quick and effective. Cross sections of Mg , versus θ e , and EPV are compared for two cases to demonstrate the effectiveness of using EPV cross sections to diagnose CSI. The distinction between slantwise convection and upright convection is also demonstrated by these case studies.
Abstract
A simple two-layer primitive equation (PE) model is used to study the effect of curvature on jet-streak kinematics, specifically vertical motion. Three types of vertical motion are studied: kinematic (PE) vertical motion, quasigeostrophic (QG) vertical motion, and vertical motion associated with an unbalanced component of the flow, partially due to inertial-gravity waves (IGW). The latter vertical motion is computed as the difference between the PE vertical motion and “balanced” vertical motion derived from Krishnamurti's balanced omega equation. In addition, the upper-level ageostrophic flow is discussed as it relates to the patterns of divergence associated with various jet curvatures. The PE model was run out to 12 h to we how straight-line (SL), cyclonic (CY), and anticyclonic (AC) curvature affects jet-streak kinematics.
At the initial time, a two-cell pattern of vertical motion was found for the CY and AC jet streaks as opposed to the four-cell pattern associated with the SL jet streak. Also, the vertical-motion centers for the AC and CY jet streaks were aligned more along the jet axis than across it, in contrast with the SL case. Quasigeostrophic vertical motion for the SL and CY jet cases agreed well with the PE vertical motion but were much weaker than the PE vertical motion for the AC case.
Results for 12 h into the model run showed that unbalanced or IGW vertical motions were strongest for the CY case where they were equal to about one-half of the PE vertical motions. A comparison of QG vertical motion with balanced vertical motion illustrates the effect of curvature most dramatically. For cyclonic curvature, QG vertical motions are 50% stronger than the balanced vertical motions, while for anticyclonic curvature, they are 50% weaker than the balanced vertical motion. For the SL and AC cases, unbalanced vertical motions were smaller but were still a significant part of the total PE vertical motion. Thus, the greatest mutual adjustment between the mass and momentum fields occurs with cyclonically curved jet streaks. A comparison of vertical motions from runs in which the curvature was varied revealed that the magnitude of the vertical motion is strongest with cyclonic jet streaks, more modest with anticyclonic jet streaks, and weakest with straight-line jet streaks. This is reflected in the maximum Rossby numbers for these cases that were 1.0, 0.40, and 0.12, respectively.
Abstract
A simple two-layer primitive equation (PE) model is used to study the effect of curvature on jet-streak kinematics, specifically vertical motion. Three types of vertical motion are studied: kinematic (PE) vertical motion, quasigeostrophic (QG) vertical motion, and vertical motion associated with an unbalanced component of the flow, partially due to inertial-gravity waves (IGW). The latter vertical motion is computed as the difference between the PE vertical motion and “balanced” vertical motion derived from Krishnamurti's balanced omega equation. In addition, the upper-level ageostrophic flow is discussed as it relates to the patterns of divergence associated with various jet curvatures. The PE model was run out to 12 h to we how straight-line (SL), cyclonic (CY), and anticyclonic (AC) curvature affects jet-streak kinematics.
At the initial time, a two-cell pattern of vertical motion was found for the CY and AC jet streaks as opposed to the four-cell pattern associated with the SL jet streak. Also, the vertical-motion centers for the AC and CY jet streaks were aligned more along the jet axis than across it, in contrast with the SL case. Quasigeostrophic vertical motion for the SL and CY jet cases agreed well with the PE vertical motion but were much weaker than the PE vertical motion for the AC case.
Results for 12 h into the model run showed that unbalanced or IGW vertical motions were strongest for the CY case where they were equal to about one-half of the PE vertical motions. A comparison of QG vertical motion with balanced vertical motion illustrates the effect of curvature most dramatically. For cyclonic curvature, QG vertical motions are 50% stronger than the balanced vertical motions, while for anticyclonic curvature, they are 50% weaker than the balanced vertical motion. For the SL and AC cases, unbalanced vertical motions were smaller but were still a significant part of the total PE vertical motion. Thus, the greatest mutual adjustment between the mass and momentum fields occurs with cyclonically curved jet streaks. A comparison of vertical motions from runs in which the curvature was varied revealed that the magnitude of the vertical motion is strongest with cyclonic jet streaks, more modest with anticyclonic jet streaks, and weakest with straight-line jet streaks. This is reflected in the maximum Rossby numbers for these cases that were 1.0, 0.40, and 0.12, respectively.
