Warm Occlusion Research Articles

Evolution of Mesoscale Precipitation Band Environments within the Comma Head of Northeast U.S. Cyclones

Abstract This paper explores the mesoscale forcing and stability evolution of intense precipitation bands in the comma head sector of extratropical cyclones using the 32-km North American Regional Reanalysis, hourly 20-km Rapid Update Cycle analyses, and 2-km composite radar reflectivity data. A statistical and composite analysis of 36 banded events occurring during the 2002–08 cool seasons reveals a common cyclone evolution and associated band life cycle. A majority (61%) of banded events develop along the northern portion of a hook-shaped upper-level potential vorticity (PV) anomaly. During the 6 h leading up to band formation, lower-tropospheric frontogenesis nearly doubles and the conditional stability above the frontal zone is reduced. The frontogenesis increase is primarily due to changes in the kinematic flow associated with the development of a mesoscale geopotential height trough. This trough extends poleward of the 700-hPa low, and is the vertical extension of the surface warm front (and surface warm occlusion when present). The conditional stability near 500 hPa is reduced by differential horizontal potential temperature advection. During band formation, layers of conditional instability above the frontal zone are present nearly 3 times as often as layers of conditional symmetric instability. The frontogenetical forcing peaks during band maturity and is offset by an increase in conditional stability. Band dissipation occurs as the conditional stability continues to increase, and the frontogenesis weakens in response to changes in the kinematic flow. A set of 22 null events, in which band formation was absent in the comma head, were also examined. Although exhibiting similar synoptic patterns as the banded events, the null events were characterized by weaker frontogenesis. However, statistically significant differences between the midlevel frontogenesis maximum of the banded and null events only appear ~2 h prior to band formation, illustrating the challenge of predicting band formation.

The IMPROVE-1 Storm of 1–2 February 2001. Part I: Development of a Forward-Tilted Cold Front and a Warm Occlusion

Abstract Analysis of observations and the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are used to study the development of a forward-tilted cold front off the coast of Washington State. The vertical velocity associated with the cold front produced a wide cold-frontal rainband. In the early stage of development the midtropospheric baroclinic zone (or upper cold front) moved forward with time over the warm sector to produce a structure similar to a split front. The movement of the upper cold front was due to horizontal transport and frontogenetical propagation. The frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis, which resulted in a negative/positive couplet of frontogenesis straddling the baroclinic zone. The lower-tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion. In the initial occluding process the converging frontal zones tilted into a warm-type occlusion configuration due to the presence of a background vertical shear of the horizontal wind component perpendicular to the occluded front. Consequently, as the storm moved over the observing network, the occluded front had the structure of a warm occlusion (tilted forward) in the lower levels. Above the occlusion, the cold front was also tilted forward because it retained its split-front-like structure. Thus, the development of the split front and the warm occlusion were separate processes that occurred in sequence. Although the MM5 captured the basic forward tilt with time of the cold front, some key aspects of the midtropospheric frontal structure were not well simulated. Because diabatic heating was an important contributor to the maintenance and movement of the upper cold front, it is hypothesized that discrepancies in diabatic heating associated with deficiencies in the model’s explicit microphysical scheme may be responsible for deficiencies in reproducing the structure of the upper cold front.

The IMPROVE-1 Storm of 1–2 February 2001. Part II: Cloud Structures and the Growth of Precipitation

Abstract On 1–2 February 2001, a strong cyclonic storm system developed over the northeastern Pacific Ocean and moved onto the Washington coast. This storm was one of several that were documented during the first field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). In the 1–2 February case, soundings and wind profiler measurements showed that a wide cold-frontal rainband was coincident with the leading edge of an upper-level cold front in a classical warm occlusion. Ground-based radar observations revealed the presence of subbands within the wide cold-frontal rainband and two layers of precipitation generating cells within this rainband: one at 5–7 km MSL and the other at 9–10 km MSL. The lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km MSL. Observations suggest a connection between the subbands and the lower layer of generating cells. A research aircraft, equipped for cloud microphysical measurements, passed through at least two generating cells in the 5–7-km region. These cells were in their formative stage, with elevated liquid water contents and low ice particle concentrations. The microphysical structure of the wide cold-frontal rainband was elucidated by particle imagery from a Cloud Particle Imaging (CPI) probe aboard the research aircraft. These images provide detailed information on crystal habits and degrees of riming throughout the depth of the rainband. The crystal habits are used to deduce the temperature and saturation conditions under which the crystals grew and, along with in situ measurements of particle size spectra, they are used to estimate particle terminal fall velocities, precipitation rates, radar reflectivities, and vertical air motions. The radar reflectivity derived in this way generally compared well with direct measurement. Both the derived and directly measured parameters are used to determine the primary particle growth processes in the wide cold-frontal rainband. Above the melting layer, vapor deposition was the dominant growth process in the rainband; growth of ice particles by riming was small. Significant aggregation of ice particles occurred in the region just above the melting layer. A doubling in the air-relative vertical precipitation mass flux occurred between the region where sheath ice crystals formed (−3° ≤ T ≤ −8°C) and the surface. Substantial amounts of liquid water were found within the melting layer where growth occurred by the accretion of cloud droplets and also by condensation. Growth by the collision and coalescence of raindrops was not significant below the melting layer.

Re‐examination of the split cold front in the British Isles cyclone of 17 July 1980

AbstractAn analysis of an MM5 model simulation of a cyclone that passed over the British Isles on 17 July 1980 indicates that the cyclone had some characteristics of both the split‐cold‐front model of Browning and Monk (1982) and of a schematic by Browning and Roberts (1996). Most importantly, the cyclone had the characteristic upper‐level baroclinic zone and its associated vertical velocity in advance of the surface front. This is an integral part of both the split‐cold‐front model of Browning and Monk and Browning and Roberts's schematic, and confirms the analysis of Browning and Monk. Simultaneously at different locations, the cyclone also had the characteristics of a warm occlusion in the lower levels with the portion of the cold front that was above the warm front tipped forward with height. The lower and upper portions of the cold front were not only fronts in equivalent potential temperature, as proposed by Browning and Monk, but true baroclinic zones with significant associated vertical velocity.The complex structure of this cyclone in the British Isles was similar to that of some storms in the Pacific Northwest of the United States (Locatelli et al. 2005). Copyright © 2005 Royal Meteorological Society.

Numerical simulations of a large-amplitude mesoscale gravity wave event

Numerical simulations with the NCAR/PSU Mesoscale Model 5 (MM5) were performed to study a large-amplitude gravity wave event that occurred on 4 January 1994 along the East Coast of the United States. Results from the MM5 control simulation using a 12-km mesh resolution compared well with the synoptic and mesoscale observational analysis. The simulated gravity waves displayed timing, location, wavelength, and propagation speed similar to those observed in a synoptic-scale environment described by the Uccellini and Koch (1987) conceptual model. Additional features existing upstream of the wave generation region not contained within their conceptual model were a warm occlusion and tropopause fold prior to and during the gravity wave generation. Wave ducting criteria were nearly satisfied along the path of the gravity waves.

The Structure and Evolution of a Continental Winter Cyclone. Part II: Frontal Forcing of an Extreme Snow Event

The production of a narrow, heavy, occasionally convective snowband that fell within a modest surface cyclone on 19 January 1995 is examined using gridded model output from a successful numerical simulation performed using the University of Wisconsin–Nonhydrostatic Modeling System. It is found that the snowband was produced by a thermally direct vertical circulation forced by significant lower-tropospheric warm frontogenesis in the presence of across-front effective static stability differences as measured in terms of the equivalent potential vorticity (PVe). The sometimes convective nature of the snowband resulted from the development of freely convective motions forced by frontal lifting of the environmental stratification. Model trajectories demonstrate that a stream of warm, moist air ascended through the trowal portion of the warm-occluded structure that developed during the cyclone life cycle. The lifting of air in the trowal was, in this case, forced by lower-tropospheric frontogenesis occurring in the warm-frontal portion of the warm occlusion. This trowal airstream accounts for the production of the so-called wrap-around precipitation often associated with occluded cyclones and, in this case, accounted for the northern third of the heavy snowband.

Surface Convergence Induced by Cold Fronts Aloft and Prefrontal Surges

Abstract The magnitude of surface convergence, produced by the movement of cold fronts aloft and prefrontal surges, is derived by applying the linear divergence equation to observed surface pressure traces for Pacific Northwest warm occlusions and from a mesoscale model simulation of a warm occlusion–like structure in the central United States. Convergence values of approximately 10−4 s−1 are found to be generated locally for periods of about 1 h, yielding vertical displacements of 10–50 hPa. It is hypothesized that such convergences should noticeably enhance condensation rates in the widespread lower stratiform clouds associated with warm occlusions and could be a key mechanism for the triggering of squall lines by cold fronts aloft in cyclones in the central United States.

Structure and Evolution of Winter Cyclones in the Central United States and Their Effects on the Distribution of Precipitation. Part IV: The Evolution of a Drytrough on 8–9 March 1992

The structure and evolution of a drytrough (i.e., a surface pressure trough that has characteristics of both a lee trough and a dryline) from the southern Great Plains to the lower Mississippi Valley are described using both observational data and outputs from a mesoscale model. An elevated zone of cold-air advection associated with a cold front aloft interacted with the drytrough over the southern Great Plains to form a structure similar to a warm occlusion. This type of structure figures importantly in a new conceptual model that has been proposed for cyclones in the central United States.

Organization and Structure of Clouds and Precipitation on the Mid-Atlantic Coast of the United States. Part I: Synoptic Evolution of a Frontal System from the Rockies to the Atlantic Coast

Abstract The frontal structure of a cyclone that developed in the lee of the Rocky Mountains and moved eastward across the United States is examined. The evolutions and interactions of three frontal features are traced: the primary cold front, a shallow secondary arctic front, and a leeside trough. The zone of warm advection associated with the lee trough became more concentrated with time, and eventually resembled a warm front. The primary cold front had a tipped-forward structure, with cold advection aloft preceding cold advection at lower levels. This front overran the trough to form on the East Coast a structure that was similar to a warm occlusion or a split cold front. Two rainbands, parallel to and approximately 225 km ahead of the surface front, formed and dissipated within the inner network of the Genesis of Atlantic Lows Experiment. These rainbands developed at the leading edge of cold advection aloft, and they dissipated as they approached a region of strong convection over the Gulf Stream. Thi...

Organization and Structure of Clouds and Precipitation on the Mid-Atlantic Coast of the United States. Part II: The Mesoscale and Microscale Structures of Some Frontal Rainbands

Abstract The mesoscale and microscale structures of the clouds and precipitation associated with a frontal system on the mid-Atlantic Coast of the United States are investigated using radar reflectivity and Doppler velocity data, surface mesonet, conventional surface, upper-air and microphysical data. The frontal structure showed similarities to a warm occlusion, with code-air advection aloft preceding cold-air advection at lower levels. Six rainbands were observed in association with the frontal system. All of these rainbands developed within the region of coverage of the NWS WSR-57 radar at Cape Hatteras, North Carolina. Two were upper-level features, associated with a prefrontal surge of cold air and the main push of cold dry air aloft. These rainbands were similar in structure to prefrontal surge and wide cold-frontal rainbands, respectively, observed on the Pacific Northwest Coast. The microphysical and small mesoscale structures of the wide cold-frontal rainband are examined. Three of the rainbands ...

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. Part XIV: Three-Dimensional Airflow and Vorticity Budget of Rainbands in a Warm Occlusion

Two rainbands in a young, warm occlusion are examined using dual-Doppler radar data and supporting mesoscale measurements. The high density of measurements permits the structure of the weak vertical motions associated with the precipitation to be resolved. Kinematic fields are described with respect to the small-scale precipitation cores that made up two rainbands. The three-dimensional motions in this core-relative frame provide insights into the relationship between the kinematic and precipitation fields. The rainbands were situated near the centers of cyclonic vorticity and upward motion in the occlusion. They were oriented perpendicular to the wind shear in the vertical and they appeared to be maintained through the interaction of small-scale (∼5 km) updrafts and band-scale (∼40 km), nondivergent, cyclonic circulations. Updrafts, forced by convergence in the boundary layer ahead of the surface occlusion and ahead of the cold air aloft, enhanced the growth of precipitation at and above the level of the band-scale cyclonic circulations. These circulations played an important role in determining the widths of the rainbands. The primary source of vertical vorticity in one of the rainbands was the tilting of the horizontal vorticity associated with the shear in the vertical of the wind across the rainband.

Numerical simulations of polar lows in the Norwegian Sea

A high resolution version of an operational weather prediction model has been developed. An explicit time integration scheme is used on a limited area polar stereographic grid. The physical parameterization of surface fluxes, fluxes in the free atmosphere, stratiform and convective precipitation are relatively simple and no radiation processes are involved. Simulations are presented for six synoptic situations when polar lows occurred in The Norwegian Sea. Analyses from the European Centre for Medium Range Weather Forecasts(ECMWF) are used as initial analyses. The simulations are verified against published subjective analyses. In one case, analyses from detailed aircraft measurements were available. In most cases the horizontal grid distance has been 50 km; in one case 25 km resolution was used. The integrations started before any polar low was observed and lasted for 36 or 48 hours. The model creates polar low disturbances at approximately the right position at the right time. The predicted intensity is, however, generally too weak. All the simulated polar lows show an initial phase where a baroclinic development takes place in a reversed shear flow. Usually the structure is relatively shallow. The low-level trough or low is found to be warm, and conditional vertical instability is connected to it. In this way, a synoptic situation is prepared which is favourable for some CISK-like mechanism which sometimes further develop the polar low. The model does not handle this rapid growth properly, probably due to lack of resolution or incomplete parameterization of convection. In all cases, cross-sections perpendicular to the trough-like disturbance show a warm occlusion, or a double-front system. The front also exists when precipitation processes are excluded in the model; however, release of latent heat strengthens the vertical circulations generally connected to each branch of the occlusion. A rapid development of a polar low might be connected to locally descending air in between the two circulations; satellite pictures indeed show a cloud-free eye in some cases. DOI: 10.1111/j.1600-0870.1987.tb00312.x