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Abstract
A formulation of Kuo's cumulus parameterization is described which satisfies arbitrary vertical profiles of apparent heat source (Q 1) and apparent moisture sink (Q 2). The approach requires little calculation, and for a given Q 1 and Q 2, contains only one parameter, the partitioning of available moisture between storage and precipitation. The proposed method is tested in the prediction of a mesoscale convective complex and its advantages and limitations are discussed.
Abstract
A formulation of Kuo's cumulus parameterization is described which satisfies arbitrary vertical profiles of apparent heat source (Q 1) and apparent moisture sink (Q 2). The approach requires little calculation, and for a given Q 1 and Q 2, contains only one parameter, the partitioning of available moisture between storage and precipitation. The proposed method is tested in the prediction of a mesoscale convective complex and its advantages and limitations are discussed.
Abstract
Rainfall rates determined from airborne radar and infrared satellite images are combined to construct a space- and time-dependent heating function for Hurricane Anita (1977). The heating is assimilated into a three-dimensional primitive equation prediction during a 12 h pre-forecast integration, after which the heating rate is computed internally by the model. The specified heating forces initial wind and mass fields toward their observed values, and produces improved 12 and 24 h forecasts of both track and intensity compared to a control integration, for which the heating is computed internally for the entire period.
Calculations indicate that model adjustment during the period of heating can be viewed as a slow response of the vorticity field to continuous forcing of the divergence by the heating. The location and pattern of the heating relative to the center appear to be of greater importance than the magnitude of the heating. This may be of significance because remotely-sensed rainfall estimates are more likely to be accurate in the positioning of heavy rainfall than in its intensity. The initialization procedure appears capable of producing useful improvement in short-term hurricane prediction, particularly prior to landfall, when data coverage is best and accuracy is of greatest concern.
A number of authors have noted the importance of upper-level inward eddy momentum fluxes for hurricane intensification. Calculations from the simulated storm indicate that such eddy fluxes are present in Hurricane Anita and are associated in part with an anticyclonic outflow eddy over an intense local rainfall area 300 km east of the center.
Abstract
Rainfall rates determined from airborne radar and infrared satellite images are combined to construct a space- and time-dependent heating function for Hurricane Anita (1977). The heating is assimilated into a three-dimensional primitive equation prediction during a 12 h pre-forecast integration, after which the heating rate is computed internally by the model. The specified heating forces initial wind and mass fields toward their observed values, and produces improved 12 and 24 h forecasts of both track and intensity compared to a control integration, for which the heating is computed internally for the entire period.
Calculations indicate that model adjustment during the period of heating can be viewed as a slow response of the vorticity field to continuous forcing of the divergence by the heating. The location and pattern of the heating relative to the center appear to be of greater importance than the magnitude of the heating. This may be of significance because remotely-sensed rainfall estimates are more likely to be accurate in the positioning of heavy rainfall than in its intensity. The initialization procedure appears capable of producing useful improvement in short-term hurricane prediction, particularly prior to landfall, when data coverage is best and accuracy is of greatest concern.
A number of authors have noted the importance of upper-level inward eddy momentum fluxes for hurricane intensification. Calculations from the simulated storm indicate that such eddy fluxes are present in Hurricane Anita and are associated in part with an anticyclonic outflow eddy over an intense local rainfall area 300 km east of the center.
Abstract
A closure is proposed for the b parameter of Kuo (1974), using the framework developed by Krishnamurti et al. (1976). Emphasis is placed on the time-dependent behavior of the solutions. The proposed closure is found to be the only one of several tested to produce an approach to moist neutrality in both temperature and moisture under strong external forcing. The sensitivity of the grid-scale evolution to the partitioning of moisture defined by the b parameter suggests that such partitioning must be carefully dealt with in any method for computing the effects of cumulus convection, whether or not b is explicitly present.
By including entrainment in the cloud lapse rate, the observed large-scale behavior of the vertical profile of moist static energy under disturbed conditions is simulated. The approach is shown to be easily invertible when precipitation rate is specified, thus insuring internal consistency in a model when such a procedure is used as part of a dynamic initialization.
Because it is relatively simple and general, and reproduces observed large-scale θ e variations under strong forcing, the approach may be particularly suitable for large-scale models. An economical way to extend the procedure to mesoscale models is proposed.
Abstract
A closure is proposed for the b parameter of Kuo (1974), using the framework developed by Krishnamurti et al. (1976). Emphasis is placed on the time-dependent behavior of the solutions. The proposed closure is found to be the only one of several tested to produce an approach to moist neutrality in both temperature and moisture under strong external forcing. The sensitivity of the grid-scale evolution to the partitioning of moisture defined by the b parameter suggests that such partitioning must be carefully dealt with in any method for computing the effects of cumulus convection, whether or not b is explicitly present.
By including entrainment in the cloud lapse rate, the observed large-scale behavior of the vertical profile of moist static energy under disturbed conditions is simulated. The approach is shown to be easily invertible when precipitation rate is specified, thus insuring internal consistency in a model when such a procedure is used as part of a dynamic initialization.
Because it is relatively simple and general, and reproduces observed large-scale θ e variations under strong forcing, the approach may be particularly suitable for large-scale models. An economical way to extend the procedure to mesoscale models is proposed.
Abstract
Helicity was calculated in Hurricane Bonnie (1998) using tropospheric-deep dropsonde soundings from the NASA Convection and Moisture Experiment. Large helicity existed downshear of the storm center with respect to the ambient vertical wind shear. It was associated with veering, semicircular hodographs created by strong, vortex-scale, radial-vertical flow induced by the shear. The most extreme values of helicity, among the largest ever reported in the literature, occurred in the vicinity of deep convective cells in the downshear-left quadrant. These cells reached as high as 17.5 km and displayed the temporal and spatial scales of supercells.
Convective available potential energy (CAPE) averaged 861 J kg−1 downshear, but only about one-third as large upshear. The soundings nearest the deep cells were evaluated using two empirical supercell parameters that make use of CAPE, helicity, and/or shear. These parameters supported the possible existence of supercells as a consequence of the exceptional helicity combined with moderate but sufficient CAPE. Ambient vertical wind shear exceeded 12 m s−1 for 30 h, yet the hurricane maintained 50 m s−1 maximum winds. It is hypothesized that the long-lived convective cells enabled the storm to resist the negative impact of the shear.
Supercells in large-helicity, curved-hodograph environments appear to provide a useful conceptual model for intense convection in the hurricane core. Helicity calculations might also give some insight into the behavior of vortical hot towers, which share some characteristics with supercells.
Abstract
Helicity was calculated in Hurricane Bonnie (1998) using tropospheric-deep dropsonde soundings from the NASA Convection and Moisture Experiment. Large helicity existed downshear of the storm center with respect to the ambient vertical wind shear. It was associated with veering, semicircular hodographs created by strong, vortex-scale, radial-vertical flow induced by the shear. The most extreme values of helicity, among the largest ever reported in the literature, occurred in the vicinity of deep convective cells in the downshear-left quadrant. These cells reached as high as 17.5 km and displayed the temporal and spatial scales of supercells.
Convective available potential energy (CAPE) averaged 861 J kg−1 downshear, but only about one-third as large upshear. The soundings nearest the deep cells were evaluated using two empirical supercell parameters that make use of CAPE, helicity, and/or shear. These parameters supported the possible existence of supercells as a consequence of the exceptional helicity combined with moderate but sufficient CAPE. Ambient vertical wind shear exceeded 12 m s−1 for 30 h, yet the hurricane maintained 50 m s−1 maximum winds. It is hypothesized that the long-lived convective cells enabled the storm to resist the negative impact of the shear.
Supercells in large-helicity, curved-hodograph environments appear to provide a useful conceptual model for intense convection in the hurricane core. Helicity calculations might also give some insight into the behavior of vortical hot towers, which share some characteristics with supercells.
Abstract
A weak tropical storm (Gabrielle in 2001) experienced a 22-hPa pressure fall in less than 3 h in the presence of 13 m s−1 ambient vertical wind shear. A convective cell developed downshear left of the center and moved cyclonically and inward to the 17-km radius during the period of rapid intensification. This cell had one of the most intense 85-GHz scattering signatures ever observed by the Tropical Rainfall Measuring Mission (TRMM). The cell developed at the downwind end of a band in the storm core. Maximum vorticity in the cell exceeded 2.5 × 10−2 s−1. The cell structure broadly resembled that of a vortical hot tower rather than a supercell.
At the time of minimum central pressure, the storm consisted of a strong vortex adjacent to the cell with a radius of maximum winds of about 10 km that exhibited almost no tilt in the vertical. This was surrounded by a broader vortex that tilted approximately left of the ambient shear vector, in a similar direction as the broad precipitation shield. This structure is consistent with the recent results of Riemer et al.
The rapid deepening of the storm is attributed to the cell growth within a region of high efficiency of latent heating following the theories of Nolan and Vigh and Schubert. This view is supported by a rapid growth of wind speed and vorticity in the storm core during the 1-h lifetime of the cell, and by the creation of a narrow 7°C spike in 700-hPa temperature adjacent to the cell and coincident with the lowest pressure. The cell is not seen as the cause of rapid intensification. Rather, it is part of a multiscale process: (i) development of a new circulation center within the downshear precipitation shield, (ii) continued ambient shear creating a favored region for cell formation just downshear of the new center, and (iii) the development of the intense cell that enhanced diabatic heating close to the center in a region of high efficiency of kinetic energy production. This sheared, asymmetric rapid intensification of Tropical Storm Gabrielle is contrasted with the nearly symmetric composite given by Kaplan and DeMaria.
Abstract
A weak tropical storm (Gabrielle in 2001) experienced a 22-hPa pressure fall in less than 3 h in the presence of 13 m s−1 ambient vertical wind shear. A convective cell developed downshear left of the center and moved cyclonically and inward to the 17-km radius during the period of rapid intensification. This cell had one of the most intense 85-GHz scattering signatures ever observed by the Tropical Rainfall Measuring Mission (TRMM). The cell developed at the downwind end of a band in the storm core. Maximum vorticity in the cell exceeded 2.5 × 10−2 s−1. The cell structure broadly resembled that of a vortical hot tower rather than a supercell.
At the time of minimum central pressure, the storm consisted of a strong vortex adjacent to the cell with a radius of maximum winds of about 10 km that exhibited almost no tilt in the vertical. This was surrounded by a broader vortex that tilted approximately left of the ambient shear vector, in a similar direction as the broad precipitation shield. This structure is consistent with the recent results of Riemer et al.
The rapid deepening of the storm is attributed to the cell growth within a region of high efficiency of latent heating following the theories of Nolan and Vigh and Schubert. This view is supported by a rapid growth of wind speed and vorticity in the storm core during the 1-h lifetime of the cell, and by the creation of a narrow 7°C spike in 700-hPa temperature adjacent to the cell and coincident with the lowest pressure. The cell is not seen as the cause of rapid intensification. Rather, it is part of a multiscale process: (i) development of a new circulation center within the downshear precipitation shield, (ii) continued ambient shear creating a favored region for cell formation just downshear of the new center, and (iii) the development of the intense cell that enhanced diabatic heating close to the center in a region of high efficiency of kinetic energy production. This sheared, asymmetric rapid intensification of Tropical Storm Gabrielle is contrasted with the nearly symmetric composite given by Kaplan and DeMaria.
Abstract
It is frequently stated that 70%–80% of western North Pacific tropical cyclones form “within the monsoon trough,” but without an objective definition of the term. Several definitions are tested here. When the monsoon trough (MT) is defined as the contiguous region where long-term (1988–2010) mean July–November 850-hPa relative vorticity is positive, 73% of all July–November tropical cyclones form within the MT. This percentage varies interannually, however, from as low as 50% to nearly 100%. The percentage correlates with the Niño-3.4 index, with more storms forming within the MT during warm periods. When the MT is defined instead using long-term monthly mean ζ 850, more than 80% of tropical cyclones form within the MT in all months except July and August, when more than 30% of storms form poleward of the MT. It is hypothesized that the known peak in the frequency of upper-tropospheric midlatitude wave breaking in July and August is responsible. It is argued that any long-term mean provides a suitable definition of the MT. Defining it on less than seasonal time scales, however, creates a lack of conceptual separation between the MT and other tropical disturbances such as the MJO, equatorial waves, and easterly waves. The term monsoon trough should represent a climatological feature that provides an asymmetric background state within which other disturbances evolve.
Abstract
It is frequently stated that 70%–80% of western North Pacific tropical cyclones form “within the monsoon trough,” but without an objective definition of the term. Several definitions are tested here. When the monsoon trough (MT) is defined as the contiguous region where long-term (1988–2010) mean July–November 850-hPa relative vorticity is positive, 73% of all July–November tropical cyclones form within the MT. This percentage varies interannually, however, from as low as 50% to nearly 100%. The percentage correlates with the Niño-3.4 index, with more storms forming within the MT during warm periods. When the MT is defined instead using long-term monthly mean ζ 850, more than 80% of tropical cyclones form within the MT in all months except July and August, when more than 30% of storms form poleward of the MT. It is hypothesized that the known peak in the frequency of upper-tropospheric midlatitude wave breaking in July and August is responsible. It is argued that any long-term mean provides a suitable definition of the MT. Defining it on less than seasonal time scales, however, creates a lack of conceptual separation between the MT and other tropical disturbances such as the MJO, equatorial waves, and easterly waves. The term monsoon trough should represent a climatological feature that provides an asymmetric background state within which other disturbances evolve.
Abstract
This paper describes a large cyclonic gyre that lasted several days in the northwest Pacific during July 1988. Cyclonic winds at 850 hPa extended beyond the 2000-km radius with a radius of maximum winds of 700–800 km. The gyre exhibited clear skies within and north of its center. Active convection extended 4000 km in longitude to its south.
The Madden–Julian oscillation (MJO) was in its active phase in the Indian Ocean prior to gyre formation. Consistent with earlier studies, diabatic heating in the MJO was associated with an anomalous upper-tropospheric westerly jet over the northeast Asian coast and a jet exit region over the northwest Pacific. Repeated equatorward wave-breaking events developed downwind of the jet exit region. One such event left behind a region of lower-tropospheric cyclonic vorticity and convection in the subtropics that played a key role in the gyre formation. A second wave-breaking event produced strong subsidence north of the mature gyre that contributed to its convective asymmetry.
Gyres from 1985 and 1989 were compared to the 1988 case. All three gyres developed during an active MJO in the Indian Ocean. Each gyre displayed the same strong convective asymmetry. Each developed in July or August during the climatological peak in breaking Rossby waves in the northwest Pacific. Finally, all of the gyres developed during La Niña at nearly the same location. This location and the convective structure of the gyres closely matched composite La Niña anomalies during boreal summer.
Abstract
This paper describes a large cyclonic gyre that lasted several days in the northwest Pacific during July 1988. Cyclonic winds at 850 hPa extended beyond the 2000-km radius with a radius of maximum winds of 700–800 km. The gyre exhibited clear skies within and north of its center. Active convection extended 4000 km in longitude to its south.
The Madden–Julian oscillation (MJO) was in its active phase in the Indian Ocean prior to gyre formation. Consistent with earlier studies, diabatic heating in the MJO was associated with an anomalous upper-tropospheric westerly jet over the northeast Asian coast and a jet exit region over the northwest Pacific. Repeated equatorward wave-breaking events developed downwind of the jet exit region. One such event left behind a region of lower-tropospheric cyclonic vorticity and convection in the subtropics that played a key role in the gyre formation. A second wave-breaking event produced strong subsidence north of the mature gyre that contributed to its convective asymmetry.
Gyres from 1985 and 1989 were compared to the 1988 case. All three gyres developed during an active MJO in the Indian Ocean. Each gyre displayed the same strong convective asymmetry. Each developed in July or August during the climatological peak in breaking Rossby waves in the northwest Pacific. Finally, all of the gyres developed during La Niña at nearly the same location. This location and the convective structure of the gyres closely matched composite La Niña anomalies during boreal summer.
Abstract
The structure and evolution of lowpass-filtered background flow and synoptic-scale easterly waves were examined during the 1991 eastern Pacific hurricane season. Active and inactive cyclogenesis periods conformed well to the sign of the near-equatorial, lowpass-filtered, 850-mb zonal wind anomaly, consistent with the recent results of Maloney and Hartmann. This behavior emphasizes the importance of westerly wind bursts associated with the Madden–Julian oscillation (MJO) in creating an environment favorable for eastern Pacific tropical cyclogenesis.
Synoptic-scale easterly waves reached the western Caribbean and eastern Pacific regularly from upstream, usually from Africa. The amplitude of waves leaving Africa had little correlation with the likelihood of a wave producing an eastern Pacific storm. Rather, easterly waves intensified, and tropical depressions formed, during the convectively active phase of the MJO in the western Caribbean and eastern Pacific. Wave growth, measured by strengthening of convection within the waves, occurred in the regions of sign reversal of the meridional potential vorticity gradient found previously. For the 1991 season cyclogenesis occurs when westward-moving synoptic-scale waves amplify within the superclusters that represent the favorable MJO envelope. Analogously, waves existed but failed to grow during the unfavorable part of the MJO.
During each active period of the MJO, the region of active convection moved eastward and northward with time in the eastern Pacific, with strongest convection reaching as far as the southwestern Gulf of Mexico by the end of such periods. The locations of tropical depression formation followed a similar path, shifting eastward with time following the MJO, and northward following the eastern Pacific intertropical convergence zone. The latter was defined by the locations of low-pass-filtered background vorticity maxima at 1000 mb.
It is argued based on previous work in the literature that the western Pacific might behave similarly, with upstream easterly waves growing and producing depressions within the convectively active envelope of the MJO.
Abstract
The structure and evolution of lowpass-filtered background flow and synoptic-scale easterly waves were examined during the 1991 eastern Pacific hurricane season. Active and inactive cyclogenesis periods conformed well to the sign of the near-equatorial, lowpass-filtered, 850-mb zonal wind anomaly, consistent with the recent results of Maloney and Hartmann. This behavior emphasizes the importance of westerly wind bursts associated with the Madden–Julian oscillation (MJO) in creating an environment favorable for eastern Pacific tropical cyclogenesis.
Synoptic-scale easterly waves reached the western Caribbean and eastern Pacific regularly from upstream, usually from Africa. The amplitude of waves leaving Africa had little correlation with the likelihood of a wave producing an eastern Pacific storm. Rather, easterly waves intensified, and tropical depressions formed, during the convectively active phase of the MJO in the western Caribbean and eastern Pacific. Wave growth, measured by strengthening of convection within the waves, occurred in the regions of sign reversal of the meridional potential vorticity gradient found previously. For the 1991 season cyclogenesis occurs when westward-moving synoptic-scale waves amplify within the superclusters that represent the favorable MJO envelope. Analogously, waves existed but failed to grow during the unfavorable part of the MJO.
During each active period of the MJO, the region of active convection moved eastward and northward with time in the eastern Pacific, with strongest convection reaching as far as the southwestern Gulf of Mexico by the end of such periods. The locations of tropical depression formation followed a similar path, shifting eastward with time following the MJO, and northward following the eastern Pacific intertropical convergence zone. The latter was defined by the locations of low-pass-filtered background vorticity maxima at 1000 mb.
It is argued based on previous work in the literature that the western Pacific might behave similarly, with upstream easterly waves growing and producing depressions within the convectively active envelope of the MJO.
Abstract
A 10-yr climatology (1986–95) was performed using ECMWF gridded analyses on isentropic surfaces to identify regions where the lower-tropospheric meridional potential vorticity (PV) gradient changes sign across Africa and Australia during their respective summer seasons. While an African sign reversal has been documented, no similar study has been performed for the Australian region, which also has desert on the poleward side of open ocean. In each hemisphere, a northward decrease of PV is sufficient to produce a sign reversal.
It was found that PV decreases northward in the lower troposphere across northern Australia, with the maximum reversal on the 315-K surface. It had comparable magnitude but smaller zonal extent (∼3000 km) than that on the 320-K surface in Africa (∼5000 km). In each region the sign reversal was associated with cyclonic PV anomalies on the equatorward side and anticyclonic anomalies on the poleward side.
OLR was used as a proxy for deep convective heating in order to evaluate the total convective forcing of PV. The vertical distribution of heating was specified. In both regions the maximum total convective forcing of PV was largest on the equatorward edge of the sign reversal region. The effects of dry convection were not included in the PV budget. Dry convection, located poleward of the maximum deep convection, acts as a lower-tropospheric PV sink and produces anticyclonic PV anomalies. In both regions these anticyclonic anomalies were larger in magnitude and areal coverage than the cyclonic anomalies associated with deep convection.
The potential instability implied by the sign reversal regions has traditionally been associated with the growth of easterly waves. In support of this argument, bandpass-filtered (2–6 day) meridional wind variance on the 320-K surface nearly triples from east to west along the African sign reversal. In Australia, little evidence was found of such waves in the 2–10-day meridional wind variance. Possible explanations for the lack of growing disturbances over Australia are discussed.
Abstract
A 10-yr climatology (1986–95) was performed using ECMWF gridded analyses on isentropic surfaces to identify regions where the lower-tropospheric meridional potential vorticity (PV) gradient changes sign across Africa and Australia during their respective summer seasons. While an African sign reversal has been documented, no similar study has been performed for the Australian region, which also has desert on the poleward side of open ocean. In each hemisphere, a northward decrease of PV is sufficient to produce a sign reversal.
It was found that PV decreases northward in the lower troposphere across northern Australia, with the maximum reversal on the 315-K surface. It had comparable magnitude but smaller zonal extent (∼3000 km) than that on the 320-K surface in Africa (∼5000 km). In each region the sign reversal was associated with cyclonic PV anomalies on the equatorward side and anticyclonic anomalies on the poleward side.
OLR was used as a proxy for deep convective heating in order to evaluate the total convective forcing of PV. The vertical distribution of heating was specified. In both regions the maximum total convective forcing of PV was largest on the equatorward edge of the sign reversal region. The effects of dry convection were not included in the PV budget. Dry convection, located poleward of the maximum deep convection, acts as a lower-tropospheric PV sink and produces anticyclonic PV anomalies. In both regions these anticyclonic anomalies were larger in magnitude and areal coverage than the cyclonic anomalies associated with deep convection.
The potential instability implied by the sign reversal regions has traditionally been associated with the growth of easterly waves. In support of this argument, bandpass-filtered (2–6 day) meridional wind variance on the 320-K surface nearly triples from east to west along the African sign reversal. In Australia, little evidence was found of such waves in the 2–10-day meridional wind variance. Possible explanations for the lack of growing disturbances over Australia are discussed.
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