Major river flooding affected the United Kingdom in late September 2012 as a slow-moving extratropical cyclone brought over 150 mm of rain to parts of northern England and north Wales. The cyclone ...deepened over the United Kingdom on 24–26 September as a potential vorticity (PV) anomaly approached from the northwest, elongated into a PV streamer, and wrapped around the cyclone. The strength and position of the PV anomaly is modified in the initial conditions of Weather Research and Forecasting Model simulations, using PV surgery, to examine whether different upper-level forcing, or different phasing between the PV anomaly and cyclone, could have produced an even more extreme event. These simulations reveal that quasigeostrophic (QG) forcing for ascent ahead of the anomaly contributed to the persistence of the rainfall over the United Kingdom. Moreover, weakening the anomaly resulted in lower rainfall accumulations across the United Kingdom, suggesting that the impact of the event might be proportional to the strength of the upper-level QG forcing. However, when the anomaly was strengthened, it rotated cyclonically around a large-scale trough over Iceland rather than moving eastward as in the verifying analysis, with strongly reduced accumulated rainfall across the United Kingdom. A similar evolution developed when the anomaly was moved farther away from the cyclone. Conversely, moving the anomaly nearer to the cyclone produced a similar solution to the verifying analysis, with slightly increased rainfall totals. These counterintuitive results suggest that the verifying analysis represented almost the highest-impact scenario possible for this flooding event when accounting for sensitivity to the initial position and strength of the PV anomaly.
Abstract
Precipitation patterns along cold fronts can exhibit a variety of morphologies including narrow cold-frontal rainbands and core-and-gap structures. A three-dimensional primitive equation ...model is used to investigate alongfront variability of precipitation in an idealized baroclinic wave. Along the poleward part of the cold front, a narrow line of precipitation develops. Along the equatorward part of the cold front, precipitation cores and gaps form. The difference between the two evolutions is due to differences in the orientation of vertical shear near the front in the lower troposphere: at the poleward end the along-frontal shear is dominant and the front is in near-thermal wind balance, while at the equatorward end the cross-frontal shear is almost as large. At the poleward end, the thermal structure remains erect with the front well defined up to the midtroposphere, hence updrafts remain erect and precipitation falls in a continuous line along the front. At the equatorward end, the cores form as undulations appear in both the prefrontal and postfrontal lighter precipitation, associated with vorticity maxima moving along the front on either side. Cross-frontal winds aloft tilt updrafts, so that some precipitation falls ahead of the surface cold front, forming the cores. Sensitivity simulations are also presented in which SST and roughness length are varied between simulations. Larger SST reduces cross-frontal winds aloft and leads to a more continuous rainband. Larger roughness length destroys the surface wind shift and thermal gradient, allowing mesovortices to dominate the precipitation distribution, leading to distinctive and irregularly shaped, quasi-regularly spaced precipitation maxima.
OCCLUDED FRONTS AND THE OCCLUSION PROCESS Schultz, David M.; Vaughan, Geraint
Bulletin of the American Meteorological Society,
04/2011, Letnik:
92, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Traditionally, the formation of an occluded front during the occlusion process in extratropical cyclones has been viewed as the catch-up of a faster-moving cold front to a slower-moving warm front ...separating the warm-sector air from the low center, as first described in the Norwegian cyclone model over 90 yr ago. In this article, the conventional wisdom, or the commonly held beliefs originating from the Norwegian cyclone model, about occluded fronts and the occlusion process are critically examined. The following four tenets of this conventional wisdom are addressed. First, the occlusion process is better described not by catch-up, but by the wrapping up and lengthening of the warm-air tongue as a result of deformation and rotation around the low center. Second, the merger of the cold front and warm front does not result in the frontal zone with the warmer air ascending over the other frontal zone. Instead, the occluded frontal zone tilts over the more statically stable frontal zone. Because a warmfrontal zone tends to be more stable than a cold-frontal zone, this process usually produces a warm-type occlusion, confirming that cold-type occlusions are less common than warm-type occlusions. Third, occlusion does not mean that the cyclone has stopped deepening, because many cyclones continue to deepen 10–30 mb for 12–36 h after the formation of the occluded front. Fourth, clouds and precipitation associated with occluded fronts differ from their widespread stratiform depiction in textbooks. Embedded precipitation bands may be parallel to the front, and little relationship may exist between the fronts and the cloud mass. These four tenets help to explain anomalies in the Norwegian cyclone model, such as how occluded fronts that spiral around the low center do not require catch-up to form, how Shapiro–Keyser cyclones undergo occlusion, why some cyclones do not form occluded fronts, how some cyclones deepen after occlusion, why few cold-type occlusions have been observed, and why occluded cyclones are often associated with heavy precipitation. This reexamination of conventional wisdom leads to a new paradigm for occluded fronts and occluded cyclones.
Abstract
Major river flooding affected the United Kingdom in late September 2012 as a slow-moving extratropical cyclone brought over 100 mm of rain to a large swath of northern England and north ...Wales, with local accumulations approaching 200 mm. The cyclone developed on 20–22 September following the interaction between an equatorward-moving potential vorticity (PV) streamer and Tropical Storm Nadine, near the Azores. A plume of tropical moisture was drawn poleward ahead of the PV streamer over a low-level baroclinic zone, allowing deep convection to develop. Convectively driven latent heat release reduced upper-tropospheric PV near the streamer, causing it to fracture and cut off from the reservoir of high PV over the United Kingdom. Simulations using the Weather Research and Forecasting Model with 4-km horizontal grid spacing in which microphysical heating and cooling tendencies are set to zero, alongside calculations of instantaneous diabatic heating rates and PV tendencies along trajectories, reveal that deposition heating contributed strongly to the fracturing of the PV streamer into a discrete anomaly by directly reducing upper-tropospheric PV to the streamer’s east. Condensation heating contributed to lower-tropospheric PV generation along the cold front as the cyclone developed, while cooling due to sublimation, evaporation, and melting modified the PV much less strongly. The results of this case study show that the collocation of strong deposition heating with positive absolute vorticity in the upper troposphere can lead to substantial PV modification and a very different cyclone evolution to that when deposition heating is suppressed.
A rapidly developing extratropical cyclone named Tini brought strong winds to Ireland and the United Kingdom on 12 February 2014. A mesoscale‐model simulation is used to analyze the development of ...the strong winds through the terms in the horizontal momentum equation. The maximum of near‐surface wind speed equatorward of the cyclone was composed of two different airstreams that underwent different paths to acceleration. First, horizontally moving air in the cold conveyor belt was accelerated by the along‐flow pressure gradient force but was decelerated by friction. Second, descending air accelerated into the eastern end of the maximum of near‐surface wind speed and was associated, in part, with a sting jet, caused by the increasing along‐flow horizontal pressure gradient force at lower levels. When this descending air entered the boundary layer, it too was decelerated by surface friction. Surface fluxes of heat and moisture were necessary to destabilize and deepen the boundary layer, allowing mixing of the strongest winds from the free troposphere down to the surface. A simulation with the surface fluxes turned off during cyclogenesis showed a more stable boundary layer around the bent‐back front, which inhibited the strongest winds from reaching the surface. The descent of the sting‐jet air was associated with a maximum in quasigeostrophic omega, which consisted of both synoptic‐scale and mesoscale descent, the latter associated with frontolysis occurring at the end of the bent‐back front. Thus, the near‐surface wind maximum was created by the synoptic‐scale and mesoscale dynamics, whereas localized moist processes were negligible.
Persistent northerly‐to‐easterly cold‐air outbreaks affected the UK during the winters of 2009–10 and 2010–11, with the resulting convection frequently organizing into snowbands over the English ...Channel and Irish Sea. Sounding data and composite radar reflectivity images from the Met Office Nimrod precipitation radar network reveal that these bands formed along the major axis of each body of water (or sea) when the boundary‐layer flow was roughly parallel to each of those axes (along‐channel). For both seas, a band was present the majority of times that the 850 hPa flow was along‐channel. Of these times of along‐channel flow, the 850 hPa wind speed and surface‐to‐850 hPa temperature difference were significantly greater when bands were present than when they were not. For the English Channel only, the land–sea temperature difference was also significantly greater when bands were present than when they were not. In a real‐data Weather Research and Forecasting model (WRF) control simulation of a typical band over the English Channel, a trough develops over the water and offshore air streams from either side converge along it. In the absence of surface fluxes, the trough, convergence and organized precipitation fail to develop altogether. Orography and roughness‐length variations are less important in band development, affecting only the location and morphology.
A low-power lidar system based in Manchester, United Kingdom has been developed to measure temperature profiles in the nocturnal urban boundary layer. The lidar transmitter uses a 355
nm
diode-pumped ...solid state Nd:YAG laser and two narrow-band interference filters in the receiver filter out rotational Raman lines that are dependent on temperature. The spectral response of the lidar is calibrated using a monochromator. Temperature profiles measured by the system are calibrated by comparison to co-located radiosondes.
A comprehensive dataset describing tropical cloud systems and their environmental setting and impacts has been collected during the Tropical Warm Pool International Cloud Experiment (TWP-ICE) and ...Aerosol and Chemical Transport in Tropical Convection (ACTIVE) campaign in the area around Darwin, Northern Australia, in January and February 2006. The aim of the experiment was to observe the evolution of tropical cloud systems and their interaction with the environment within an observational framework optimized for a range of modeling activities with the goal of improving the representation of cloud and aerosol process in a range of models. The experiment design utilized permanent observational facilities in Darwin, including a polarimetric weather radar and a suite of cloud remote-sensing instruments. This was augmented by a dense network of soundings, together with radiation, f lux, lightning, and remote-sensing measurements, as well as oceanographic observations. A fleet of five research aircraft, including two high-altitude aircraft, were taking measurements of fluxes, cloud microphysics, and chemistry; cloud radar and lidar were carried on a third aircraft. Highlights of the experiment include an intense mesoscale convective system (MCS) developed within the network, observations used to analyze the impacts of aerosol on convective systems, and observations used to relate cirrus properties to the parent storm properties.
A climatology of midlatitude 200- and 500-hPa cut-off low systems in the Northern and Southern Hemispheres is constructed from the NCEP–NCAR reanalysis by detecting and tracking, under one consistent ...method, all of the systems that persisted for more than 36 h for the 58 years of 1960–2017. This method identifies a cut-off low as a cold-core geopotential height minimum that is isolated from the main westerlies and with a strong temperature gradient on its eastern flank. The obtained spatial and seasonal distributions show preferred regions of occurrence and that within these regions there is a level-dependent seasonality of cut-off lows. Whereas 200-hPa systems are more frequent in summer and autumn, 500-hPa systems are more evenly distributed throughout the seasons. Within each region and at each level, the annual number of cut-off lows has been increasing over time, trends that are consistent with documented signals of climate change such as a weakening and poleward shift of the subtropical jets and an increase in blocking frequency. These trends explain as much as 64% of the variance in the annual number of cut-off lows. The contribution of the annular modes and El Niño–Southern Oscillation to the interannual variability of the number of cut-off lows per season in each hemisphere is also investigated. Only the Northern Hemisphere annular mode has a statistically significant negative correlation throughout all seasons that explains 18%–45% of the variance in the yearly number of Northern Hemisphere 500-hPa cut-off lows.
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