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Abstract
The structure of near-tropopause potential vorticity (PV) acts as a primary control on the evolution of extratropical cyclones. Diabatic processes such as the latent heating found in ascending moist warm conveyor belts modify PV. A dipole in diabatically generated PV (hereafter diabatic PV) straddling the extratropical tropopause, with the positive pole above the negative pole, was diagnosed in a recently published analysis of a simulated extratropical cyclone. This PV dipole has the potential to significantly modify the propagation of Rossby waves and the growth of baroclinically unstable waves. This previous analysis was based on a single case study simulated with 12-km horizontal grid spacing and parameterized convection. Here the dipole is investigated in three additional cold-season extratropical cyclones simulated in both convection-parameterizing and convection-permitting model configurations. A diabatic PV dipole across the extratropical tropopause is diagnosed in all three cases. The amplitude of the dipole saturates approximately 36 h from the time diabatic PV is accumulated. The node elevation of the dipole varies between 2 and 4 PVU (1 PVU = 106 K kg−1 m2 s−1) in the three cases, and the amplitude of the system-averaged dipole varies between 0.2 and 0.4 PVU. The amplitude of the negative pole is similar in the convection-parameterizing and convection-permitting simulations. The positive pole, which is generated by longwave radiative cooling, is weak in the convection-permitting simulations due to the small domain size, which limits the accumulation of diabatic tendencies within the interior of the domain. The possible correspondence between the diabatic PV dipole and the extratropical tropopause inversion layer is discussed.
Abstract
The structure of near-tropopause potential vorticity (PV) acts as a primary control on the evolution of extratropical cyclones. Diabatic processes such as the latent heating found in ascending moist warm conveyor belts modify PV. A dipole in diabatically generated PV (hereafter diabatic PV) straddling the extratropical tropopause, with the positive pole above the negative pole, was diagnosed in a recently published analysis of a simulated extratropical cyclone. This PV dipole has the potential to significantly modify the propagation of Rossby waves and the growth of baroclinically unstable waves. This previous analysis was based on a single case study simulated with 12-km horizontal grid spacing and parameterized convection. Here the dipole is investigated in three additional cold-season extratropical cyclones simulated in both convection-parameterizing and convection-permitting model configurations. A diabatic PV dipole across the extratropical tropopause is diagnosed in all three cases. The amplitude of the dipole saturates approximately 36 h from the time diabatic PV is accumulated. The node elevation of the dipole varies between 2 and 4 PVU (1 PVU = 106 K kg−1 m2 s−1) in the three cases, and the amplitude of the system-averaged dipole varies between 0.2 and 0.4 PVU. The amplitude of the negative pole is similar in the convection-parameterizing and convection-permitting simulations. The positive pole, which is generated by longwave radiative cooling, is weak in the convection-permitting simulations due to the small domain size, which limits the accumulation of diabatic tendencies within the interior of the domain. The possible correspondence between the diabatic PV dipole and the extratropical tropopause inversion layer is discussed.
Abstract
The Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project aims to improve forecasts of high-impact weather in extratropical cyclones through field measurements, high-resolution numerical modeling, and improved design of ensemble forecasting and data assimilation systems. This article introduces DIAMET and presents some of the first results. Four field campaigns were conducted by the project, one of which, in late 2011, coincided with an exceptionally stormy period marked by an unusually strong, zonal North Atlantic jet stream and a succession of severe windstorms in northwest Europe. As a result, December 2011 had the highest monthly North Atlantic Oscillation index (2.52) of any December in the last 60 years. Detailed observations of several of these storms were gathered using the U.K.’s BAe 146 research aircraft and extensive ground-based measurements. As an example of the results obtained during the campaign, observations are presented of Extratropical Cyclone Friedhelm on 8 December 2011, when surface winds with gusts exceeding 30 m s–1 crossed central Scotland, leading to widespread disruption to transportation and electricity supply. Friedhelm deepened 44 hPa in 24 h and developed a pronounced bent-back front wrapping around the storm center. The strongest winds at 850 hPa and the surface occurred in the southern quadrant of the storm, and detailed measurements showed these to be most intense in clear air between bands of showers. High-resolution ensemble forecasts from the Met Office showed similar features, with the strongest winds aligned in linear swaths between the bands, suggesting that there is potential for improved skill in forecasts of damaging winds.
Abstract
The Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project aims to improve forecasts of high-impact weather in extratropical cyclones through field measurements, high-resolution numerical modeling, and improved design of ensemble forecasting and data assimilation systems. This article introduces DIAMET and presents some of the first results. Four field campaigns were conducted by the project, one of which, in late 2011, coincided with an exceptionally stormy period marked by an unusually strong, zonal North Atlantic jet stream and a succession of severe windstorms in northwest Europe. As a result, December 2011 had the highest monthly North Atlantic Oscillation index (2.52) of any December in the last 60 years. Detailed observations of several of these storms were gathered using the U.K.’s BAe 146 research aircraft and extensive ground-based measurements. As an example of the results obtained during the campaign, observations are presented of Extratropical Cyclone Friedhelm on 8 December 2011, when surface winds with gusts exceeding 30 m s–1 crossed central Scotland, leading to widespread disruption to transportation and electricity supply. Friedhelm deepened 44 hPa in 24 h and developed a pronounced bent-back front wrapping around the storm center. The strongest winds at 850 hPa and the surface occurred in the southern quadrant of the storm, and detailed measurements showed these to be most intense in clear air between bands of showers. High-resolution ensemble forecasts from the Met Office showed similar features, with the strongest winds aligned in linear swaths between the bands, suggesting that there is potential for improved skill in forecasts of damaging winds.
Abstract
Three case studies in frontal clouds from the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project are described to understand the microphysical development of the mixed phase regions of these clouds. The cases are a kata-type cold front, a wintertime warm front, and a summertime occluded frontal system. The clouds were observed by radar, satellite, and in situ microphysics measurements from the U.K. Facility for Airborne Atmospheric Measurements (FAAM) research aircraft. The kata cold front cloud was shallow with a cloud-top temperature of approximately −13°C. Cloud-top heterogeneous ice nucleation was found to be consistent with predictions by a primary ice nucleation scheme. The other case studies had high cloud tops (< −40°C) and despite no direct cloud-top measurements in these regions, homogeneous ice nucleation would be expected. The maximum ice crystal concentrations and ice water contents in all clouds were observed at temperatures around −5°C. Graupel was not observed, hence, secondary ice was produced by riming on snow falling through regions of supercooled liquid water. Within these regions substantial concentrations (10–150 L−1) of supercooled drizzle were observed. The freezing of these drops increases the riming rate due to the increase in rimer surface area. Increasing rime accretion has been shown to lead to higher ice splinter production rates. Despite differences in the cloud structure, the maximum ice crystal number concentration in all three clouds was ~100 L−1. Ice water contents were similar in the warm and occluded frontal cases, where median values in both cases reached ~0.2–0.3 g m−3, but lower in the cold front case.
Abstract
Three case studies in frontal clouds from the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project are described to understand the microphysical development of the mixed phase regions of these clouds. The cases are a kata-type cold front, a wintertime warm front, and a summertime occluded frontal system. The clouds were observed by radar, satellite, and in situ microphysics measurements from the U.K. Facility for Airborne Atmospheric Measurements (FAAM) research aircraft. The kata cold front cloud was shallow with a cloud-top temperature of approximately −13°C. Cloud-top heterogeneous ice nucleation was found to be consistent with predictions by a primary ice nucleation scheme. The other case studies had high cloud tops (< −40°C) and despite no direct cloud-top measurements in these regions, homogeneous ice nucleation would be expected. The maximum ice crystal concentrations and ice water contents in all clouds were observed at temperatures around −5°C. Graupel was not observed, hence, secondary ice was produced by riming on snow falling through regions of supercooled liquid water. Within these regions substantial concentrations (10–150 L−1) of supercooled drizzle were observed. The freezing of these drops increases the riming rate due to the increase in rimer surface area. Increasing rime accretion has been shown to lead to higher ice splinter production rates. Despite differences in the cloud structure, the maximum ice crystal number concentration in all three clouds was ~100 L−1. Ice water contents were similar in the warm and occluded frontal cases, where median values in both cases reached ~0.2–0.3 g m−3, but lower in the cold front case.
Abstract
Observations from a mesoscale network of automatic weather stations are analyzed for 15 U.K. cold fronts exhibiting narrow cold frontal rainbands (NCFRs). Seven of the NCFRs produced tornadoes. A time-compositing approach is applied to the minute-resolution data using the radar-observed motion vectors of NCFR precipitation segments. Interpolated onto a 5-km grid, the analyses resolve much of the small-mesoscale structure in surface wind, temperature, and pressure fields. Postfrontal winds varied substantially between cases. Tornadic NCFRs exhibited a near-90° wind veer and little or no reduction in wind speed on NCFR passage; these attributes were generally associated with large vertical vorticity, horizontal convergence, and vorticity stretching at the NCFR. Nontornadic NCFRs exhibited smaller wind veers and/or marked decreases in wind speed across the NCFR, and weaker vorticity, convergence, and vorticity stretching. In at least four tornadic NCFRs, increases in vorticity stretching preceded tornadogenesis. Doppler radar observations of two tornadic NCFRs revealed the development of misocyclones, some tornadic, during the latter stages of vorticity-stretching increase. The presence of cyclonic vortices only, in one case occurring at regular intervals along the NCFR, provides limited circumstantial evidence for horizontal shearing instability (HSI), though other vortex-genesis mechanisms cannot be discounted. Vorticity-stretching increases were associated with coherent mesoscale structures in the postfrontal wind field, which modified the cross-frontal convergence. Where cross-frontal convergence was large, extremely narrow, intense shear zones were observed; results suggest that tornadoes occurred when such shear zones developed in conjunction with conditional instability in the prefrontal environment.
Abstract
Observations from a mesoscale network of automatic weather stations are analyzed for 15 U.K. cold fronts exhibiting narrow cold frontal rainbands (NCFRs). Seven of the NCFRs produced tornadoes. A time-compositing approach is applied to the minute-resolution data using the radar-observed motion vectors of NCFR precipitation segments. Interpolated onto a 5-km grid, the analyses resolve much of the small-mesoscale structure in surface wind, temperature, and pressure fields. Postfrontal winds varied substantially between cases. Tornadic NCFRs exhibited a near-90° wind veer and little or no reduction in wind speed on NCFR passage; these attributes were generally associated with large vertical vorticity, horizontal convergence, and vorticity stretching at the NCFR. Nontornadic NCFRs exhibited smaller wind veers and/or marked decreases in wind speed across the NCFR, and weaker vorticity, convergence, and vorticity stretching. In at least four tornadic NCFRs, increases in vorticity stretching preceded tornadogenesis. Doppler radar observations of two tornadic NCFRs revealed the development of misocyclones, some tornadic, during the latter stages of vorticity-stretching increase. The presence of cyclonic vortices only, in one case occurring at regular intervals along the NCFR, provides limited circumstantial evidence for horizontal shearing instability (HSI), though other vortex-genesis mechanisms cannot be discounted. Vorticity-stretching increases were associated with coherent mesoscale structures in the postfrontal wind field, which modified the cross-frontal convergence. Where cross-frontal convergence was large, extremely narrow, intense shear zones were observed; results suggest that tornadoes occurred when such shear zones developed in conjunction with conditional instability in the prefrontal environment.
Diabatic Heating and Cooling Rates Derived from In Situ Microphysics Measurements: A Case Study of a Wintertime U.K. Cold Front
Abstract
In situ measurements associated with the passage of a kata cold front over the United Kingdom on 29 November 2011 are used to initialize a Lagrangian parcel model for the purpose of calculating rates of diabatic heating and cooling associated with the phase changes of water within the cloud system. The parcel model calculations are performed with both bin-resolved and bulk treatments of microphysical processes. The in situ data from this case study reveal droplet number concentrations up to 100 cm−3, with planar ice crystals detected at cloud top, as well as columnar crystals produced by rime splinter ejection within the prefrontal warm sector. The results show that in terms of magnitude, the most significant rates of diabatic heating and cooling are produced by condensation growth of liquid water within the convective updrafts at the leading edge of the front. The peak temperature tendencies associated with condensation are typically found to be at least an order of magnitude larger than those associated with the ice phase, although the cooling effect from sublimation and melting occurs over a wide region. The parcel model framework is used in conjunction with the observations to assess the suitability of existing bulk microphysical treatments, of the kind used in operational weather forecast models. It is found that the assumption of spherical ice crystals (with diameters equal to the maximum dimension of those sampled), along with the use of negative exponential functions to describe ice particle size distributions, can lead to an overestimation of local diabatic heating and cooling rates by a factor of 2 or more.
Abstract
In situ measurements associated with the passage of a kata cold front over the United Kingdom on 29 November 2011 are used to initialize a Lagrangian parcel model for the purpose of calculating rates of diabatic heating and cooling associated with the phase changes of water within the cloud system. The parcel model calculations are performed with both bin-resolved and bulk treatments of microphysical processes. The in situ data from this case study reveal droplet number concentrations up to 100 cm−3, with planar ice crystals detected at cloud top, as well as columnar crystals produced by rime splinter ejection within the prefrontal warm sector. The results show that in terms of magnitude, the most significant rates of diabatic heating and cooling are produced by condensation growth of liquid water within the convective updrafts at the leading edge of the front. The peak temperature tendencies associated with condensation are typically found to be at least an order of magnitude larger than those associated with the ice phase, although the cooling effect from sublimation and melting occurs over a wide region. The parcel model framework is used in conjunction with the observations to assess the suitability of existing bulk microphysical treatments, of the kind used in operational weather forecast models. It is found that the assumption of spherical ice crystals (with diameters equal to the maximum dimension of those sampled), along with the use of negative exponential functions to describe ice particle size distributions, can lead to an overestimation of local diabatic heating and cooling rates by a factor of 2 or more.
Precipitation Banding in Idealized Baroclinic Waves
Abstract
Moist idealized baroclinic-wave simulations show the development of precipitation bands from a zonally uniform initial midlatitude jet. For a frictionless lower boundary, and with no latent-heat release or surface heat and moisture fluxes, warm advection is strong and a bent-back warm front forms. Although a narrow vertical-velocity maximum forms within the area of synoptic-scale ascent near the triple point, only a wide warm-frontal band forms. As surface roughness length increases between simulations to that of an ocean then a land surface, warm advection is reduced and the cold front becomes stronger relative to the warm front. A separate narrow rainband forms along the cold front, which is more intense and farther removed from the wide warm-frontal band when roughness length is greater. In the simulation with roughness length appropriate to the ocean, after the narrow band decays, the precipitation becomes oriented along the warm conveyor belt in the warm sector. When latent-heat release is included, this warm-sector precipitation evolves into multiple bands, which eventually weaken with the cyclone. When surface heat and moisture fluxes are included, the ascent at the surface cold front stays strong and a well-defined cold front of the anafront variety persists through this mature stage. The surface precipitation remains in a single intense band along and ahead of the cold front. Therefore, strong surface heat and moisture fluxes inhibit multiple bands, but a simulation with lower sea surface temperature (SST) more closely resembles the simulation without surface heat and moisture fluxes, demonstrating that the detailed structure and evolution of precipitation banding is sensitive to SST.
Abstract
Moist idealized baroclinic-wave simulations show the development of precipitation bands from a zonally uniform initial midlatitude jet. For a frictionless lower boundary, and with no latent-heat release or surface heat and moisture fluxes, warm advection is strong and a bent-back warm front forms. Although a narrow vertical-velocity maximum forms within the area of synoptic-scale ascent near the triple point, only a wide warm-frontal band forms. As surface roughness length increases between simulations to that of an ocean then a land surface, warm advection is reduced and the cold front becomes stronger relative to the warm front. A separate narrow rainband forms along the cold front, which is more intense and farther removed from the wide warm-frontal band when roughness length is greater. In the simulation with roughness length appropriate to the ocean, after the narrow band decays, the precipitation becomes oriented along the warm conveyor belt in the warm sector. When latent-heat release is included, this warm-sector precipitation evolves into multiple bands, which eventually weaken with the cyclone. When surface heat and moisture fluxes are included, the ascent at the surface cold front stays strong and a well-defined cold front of the anafront variety persists through this mature stage. The surface precipitation remains in a single intense band along and ahead of the cold front. Therefore, strong surface heat and moisture fluxes inhibit multiple bands, but a simulation with lower sea surface temperature (SST) more closely resembles the simulation without surface heat and moisture fluxes, demonstrating that the detailed structure and evolution of precipitation banding is sensitive to SST.
Abstract
Strong winds equatorward and rearward of a cyclone core have often been associated with two phenomena: the cold conveyor belt (CCB) jet and sting jets. Here, detailed observations of the mesoscale structure in this region of an intense cyclone are analyzed. The in situ and dropsonde observations were obtained during two research flights through the cyclone during the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) field campaign. A numerical weather prediction model is used to link the strong wind regions with three types of “airstreams” or coherent ensembles of trajectories: two types are identified with the CCB, hooking around the cyclone center, while the third is identified with a sting jet, descending from the cloud head to the west of the cyclone. Chemical tracer observations show for the first time that the CCB and sting jet airstreams are distinct air masses even when the associated low-level wind maxima are not spatially distinct. In the model, the CCB experiences slow latent heating through weak-resolved ascent and convection, while the sting jet experiences weak cooling associated with microphysics during its subsaturated descent. Diagnosis of mesoscale instabilities in the model shows that the CCB passes through largely stable regions, while the sting jet spends relatively long periods in locations characterized by conditional symmetric instability (CSI). The relation of CSI to the observed mesoscale structure of the bent-back front and its possible role in the cloud banding is discussed.
Abstract
Strong winds equatorward and rearward of a cyclone core have often been associated with two phenomena: the cold conveyor belt (CCB) jet and sting jets. Here, detailed observations of the mesoscale structure in this region of an intense cyclone are analyzed. The in situ and dropsonde observations were obtained during two research flights through the cyclone during the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) field campaign. A numerical weather prediction model is used to link the strong wind regions with three types of “airstreams” or coherent ensembles of trajectories: two types are identified with the CCB, hooking around the cyclone center, while the third is identified with a sting jet, descending from the cloud head to the west of the cyclone. Chemical tracer observations show for the first time that the CCB and sting jet airstreams are distinct air masses even when the associated low-level wind maxima are not spatially distinct. In the model, the CCB experiences slow latent heating through weak-resolved ascent and convection, while the sting jet experiences weak cooling associated with microphysics during its subsaturated descent. Diagnosis of mesoscale instabilities in the model shows that the CCB passes through largely stable regions, while the sting jet spends relatively long periods in locations characterized by conditional symmetric instability (CSI). The relation of CSI to the observed mesoscale structure of the bent-back front and its possible role in the cloud banding is discussed.
Abstract
According to the Norwegian cyclone model, whether a warm-type or cold-type occluded front forms depends upon which cold air mass is colder: the prewarm-frontal air mass or the postcold-frontal air mass. For example, a cold-type occlusion is said to occur when the occluded front slopes rearward with height because the prewarm-frontal air mass is warmer than the postcold-frontal air mass. This temperature difference and the resulting occluded-frontal structure in the Norwegian cyclone model is part of what is called the temperature rule. Paradoxically, no clear example of a rearward-sloping, cold-type occluded front has been found in the literature, even though the required temperature difference has been documented in several cases. This article presents the first documented, rearward-sloping, cold-type occluded front. This occluded front forms in a cyclone over the North Atlantic Ocean on 3–5 January 2003 and is documented in model output from the European Centre for Medium-Range Weather Forecasts. Cross sections through the evolving cyclone show the occluded front forms as the less statically stable warm-frontal zone ascends over the more stable cold-frontal zone. Such a stability difference between the cold- and warm-frontal zones is consistent with a previously published hypothesis that the less stable air is lifted by the more stable air to form occluded fronts, in disagreement with the temperature rule. Because warm-frontal zones and the cold air underneath tend to be more stable than cold-frontal zones and the postcold-frontal air, warm-type occluded fronts are much more common than cold-type occluded fronts, explaining why well-defined, rearward-sloping, cold-type occluded fronts are not common in the meteorological literature.
Abstract
According to the Norwegian cyclone model, whether a warm-type or cold-type occluded front forms depends upon which cold air mass is colder: the prewarm-frontal air mass or the postcold-frontal air mass. For example, a cold-type occlusion is said to occur when the occluded front slopes rearward with height because the prewarm-frontal air mass is warmer than the postcold-frontal air mass. This temperature difference and the resulting occluded-frontal structure in the Norwegian cyclone model is part of what is called the temperature rule. Paradoxically, no clear example of a rearward-sloping, cold-type occluded front has been found in the literature, even though the required temperature difference has been documented in several cases. This article presents the first documented, rearward-sloping, cold-type occluded front. This occluded front forms in a cyclone over the North Atlantic Ocean on 3–5 January 2003 and is documented in model output from the European Centre for Medium-Range Weather Forecasts. Cross sections through the evolving cyclone show the occluded front forms as the less statically stable warm-frontal zone ascends over the more stable cold-frontal zone. Such a stability difference between the cold- and warm-frontal zones is consistent with a previously published hypothesis that the less stable air is lifted by the more stable air to form occluded fronts, in disagreement with the temperature rule. Because warm-frontal zones and the cold air underneath tend to be more stable than cold-frontal zones and the postcold-frontal air, warm-type occluded fronts are much more common than cold-type occluded fronts, explaining why well-defined, rearward-sloping, cold-type occluded fronts are not common in the meteorological literature.
Abstract
Sting jets, or surface wind maxima at the end of bent-back fronts in Shapiro–Keyser cyclones, are one cause of strong winds in extratropical cyclones. Although previous studies identified the release of conditional symmetric instability as a cause of sting jets, the mechanism to initiate its release remains unidentified. To identify this mechanism, a case study was selected of an intense cyclone over the North Atlantic Ocean during 7–8 December 2005 that possessed a sting jet detected from the NASA Quick Scatterometer (QuikSCAT). A couplet of Petterssen frontogenesis and frontolysis occurred along the bent-back front. The direct circulation associated with the frontogenesis led to ascent within the cyclonically turning portion of the warm conveyor belt, contributing to the comma-cloud head. When the bent-back front became frontolytic, an indirect circulation associated with the frontolysis, in conjunction with alongfront cold advection, led to descent within and on the warm side of the front, bringing higher-momentum air down toward the boundary layer. Sensible heat fluxes from the ocean surface and cold-air advection destabilized the boundary layer, resulting in near-neutral static stability facilitating downward mixing. Thus, descent associated with the frontolysis reaching a near-neutral boundary layer provides a physical mechanism for sting jets, is consistent with previous studies, and synthesizes existing knowledge. Specifically, this couplet of frontogenesis and frontolysis could explain why sting jets occur at the end of the bent-back front and emerge from the cloud head, why sting jets are mesoscale phenomena, and why they only occur within Shapiro–Keyser cyclones. A larger dataset of cases is necessary to test this hypothesis.
Abstract
Sting jets, or surface wind maxima at the end of bent-back fronts in Shapiro–Keyser cyclones, are one cause of strong winds in extratropical cyclones. Although previous studies identified the release of conditional symmetric instability as a cause of sting jets, the mechanism to initiate its release remains unidentified. To identify this mechanism, a case study was selected of an intense cyclone over the North Atlantic Ocean during 7–8 December 2005 that possessed a sting jet detected from the NASA Quick Scatterometer (QuikSCAT). A couplet of Petterssen frontogenesis and frontolysis occurred along the bent-back front. The direct circulation associated with the frontogenesis led to ascent within the cyclonically turning portion of the warm conveyor belt, contributing to the comma-cloud head. When the bent-back front became frontolytic, an indirect circulation associated with the frontolysis, in conjunction with alongfront cold advection, led to descent within and on the warm side of the front, bringing higher-momentum air down toward the boundary layer. Sensible heat fluxes from the ocean surface and cold-air advection destabilized the boundary layer, resulting in near-neutral static stability facilitating downward mixing. Thus, descent associated with the frontolysis reaching a near-neutral boundary layer provides a physical mechanism for sting jets, is consistent with previous studies, and synthesizes existing knowledge. Specifically, this couplet of frontogenesis and frontolysis could explain why sting jets occur at the end of the bent-back front and emerge from the cloud head, why sting jets are mesoscale phenomena, and why they only occur within Shapiro–Keyser cyclones. A larger dataset of cases is necessary to test this hypothesis.
