Dry Intrusion Research Articles

Mesoscale Substructure of Extratropical Cyclones Observed by Radar

This is the fourth in a sequence of review articles in American Meteorological Society (AMS) monographs in which we deal with the observed structure of extra-tropical cyclones. The first of these articles (Browning 1990b), following the 1988 Palmen Memorial Symposium in Helsinki, classified cyclones into comma-cloud systems, instant-occlusion systems, and frontal wave cyclones according to the location of an upper-level trough/vortex in relation to the main baroclinic zone (see also Zillman and Price 1972). It presented the system-relative airflow concepts surrounding warm and cold conveyor belts (Green et al. 1966; Harrold 1973; Carlson 1980) and described two contrasting types of frontal system according to whether the warm conveyor belt is characterized by rearward-sloping or forward-sloping ascent anabatic (ana-) and katabatic (kata-) cold fronts, respectively. A radar-oriented version of this material appeared as a chapter (Browning 1990a) in the Battan Memorial Volume, an AMS monograph edited by Dave Atlas. These articles also drew attention, following Reed and Danielsen (1959) and Danielsen (1966), to the importance in developing cyclones of air descended from the upper troposphere and lower stratosphere, which forms a third airstream referred to as the dry intrusion. Although these airstream concepts suffer from being somewhat subjective in their application, they provide a useful framework within which to in- terpret the mesoscale substructure of cyclones, and in the 1990 reviews a start was made in setting mesoscale rainband structures within such a context. In the last several years progress has been made in giving Lagrangian-airstream descriptions an objective basis (Wernli 1997; Davies and Wernli 1997; Rossa et al. 2000).

A case study of tropopause cyclogenesis

Meteorological ApplicationsVolume 10, Issue 2 p. 187-199 Research ArticleFree Access A case study of tropopause cyclogenesis Aarnout van Delden, Corresponding Author Aarnout van Delden a.j.vandelden@phys.uu.nl Institute for Marine and Atmospheric Sciences, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The NetherlandsInstitute for Marine and Atmospheric Sciences, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The NetherlandsSearch for more papers by this authorRoel Neggers, Roel Neggers Institute for Marine and Atmospheric Sciences, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The NetherlandsSearch for more papers by this author Aarnout van Delden, Corresponding Author Aarnout van Delden a.j.vandelden@phys.uu.nl Institute for Marine and Atmospheric Sciences, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The NetherlandsInstitute for Marine and Atmospheric Sciences, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The NetherlandsSearch for more papers by this authorRoel Neggers, Roel Neggers Institute for Marine and Atmospheric Sciences, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The NetherlandsSearch for more papers by this author First published: 29 December 2006 https://doi.org/10.1017/S1350482703002081Citations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume10, Issue2June 2003Pages 187-199 ReferencesRelatedInformation

Transport and mixing between airmasses in cold frontal regions during Dynamics and Chemistry of Frontal Zones (DCFZ)

The passage of two cold front systems over the United Kingdom are compared and contrasted, using the results of a detailed aircraft and ground‐based study. The measurements are interpreted by means of three‐dimensional, reverse‐domain‐filling trajectories using both global models and limited‐area mesoscale models. This method provides a three‐dimensional picture of the interleaving air‐masses in the frontal zone as defined by their Lagrangian histories. The two systems studied differ in that the first is associated with an intense surface low in January and the second is associated with a relatively weak surface low in April. In the intense surface low case the trajectory study suggests that a dry intrusion with stratospheric characteristics penetrated deep into the troposphere along the upper level front. Measurements indeed revealed an unsaturated layer with anomalously high ozone. This layer was intersected at four levels in the troposphere (at 8.5, 7.1, 5.2 and 3.7 km), and the lower the intersection, the lower the measured anomalous ozone and the higher the water vapor content. It is argued that this is best explained by the dry‐intrusion layer becoming mixed with background air by three‐dimensional turbulence, also encountered by the aircraft, along the upper level front. Evidence for this mixing is apparent on tracer‐tracer scatterplots. In the weak surface low case, by contrast, the dry intrusion has a more complex structure, with up to three separate layers of enhanced ozone and low humidity. Strong evidence for mixing was apparent only in the lowest layer. The weaker system may therefore be much more efficient at transporting upper tropospheric/stratospheric ozone to the lower troposphere. The transport of boundary layer air to the upper troposphere in the warm conveyor belt (WCB), however, was found to be around 8 times stronger in the intense system. Sonde measurements suggested that the WCB was ventilated by convection from the surface front in some regions to about 5–6 km, while it was stably stratified in other regions, suggesting layerwise long‐range transport.

A Mechanism of the Onset of the South Asian Summer Monsoon

Using the European Centre for Medium-Range Weather Forecasts reanalysis data, we examine whether an onset mechanism of the Australian summer monsoon proposed by Kawamura et al., which incorporates possible air-sea feedback processes, can apply to the South Asian summer monsoon system as well. In their mechanism, a combination of the increase in sea surface temperatures and dry intrusion into the layer at 600-850 hPa level over the ocean off, the equatorial side of a continent, plays a crucial role in enhancing potentially convective instability prior to the onset. It is found that the onset mechanism is able to apply to the abrupt onset of the Indian summer monsoon at the beginning of June, rather than the earliest onset over the Indochina peninsula in middle May. This is consistent with the observational fact that abruptness of the monsoon onset is most evident over the coastal regions of the Indian subcontinent. The Indochina peninsula is characterized by a relatively slow onset, although its date is earliest. The asymmetry of the transition speed between the onset and retreat regimes also has similar regional features. If the mechanism operates efficiently on the Indian subcontinent and adjacent oceans, it is anticipated that the onset of the Indian summer monsoon is delayed, as compared to that over Southeast Asia because subsidence in the periphery of a sub-continental scale thermal low, resulting from intensification of land-ocean thermal contrast, inhibits convection. This may be one of the possible reasons why the two major onsets seen in the South Asian summer monsoon system are clearly distinguished from each other.

Recovery Processes and Factors Limiting Cloud-Top Height following the Arrival of a Dry Intrusion Observed during TOGA COARE

Abstract This study investigates the recovery of the tropical atmosphere to moist conditions following the arrival of a dry intrusion observed during the Tropical Ocean and Global Atmosphere Program Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). A cloud-resolving model was used to quantify the processes leading to the moistening of the lower and middle troposphere. The model replicates the general recovery of the tropical atmosphere. The moisture field in the lower and middle troposphere recovered in large part from clouds repeatedly penetrating into the dry air mass. The moistening of the dry air mass in the simulation was due to lateral mixing on the edges of cloudy regions rather than mixing at cloud top. While the large-scale advection of moisture played a role in controlling the general evolution of moisture field, the large-scale thermal advection and radiation tend to directly control the evolution of the temperature field. The diurnal variations in these two terms were largely responsi...

A mechanism of the onset of the Australian summer monsoon

An onset mechanism of the Australian summer monsoon that incorporates possible air‐sea feedback processes is investigated using the National Centers for Environmental Prediction /National Center for Atmospheric Research daily reanalysis data aided by an ocean general circulation model. Rapid intensification of land‐ocean thermal contrast during the premonsoon period results in a well‐organized continental‐scale shallow vertical circulation over the Australian continent. The shallow vertical circulation is dynamically coupled both with a thermally induced low at the lower level below 850 hPa and a thermal high at 600–700 hPa level. Intensified low‐level westerly anomalies and increased solar radiation in less cloudy air induced by the subsidence in the periphery of the Australian thermal low results in increasing sea surface temperature (SST) along the northern coast of Australia. The thermal high concurrent with the shallow vertical circulation leads to dry intrusion into the layer at ∼700 hPa over the Arafura Sea and Coral Sea through the horizontal and vertical advective processes. A combination of the SST increase and the dry intrusion creates a more convectively unstable condition. When convective instability is intensified while subsidence suppresses convection, the arrival of large‐scale disturbances with ascending motion (such as the Madden‐Julian oscillation) at the domain where the instability is enhanced triggers deep cumulus convection, implying the onset of the monsoon. The onset mechanism proposed in this study may apply not only to the Australian monsoon but also to other monsoon systems that have continental masses in the subtropics.

Multi‐sensor synthesis of the mesoscale structure of a cold‐air comma cloud system

AbstractA multiscale study of a cold‐air comma cloud that produced an area of heavy rain and locally severe weather has been undertaken by synthesising data from a research microwave Doppler radar and VHF and UHF Doppler wind profilers, along with routinely available radar‐network, satellite, in situ and mesoscale‐model data. The rain area was generated in the exit region of an upper‐level jet characterised by laminated velocity perturbations. Some of the perturbations were attributable to inertia‐gravity wave activity. The rain area itself is shown to have been composed of a well‐organised set of mesoscale rainbands each being due to a mixture of upright and slantwise convection. The existence of the multiple rainbands may have been related to the multi‐layered atmospheric structure upwind. Each of the rainbands had cold‐frontal and warm‐frontal portions, so as to form a series of mini warm sectors stacked along the axis of the comma cloud at roughly 70 km intervals. The multiple rainbands were accompanied by multiple fingers of overrunning low‐θw air from part of a dry intrusion originating from just below a major tropopause fold. The fold contained an intense potential‐vorticity maximum which appeared to be the focus of the overall system. The operational mesoscale version of the Met. Office's Unified Model, with its 12 km grid, is shown to have resolved many but not all of the key features of the rainbands. It is suggested that further improvements in very‐short‐range forecasting of important local detail could be achieved by further increasing its resolution and assimilating more mesoscale observational data. Copyright © 2002 Royal Meteorological Society

Multiscale Variability of the Atmospheric Mixed Layer over the Western Pacific Warm Pool

Sounding data from Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) have provided a first opportunity to document the variability of the atmospheric mixed layer over the western Pacific warm pool on timescales ranging from diurnal to intraseasonal. Six-hourly sounding data from four sites—the atoll Kapingamarangi and R/Vs Moana Wave, Shiyan 3, and Xiangyanghong 5—are used to determine the mixed layer depth and its thermodynamic properties. Almost three-quarters of the soundings at these four sites exhibited well-mixed structures: nearly constant profiles of potential temperature and specific humidity capped by a ∼150-m-deep entrainment zone. The majority of the remaining soundings were modified by precipitation and their associated downdrafts. It is estimated that approximately 40%–50% of the total soundings in COARE were influenced by precipitation downdrafts. The mean mixed layer depth at the four sites was 512 m with large variations on multiple timescales. Mean depths decreased across the warm pool from west to east, consistent with the west-to-east increase in precipitation averaged over the 4-month Intensive Observing Period. Significant modulation of the mixed layer occurred on the timescale of the Madden–Julian oscillation (MJO): the mean depth was 562 m during the undisturbed, light-wind period prior to the strong westerly wind burst (WWB) associated with the December MJO; it decreased to 466 m during the heavy-rain period of the WWB, reflecting numerous, recovering precipitation downdraft wakes; and then increased to 629 m during the late stages of the WWB when precipitation had ended. Dry intrusions over the warm pool caused the mixed layer to deepen at times to 800 m and more. Since the surface buoyancy flux typically did not increase at these times, the deepening is linked to a suppression of shallow cumulus clouds by the dry air (reduced between-cloud subsidence) as well as a general reduction in the overall shower activity and associated precipitation downdrafts. Dry intrusions also acted to enhance radiative cooling in the mixed layer. A diurnal cycle in the mixed layer depth was observed, with maximum amplitude in undisturbed (mostly clear), light-wind conditions. The mixed layer was deepest and warmest in the afternoon in direct response to 1) an afternoon peak in the surface buoyancy flux, which, in turn, arose from the large diurnal cycle in SST (up to 2°–3°C) on light-wind days, and 2) absorption of solar shortwave radiation. Thus, the atmospheric mixed layer over the warm pool during undisturbed conditions behaves like that over land, albeit with a weaker diurnal cycle amplitude, but sufficient to generate an afternoon maximum in shallow cumulus clouds and precipitation. This pattern is distinct from the typical early morning maximum in rainfall during disturbed conditions over tropical oceans. Diagnosis of the mixed layer net radiative cooling rate during light-wind conditions indicates a large diurnal range, from ∼−1.5 K day−1 at midday to nearly −3 K day−1 at night.

A 1‐year Lagrangian “climatology” of airstreams in the northern hemisphere troposphere and lowermost stratosphere

This paper presents a 1‐year “climatology” of airstreams, based on a large number of three‐dimensional air parcel trajectories started daily from points on a grid covering the Northern Hemisphere troposphere and lowermost stratosphere. Different types of airstreams (warm conveyor belts, dry intrusions, stratosphere‐to‐troposphere flows, and troposphere‐to‐stratosphere flows) are identified, and their geographical distribution and associated mass fluxes are examined. Warm conveyor belt air parcels, which are deposited in the uppermost troposphere and even the lowermost stratosphere, originate most frequently in the boundary layer over the warm water pools at the eastern seaboards of North America and Asia, close to the worldwide highest anthropogenic emissions. Therefore their chemical characteristics likely range from very clean to highly polluted. Because warm conveyor belts rarely start over Europe, European trace gas emissions have fewer chances of reaching the upper troposphere and lowermost stratosphere with warm conveyor belts than emissions from other densely populated regions. On the other hand, the upper troposphere over Europe frequently receives outflow from warm conveyor belts originating over the North American seaboard. Dry intrusion activity is highest in winter and lowest in summer and shows maxima over western North America and eastern Asia. Deep intrusions from the stratosphere into the lower troposphere are almost nonexistent during the summer. Mass fluxes associated with each of the airstream types are reported and are an alternative to Eulerian mass flux estimates. All types of airstreams with strong vertical motions are highly incoherent. Rather than depositing compact air parcels, they stir air masses into a new environment and create filamentary tracer structures.

Diagnostic study of the severe storm over Poland on 28 March 1997 a fine mesh retrieval perspective

A cyclonic explosive vortex was captured over Poland on 28 March 1997 by meteorological radar in a domain of 400 x 400 km. The radar echo displacement allowed the wind analysis refinement. As a result, 3D mass and wind fields were retrieved with a horizontal grid step of 4 km and vertical step of 100 m on a grid domain of 100 x 100 x 100 nodes. Isentropic potential vorticity (IPV) approach supplemented by a traditional synoptic analysis, as well as a simple adjoint method of Qiu and Xu were applied for retrieving horizontal (and vertical) displacements. The thermodynamic retrieval approach of GAL-CHEN was additionally employed to deduce pressure and temperature fluctuations. The obtained results confirmed the impact of a dry stratospheric intrusion on an explosive cyclogenesis process. The vertical distribution of the pressure was fairly regularly arranged. We have got negative fluctuations in the front (eastward) lower part of the section and positive in the rear lower part of the vortex. In the upper troposphere, this structure was transposed. The retrieved potential temperature fluctuations, as compared with objective analysis, showed higher diversity, although the basic picture remained similar: a warm conveyor belt in front and cold air in the rear part of the vortex. The cold air did not form a wall but might have been regarded as a cold core of a lower troposphere jet. The pressure field related to the horizontal relative motion is also calculated, representing the rotating structure of the vortex.

Use of METEOSAT water‐vapour images for the diagnosis of a vigorous stratospheric intrusion over the central Mediterranean

AbstractThe diagnosis of a vigorous dry intrusion over the central Mediterranean is performed using water‐vapour images from METEOSAT. This dry intrusion was located on the rear side of a cold front (propagating from Italy to Greece) and played an important role in the onset of thunderstorms over the western Greek coasts. A combination of satellite imagery and potential vorticity analyses showed that the dry air originated in the lower‐stratospheric and higher‐tropospheric layers. The interaction of the dry air with the moist air masses within the warm conveyor belt ahead of the cold front (overrun of warm air by low equivalent potential temperature air) produced a potentially unstable region over the area of reported thunderstorms. Copyright © 2000 Royal Meteorological Society

Environmental Variability during TOGA COARE

Abstract This study provides quantitative estimates of the thermodynamic and kinematic structures of the troposphere during various convective regimes observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The data source is the upper air soundings from six stations in the intensive flux array. A correction algorithm has been applied to the humidity data to remove biases between the stations. The data are analyzed using the nonhierarchical clustering method known as k means. Eleven thermodynamic clusters and 20 kinematic clusters are selected. The thermodynamic clusters are grouped into four general categories based on their midtropospheric equivalent potential temperature. Deep convective activity varies with the thermodynamic structure of the environment. When the “dry intrusion” group is observed, convection is suppressed. The “fair weather” category corresponds to undisturbed periods with light winds and small mesoscale convective systems (MCSs). The largest...

The evolution of the tropical western Pacific atmosphere-ocean system following the arrival of a dry intrusion

The evolution of the tropical western Pacific atmosphere-ocean system following the arrival of a dry intrusion

The evolution of the tropical western Pacific atmosphere‐ocean system following the arrival of a dry intrusion

AbstractRecent studies using TOGA COARE data have found that extremely dry air from middle‐latitude waves frequently intrudes into the equatorial troposphere over the western Pacific. Using sounding data taken during the COARE, the magnitude of the advection of water vapour for one event is calculated, and it is estimated that the lime for the atmosphere to recover to moist conditions was ∼ 10‐20 days. From the magnitude of the drying and from the frequency of these events, it is proposed that dry intrusions must be a major contributor to the tropospheric moisture budget over the region during the COARE, making it difficult for the atmosphere to reach a radiative‐convective equilibrium, intrusions, instead, can help to recharge the tropical atmosphere by decreasing convective activity and, thus, driving the atmosphere toward unusually large values of convective available potential energy. A variety of atmospheric and oceanic measurements are also used to study the recovery process in detail. A conceptual model is proposed based on this work and previous investigations. As in past studies, the recovery of the atmosphere to moist conditions is accomplished through detrainment from convective clouds that began to form soon after the arrival of the dry air mass and slowly deepen in height as the recovery progresses. Previous investigators concluded that the entrainment of dry air into convective ceils is generally the factor that tends to suppress convective activity and limits the height of any convection that does develop under these adverse conditions. The idea that entrainment limits convective activity is consistent with the commonly held perception that the western Pacific is a region where there is little inhibition to deep convection and, when inhibition does occur, it can be removed by surface fluxes within hours. In contrast, it is found that convective inhibition can be large enough to suppress convection following dry intrusions, and that the diurnal variation in rainfall is due partly to modulations in convective inhibition. The modulations in convective inhibition are, in turn, caused by diurnal variations in the vertical profiles of radiation, in surface fluxes, and perhaps in large‐scale subsidence, leading to a minimum in convective inhibition during the late afternoon. In contrast, studies of this type of convection have generally emphasized diurnal variations in the surface fluxes, and often ignored convective inhibition and diurnal variations in atmospheric radiative heating.

Rapid cyclogenesis over Poland on 28 March 1997

A small wave on the polar front crossed the Atlantic and experienced strong explosive cyclogenesis on 28 March 1997 when passing over the North Sea. It then progressed eastwards over northern Poland causing severe damage and the death of at least 11 people. A phenomenological study of the event is presented based on a potential vorticity (PV) approach supplemented by a synoptic analysis. Results confirm previous findings about the impact of a dry stratospheric intrusion on the process of explosive cyclogenesis and fit well with the conceptual model described by Browning (1997) concerning diffluent flow cyclogenesis. Also apparent is the role of potential vorticity advection, which is associated with the rapid change of flow pattern in the upper atmosphere. The careful analysis of potential vorticity fields every 100 m suggests that further research is required into fine mesh analysis and wind field retrieval. Copyright © 1999 Royal Meteorological Society

Mesoscale analysis of arc rainbands in a dry slot

A series of almost concentric, are-shaped narrow rainbands was observed travelling at 140 km h−1 and producing severe wind gusts over parts of England. Their structure and environment have been analysed using radar and satellite imagery, mesoscale-model diagnostics, surface observations and additional rawinsonde ascents obtained during the Fronts and Atlantic Storm-Track EXperiment (FASTEX). the multiple-rainband event is shown to have been associated with part of a dry intrusion characterized by a mesoscale vortex/cold pool. the event occurred within the ‘dry slot’ region of a small frontal cyclone where the mesoscale pool of cold dry air was overrunning warm-sector air. Continuous wind profiles from a UHF radar, together with a well-located rawinsonde at the ‘epicentre’ of the arc rainbands, showed that the major anomaly in the cold pool was an area of locally stronger winds (>40 m s−1) at 800 hPa colocated with the temperature minimum at that level. the observed orientation of the rainbands is explained in terms of the cold-frontal structure at this level triggering ascent of the underlying warm moist air. the multiple nature of the arc rainbands remains unexplained; the possibilities of mesoscale gravity waves and conditional symmetric instability are both considered.

Mesoscale analysis of arc rainbands in a dry slot

AbstractA series of almost concentric, are‐shaped narrow rainbands was observed travelling at 140 km h−1 and producing severe wind gusts over parts of England. Their structure and environment have been analysed using radar and satellite imagery, mesoscale‐model diagnostics, surface observations and additional rawinsonde ascents obtained during the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). the multiple‐rainband event is shown to have been associated with part of a dry intrusion characterized by a mesoscale vortex/cold pool. the event occurred within the ‘dry slot’ region of a small frontal cyclone where the mesoscale pool of cold dry air was overrunning warm‐sector air. Continuous wind profiles from a UHF radar, together with a well‐located rawinsonde at the ‘epicentre’ of the arc rainbands, showed that the major anomaly in the cold pool was an area of locally stronger winds (>40 m s−1) at 800 hPa colocated with the temperature minimum at that level. the observed orientation of the rainbands is explained in terms of the cold‐frontal structure at this level triggering ascent of the underlying warm moist air. the multiple nature of the arc rainbands remains unexplained; the possibilities of mesoscale gravity waves and conditional symmetric instability are both considered.

Pacific and atlantic ‘bomb‐like’ deepenings in mature phase: A comparative study

AbstractThe mesoscale dynamics of a secondary cyclone sampled during the field phase of the Fronts and Atlantic Storm‐Track Experiment (FASTEX) are documented using airborne Doppler radar data. the high quality of the mesoscale airborne Doppler dataset collected within this secondary cyclone is highlighted. This study demonstrates, however, the need to combine in the near future airborne Doppler radar and dropsonde measurements, in order to access multiscale processes involved in the mature stage of this FASTEX cyclone both in clear‐air and in precipitating regions. the studied secondary cyclone developed on the trailing cold front of a parent low situated over Greenland, and underwent the most explosive deepening (roughly −54 mb in 24 hours) sampled in the multiscale sampling area during FASTEX. the main flows responsible for the mesoscale organization of the cyclone are: a warm airflow emerging from the base of the synoptic warm conveyor belt that generates the most active part of the cloud head; a dry intrusion approaching in the dry slot the cyclone centre; a cold conveyor belt that contributes to the upward mass transport towards the upper part of the polar‐front cloud band; and a cold northerly flow in the western part of the cloud head, which tends to wrap around the low centre and is likely to release convective instability through air‐sea interactions. This study also documents a slight shift in position between the high vertical‐vorticity core associated with the active part of the cloud head and the location of the pressure minimum.The detailed three‐dimensional structure of this Atlantic secondary cyclone is also compared with that gathered within another ‘bomb‐like’ deepening (defined as more than a 24 mb surface pressure drop in 24 hours) observed over the Pacific ocean during the Coastal Observation and Simulation with Topography (COAST) experiment. Some similarities are found. the main differences appear to be linked to the warm seclusion that is exclusive to the studied Pacific cyclone. More pronounced ascending motions are found within the cloud head of the Pacific ‘reference’ case, and the deepening results from a more constructive effect of geostrophic and non‐geostrophic processes than for the Atlantic ‘reference’ case.

Analysis of an ex‐tropical cyclone after its reintensification as a warm‐core extratropical cyclone

AbstractEx‐hurricane Lili reintensified as an extratropical cyclone before travelling across the data‐rich region of the British Isles on 28 October 1996. The cyclone centre passed close to a Mesosphere‐Stratosphere‐Troposphere (MST) radar, providing continuous profiles of wind etc. which were used to evaluate diagnostics from the mesoscale version of the operational UK Meteorological Office Unified Model. The paper presents a mesoanalysis of the mature extratropical cyclone using model output together with radar and satellite observations. The combined analysis reveals a vertically extensive warm core three‐quarters surrounded by a low‐level jet reaching over 40 m s−1. There was an associated eye, relatively free of cloud and partly surrounded by a hook cloud producing extensive heavy rain, which was itself encircled by cooler dry‐intrusion air. The stratospheric part of the dry intrusion (and its potential‐vorticity (PV) anomaly) descended within a tropopause fold around the cloud hook generally to below 400 hPa, with small pockets penetrating significantly lower. The cyclone's reintensification as an extratropical cyclone was related to its interaction with the stratospheric PV anomaly. This interaction commenced immediately after the decay of the strong moist ascent and associated deep column of diabatically generated positive PV that had characterized the earlier tropical‐cyclone phase. Following reintensification, the dry‐intrusion air entered the eye region of the extratropical cyclone over a deep layer. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the dryness of the dry‐intrusion air entering the eye. The MST radar vividly depicted the region of moist boundary‐layer air responsible for the hook cloud rising up into the region of the lowered tropopause.

Analysis of an ex-tropical cyclone after its reintensification as a warm-core extratropical cyclone

Ex-hurricane Lili reintensified as an extratropical cyclone before travelling across the data-rich region of the British Isles on 28 October 1996. The cyclone centre passed close to a Mesosphere-Stratosphere-Troposphere (MST) radar, providing continuous profiles of wind etc. which were used to evaluate diagnostics from the mesoscale version of the operational UK Meteorological Office Unified Model. The paper presents a mesoanalysis of the mature extratropical cyclone using model output together with radar and satellite observations. The combined analysis reveals a vertically extensive warm core three-quarters surrounded by a low-level jet reaching over 40 m s−1. There was an associated eye, relatively free of cloud and partly surrounded by a hook cloud producing extensive heavy rain, which was itself encircled by cooler dry-intrusion air. The stratospheric part of the dry intrusion (and its potential-vorticity (PV) anomaly) descended within a tropopause fold around the cloud hook generally to below 400 hPa, with small pockets penetrating significantly lower. The cyclone's reintensification as an extratropical cyclone was related to its interaction with the stratospheric PV anomaly. This interaction commenced immediately after the decay of the strong moist ascent and associated deep column of diabatically generated positive PV that had characterized the earlier tropical-cyclone phase. Following reintensification, the dry-intrusion air entered the eye region of the extratropical cyclone over a deep layer. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the dryness of the dry-intrusion air entering the eye. The MST radar vividly depicted the region of moist boundary-layer air responsible for the hook cloud rising up into the region of the lowered tropopause.