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Abstract
Large, long-lived convective systems over the United States in 1992 and 1993 have been classified according to physical characteristics observed in satellite imagery as quasi-circular [mesoscale convective complex (MCC)] or elongated [persistent elongated convective system (PECS)] and cataloged. The catalog includes the time of initiation, maximum extent, termination, duration, area of the −52°C cloud shield at the time of maximum extent, significant weather associated with each occurrence, and tracks of the −52°C cloud-shield centroid.
Both MCC and PECS favored nocturnal development and on average lasted about 12 h. In both 1992 and 1993, PECS produced −52°C cloud-shield areas of greater extent and occurred more frequently compared with MCCs. The mean position of initiation for PECS in 1992 and 1993 followed a seasonal shift similar to the climatological seasonal shift for MCC occurrences but was displaced eastward of the mean position of MCC initiation in 1992 and 1993. The spatial distribution of MCC and PECS occurrences contain a period of persistent development near 40°N in July 1992 and July 1993 that contributed to the extreme wetness experienced in the Midwest during these two months.
Both MCC and PECS initiated in environments characterized by deep, synoptic-scale ascent associated with continental-scale baroclinic waves. PECS occurrences initiated more often as vigorous waves exited the intermountain region, whereas MCCs initiated more often within a high-amplitude wave with a trough positioned over the northwestern United States and a ridge positioned over the Great Plains. The low-level jet transported moisture into the region of initiation for both MCC and PECS occurrences. The areal extent of convective initiation was limited by the orientation of low-level features for MCC occurrences.
Abstract
Large, long-lived convective systems over the United States in 1992 and 1993 have been classified according to physical characteristics observed in satellite imagery as quasi-circular [mesoscale convective complex (MCC)] or elongated [persistent elongated convective system (PECS)] and cataloged. The catalog includes the time of initiation, maximum extent, termination, duration, area of the −52°C cloud shield at the time of maximum extent, significant weather associated with each occurrence, and tracks of the −52°C cloud-shield centroid.
Both MCC and PECS favored nocturnal development and on average lasted about 12 h. In both 1992 and 1993, PECS produced −52°C cloud-shield areas of greater extent and occurred more frequently compared with MCCs. The mean position of initiation for PECS in 1992 and 1993 followed a seasonal shift similar to the climatological seasonal shift for MCC occurrences but was displaced eastward of the mean position of MCC initiation in 1992 and 1993. The spatial distribution of MCC and PECS occurrences contain a period of persistent development near 40°N in July 1992 and July 1993 that contributed to the extreme wetness experienced in the Midwest during these two months.
Both MCC and PECS initiated in environments characterized by deep, synoptic-scale ascent associated with continental-scale baroclinic waves. PECS occurrences initiated more often as vigorous waves exited the intermountain region, whereas MCCs initiated more often within a high-amplitude wave with a trough positioned over the northwestern United States and a ridge positioned over the Great Plains. The low-level jet transported moisture into the region of initiation for both MCC and PECS occurrences. The areal extent of convective initiation was limited by the orientation of low-level features for MCC occurrences.
Abstract
Large, long-lived mesoscale convective systems (MCSs) over the United States during the 1997–98 El Niño are documented. Two periods of abnormal MCS activity are identified in 1998: from March to mid-April an unusually large number of quasi-linear MCSs were observed in the Midwest; while quasi-circular MCSs in June–August of 1998 were concentrated near 37°N rather than following a seasonal shift similar to that observed in the climatological distribution. Episodic surges of northerly low-level flow were infrequent in March 1998, thereby leading to an unusually high incidence of quasi-linear MCSs and to precipitation anomalies in the central United States.
Abstract
Large, long-lived mesoscale convective systems (MCSs) over the United States during the 1997–98 El Niño are documented. Two periods of abnormal MCS activity are identified in 1998: from March to mid-April an unusually large number of quasi-linear MCSs were observed in the Midwest; while quasi-circular MCSs in June–August of 1998 were concentrated near 37°N rather than following a seasonal shift similar to that observed in the climatological distribution. Episodic surges of northerly low-level flow were infrequent in March 1998, thereby leading to an unusually high incidence of quasi-linear MCSs and to precipitation anomalies in the central United States.
Abstract
This paper quantifies the linear mass–wind field balance and its temporal variability in the global data assimilation system Data Assimilation Research Testbed/Community Atmosphere Model (DART/CAM), which is based on the ensemble adjustment Kalman filter. The part of the model state that projects onto quasigeostrophic modes represents the balanced state. The unbalanced part corresponds to inertio-gravity (IG) motions. The 80-member ensemble is diagnosed by using the normal-mode function expansion. It was found that the balanced variance in the prior ensemble is on average about 90% of the total variance and about 80% of the wave variance. Balance depends on the scale and the largest zonal scales are best balanced. For zonal wavenumbers greater than k = 30 the balanced variance stays at about the 45% level. There is more variance in the westward- than in the eastward-propagating IG modes; the difference is about 2% of the total wave variance and it is associated with the covariance inflation. The applied inflation field has a major impact on the structure of the prior variance field and its reduction by the assimilation step. The shape of the inflation field mimics the global radiosonde observation network (k = 2), which is associated with the minimum variance reduction in k = 2. Temporal variability of the ensemble variance is significant and appears to be associated with changes in the energy of the flow. A perfect-model assimilation experiment supports the findings from the real-observation experiment.
Abstract
This paper quantifies the linear mass–wind field balance and its temporal variability in the global data assimilation system Data Assimilation Research Testbed/Community Atmosphere Model (DART/CAM), which is based on the ensemble adjustment Kalman filter. The part of the model state that projects onto quasigeostrophic modes represents the balanced state. The unbalanced part corresponds to inertio-gravity (IG) motions. The 80-member ensemble is diagnosed by using the normal-mode function expansion. It was found that the balanced variance in the prior ensemble is on average about 90% of the total variance and about 80% of the wave variance. Balance depends on the scale and the largest zonal scales are best balanced. For zonal wavenumbers greater than k = 30 the balanced variance stays at about the 45% level. There is more variance in the westward- than in the eastward-propagating IG modes; the difference is about 2% of the total wave variance and it is associated with the covariance inflation. The applied inflation field has a major impact on the structure of the prior variance field and its reduction by the assimilation step. The shape of the inflation field mimics the global radiosonde observation network (k = 2), which is associated with the minimum variance reduction in k = 2. Temporal variability of the ensemble variance is significant and appears to be associated with changes in the energy of the flow. A perfect-model assimilation experiment supports the findings from the real-observation experiment.
Abstract
This paper presents the application of the normal-mode functions to diagnose the atmospheric energy spectra in terms of balanced and inertia–gravity (IG) contributions. A set of three-dimensional orthogonal normal modes is applied to four analysis datasets from July 2007. The datasets are the operational analysis systems of NCEP and ECMWF, the NCEP–NCAR reanalyses, and the Data Assimilation Research Testbed–Community Atmospheric Model (DART–CAM), an ensemble analysis system developed at NCAR. The differences between the datasets can be considered as a measure of uncertainty of the IG contribution to the global energetics.
The results show that the percentage of IG motion in the present NCEP, ECMWF, and DART–CAM analysis systems is between 1% and 2% of the total energy field. In the wave part of the flow (zonal wavenumber k ≠0), the IG energy contribution is between 9% and 15%. On the contrary, the NCEP–NCAR reanalyses contain more IG motion, especially in the Southern Hemisphere extratropics. Each analysis contains more energy in the eastward IG motion than in its westward counterpart. The difference is about 2%–3% of the total wave energy and it is associated with the motions projected onto the Kelvin wave in the tropics.
The selected truncation parameters of the expansion (zonal, meridional, and vertical truncation) ensure that the projection provides the optimal fit to the input data on model levels. This approach is different from previous applications of the normal modes and under the linearity assumption it allows the application of the inverse projection to obtain details of circulation associated with a selected type of motion. The bulk of the IG motion is confined to the tropics. For the successful reproduction of three-dimensional circulations by the normal modes it is important that the expansion includes a number of vertical modes.
Abstract
This paper presents the application of the normal-mode functions to diagnose the atmospheric energy spectra in terms of balanced and inertia–gravity (IG) contributions. A set of three-dimensional orthogonal normal modes is applied to four analysis datasets from July 2007. The datasets are the operational analysis systems of NCEP and ECMWF, the NCEP–NCAR reanalyses, and the Data Assimilation Research Testbed–Community Atmospheric Model (DART–CAM), an ensemble analysis system developed at NCAR. The differences between the datasets can be considered as a measure of uncertainty of the IG contribution to the global energetics.
The results show that the percentage of IG motion in the present NCEP, ECMWF, and DART–CAM analysis systems is between 1% and 2% of the total energy field. In the wave part of the flow (zonal wavenumber k ≠0), the IG energy contribution is between 9% and 15%. On the contrary, the NCEP–NCAR reanalyses contain more IG motion, especially in the Southern Hemisphere extratropics. Each analysis contains more energy in the eastward IG motion than in its westward counterpart. The difference is about 2%–3% of the total wave energy and it is associated with the motions projected onto the Kelvin wave in the tropics.
The selected truncation parameters of the expansion (zonal, meridional, and vertical truncation) ensure that the projection provides the optimal fit to the input data on model levels. This approach is different from previous applications of the normal modes and under the linearity assumption it allows the application of the inverse projection to obtain details of circulation associated with a selected type of motion. The bulk of the IG motion is confined to the tropics. For the successful reproduction of three-dimensional circulations by the normal modes it is important that the expansion includes a number of vertical modes.
Abstract
This paper analyzes the spectra and spatiotemporal features of the large-scale inertia-gravity (IG) circulations in four analysis systems in the tropics. Of special interest is the Kelvin wave (KW), which represents between 7% and 25% of the total IG wave (zonal wavenumber k ≠0) energy. The mixed Rossby–gravity (MRG) mode comprises between 4% and 15% of the IG wave energy. At the longest scales, the KW spectra are fitted by a 
The average circulation associated with KW is characterized by reverse flows in the upper and lower troposphere consistent with the ideas behind simple tropical models. The inverse projection is used to quantify the role of Kelvin and MRG waves in current analysis systems in the upper troposphere over the Indian Ocean. At these levels, easterlies between 10°S and 30°N are represented by the KW to a significant degree while the cross-equatorial flow toward the descending branch of the Hadley cell at 10°S is associated with the MRG waves.
The transient structure of equatorial waves is presented in the space of normal modes defined by the zonal wavenumbers, meridional Hough functions, and the vertical eigenfunctions. The difference in the depth of the model domain in DART–CAM and NCEP–NCAR on one hand and ECMWF and NCEP on the other appears to be one reason for different wave propagation properties. In the latter case the vertical energy propagation is diagnosed by filtering the propagating KW modes back to physical space. The results agree with the linear theory of vertically propagating equatorial waves.
Abstract
This paper analyzes the spectra and spatiotemporal features of the large-scale inertia-gravity (IG) circulations in four analysis systems in the tropics. Of special interest is the Kelvin wave (KW), which represents between 7% and 25% of the total IG wave (zonal wavenumber k ≠0) energy. The mixed Rossby–gravity (MRG) mode comprises between 4% and 15% of the IG wave energy. At the longest scales, the KW spectra are fitted by a
The average circulation associated with KW is characterized by reverse flows in the upper and lower troposphere consistent with the ideas behind simple tropical models. The inverse projection is used to quantify the role of Kelvin and MRG waves in current analysis systems in the upper troposphere over the Indian Ocean. At these levels, easterlies between 10°S and 30°N are represented by the KW to a significant degree while the cross-equatorial flow toward the descending branch of the Hadley cell at 10°S is associated with the MRG waves.
The transient structure of equatorial waves is presented in the space of normal modes defined by the zonal wavenumbers, meridional Hough functions, and the vertical eigenfunctions. The difference in the depth of the model domain in DART–CAM and NCEP–NCAR on one hand and ECMWF and NCEP on the other appears to be one reason for different wave propagation properties. In the latter case the vertical energy propagation is diagnosed by filtering the propagating KW modes back to physical space. The results agree with the linear theory of vertically propagating equatorial waves.
Abstract
The analysis of the rainband structure and wind fields associated with a coastal front along the North Carolina shoreline is described. Dual-Doppler radar and the augmented GALE (Genesis of Atlantic Lows Experiment) ensemble of in situ stations depict shallow, convective rainbands that overtake the front from the warm-air sector and intensify at the surface front location. Clockwise band rotation is shown to be caused by the difference in alignment between the echo motion and the rainband axes and by new development ahead of the front.
Radar measurements depict the circulation systems associated with a portion of one rainband in the cold air ahead of the front. Here shallow precipitation cores are vertically tilted due to the frontal wind shear. Circulation cells and most precipitation cores are centered just above the frontal inversion, as inferred by the wind shift line aloft. This feature is nearly horizontal in the cross-frontal direction but slopes downward in a direction roughly parallel to the front.
Ahead of the front, main updrafts in and above the cold air are found near the upwind portion of precipitation cores and along two well-defined lines aligned roughly perpendicular to the front. These lines propagate northward and affect several nearby surface sites prior to frontal passage. The speed of northward propagation is consistent with gravity wave theory, while on the larger scale the front appears to behave as the leading edge of a density current. The major features found in this case are compared and contrasted with those of a synoptic-scale warm front.
Abstract
The analysis of the rainband structure and wind fields associated with a coastal front along the North Carolina shoreline is described. Dual-Doppler radar and the augmented GALE (Genesis of Atlantic Lows Experiment) ensemble of in situ stations depict shallow, convective rainbands that overtake the front from the warm-air sector and intensify at the surface front location. Clockwise band rotation is shown to be caused by the difference in alignment between the echo motion and the rainband axes and by new development ahead of the front.
Radar measurements depict the circulation systems associated with a portion of one rainband in the cold air ahead of the front. Here shallow precipitation cores are vertically tilted due to the frontal wind shear. Circulation cells and most precipitation cores are centered just above the frontal inversion, as inferred by the wind shift line aloft. This feature is nearly horizontal in the cross-frontal direction but slopes downward in a direction roughly parallel to the front.
Ahead of the front, main updrafts in and above the cold air are found near the upwind portion of precipitation cores and along two well-defined lines aligned roughly perpendicular to the front. These lines propagate northward and affect several nearby surface sites prior to frontal passage. The speed of northward propagation is consistent with gravity wave theory, while on the larger scale the front appears to behave as the leading edge of a density current. The major features found in this case are compared and contrasted with those of a synoptic-scale warm front.
Abstract
The development and propagation of mesoscale convective systems (MCSs) was examined within the Weather Research and Forecasting (WRF) model using the Kain–Fritsch (KF) cumulus parameterization scheme and a modified version of this scheme. Mechanisms that led to propagation in the parameterized MCS are evaluated and compared between the versions of the KF scheme. Sensitivity to the convective time step is identified and explored for its role in scheme behavior. The sensitivity of parameterized convection propagation to microphysical feedback and to the shape and magnitude of the convective heating profile is also explored.
Each version of the KF scheme has a favored calling frequency that alters the scheme’s initiation frequency despite using the same convective trigger function. The authors propose that this behavior results in part from interaction with computational damping in WRF. A propagating convective system develops in simulations with both versions, but the typical flow structures are distorted (elevated ascending rear inflow as opposed to a descending rear inflow jet as is typically observed). The shape and magnitude of the heating profile is found to alter the propagation speed appreciably, even more so than the microphysical feedback. Microphysical feedback has a secondary role in producing realistic flow features via the resolvable-scale model microphysics. Deficiencies associated with the schemes are discussed and improvements are proposed.
Abstract
The development and propagation of mesoscale convective systems (MCSs) was examined within the Weather Research and Forecasting (WRF) model using the Kain–Fritsch (KF) cumulus parameterization scheme and a modified version of this scheme. Mechanisms that led to propagation in the parameterized MCS are evaluated and compared between the versions of the KF scheme. Sensitivity to the convective time step is identified and explored for its role in scheme behavior. The sensitivity of parameterized convection propagation to microphysical feedback and to the shape and magnitude of the convective heating profile is also explored.
Each version of the KF scheme has a favored calling frequency that alters the scheme’s initiation frequency despite using the same convective trigger function. The authors propose that this behavior results in part from interaction with computational damping in WRF. A propagating convective system develops in simulations with both versions, but the typical flow structures are distorted (elevated ascending rear inflow as opposed to a descending rear inflow jet as is typically observed). The shape and magnitude of the heating profile is found to alter the propagation speed appreciably, even more so than the microphysical feedback. Microphysical feedback has a secondary role in producing realistic flow features via the resolvable-scale model microphysics. Deficiencies associated with the schemes are discussed and improvements are proposed.
Abstract
In this note the forcing of cumulus cloud clearing over and downwind from lakes during the warm season is evaluated conceptually by modeling and observational approaches. It is suggested that drying by dynamically induced subsidence and suppression of the CBL over the lake mutually contribute to the cloud clearing. The effect of background flow speed and the extent of potential clearing area is illustrated. Various implications of the cloud clearing are discussed.
Abstract
In this note the forcing of cumulus cloud clearing over and downwind from lakes during the warm season is evaluated conceptually by modeling and observational approaches. It is suggested that drying by dynamically induced subsidence and suppression of the CBL over the lake mutually contribute to the cloud clearing. The effect of background flow speed and the extent of potential clearing area is illustrated. Various implications of the cloud clearing are discussed.
