Search Results

You are looking at 21 - 30 of 43 items for

  • Author or Editor: John D. Locatelli x
  • Refine by Access: All Content x
Clear All Modify Search
Christopher P. Woods, Mark T. Stoelinga, and John D. Locatelli

Abstract

Particle size spectra collected by the University of Washington’s Convair-580 research aircraft at a variety of altitudes and temperatures in winter frontal and orographic precipitation systems during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE) are analyzed in this study. The particle size spectra generally appeared to conform to an exponential size distribution, with well-correlated linear fits between the log of the number concentration and particle diameter. When the particle size spectra were grouped according to the habit composition as determined from airborne imagery, significantly improved correlations between the size spectrum parameters and temperature were obtained. This result could potentially be exploited for specifying the size distribution in a single-moment bulk microphysical scheme, if particle habit is predicted by the scheme. Analyses of “spectral trajectories” suggest that the rime-splintering process was likely responsible for the presence of needle and column habit types and the positive shift in both N 0s and λs at temperatures warmer than −10°C.

Full access
Mark T. Stoelinga, John D. Locatelli, and Christopher P. Woods

Abstract

Recent studies that have classified ice particles from airborne imaging probe data have concluded that the vast majority of ice particles in stratiform precipitation systems are of an “irregular shape.” This conclusion stands in contrast to the findings from microscope observations of snow particles at the ground during the Improvement of Microphysical Parameterization through Observational Verification Experiment from November to December 2001 in the Oregon Cascade Mountains (IMPROVE-2), which show that most snow crystals (either single crystals or the component crystals of snow aggregates) are readily identified as regular types within established crystal classification systems. This apparent contradiction is rectified by examining the definition of the term irregular as applied to ice particles and by considering limitations of different methods for observing ice particles. It is concluded that the finding of the airborne probe-based studies is a consequence of both limitations of the observing technology and an overly broad definition of irregular shape that is not consistent with the more restrictive definition established in well-known snow crystal classification schemes. When detailed microscope analysis of snow crystals is performed at the ground, and all regular types are included in the classification, the vast majority of snow crystals are of an identifiable regular type, rather than an irregular type.

The classification of the vast majority of particles as irregular implies that there is little hope to describe the important properties of these particles (such as their scattering properties, fall speeds, and temperature and humidity conditions in which they grew), when in fact, many of these particles are of known types with known properties. Instead of using the term irregular, classification studies should use a term that focuses on the limitation of the observation method as being the defining characteristic of the category, such as “unidentified” or “undetermined.”

Full access
Mark T. Stoelinga, John D. Locatelli, and Peter V. Hobbs

Abstract

A cyclonic storm that moved over the central United States on 8–9 March 1992 developed two convective rainbands, namely, a pre–dry trough rainband and a cold front aloft (CFA) rainband. This study extends the results of previous investigations of these two rainbands by examining their initiation with the use of a nested-grid mesoscale model simulation with spatial resolution down to 8.3 km. The model simulation reproduced the synoptic-scale setting in which the rainbands developed, as well as the mesoscale processes that initiated the rainbands.

The pre–dry trough rainband was produced by the gradual ascent of a convectively unstable airstream above a gently sloping warm-frontal zone east of the dryline. After sufficient lifting, the instability was released through upright convection. The gradual ascent is well estimated by quasigeostrophic diagnosis, but the location and timing of the rainband are very sensitive to the convective stability characteristics within the airstream.

The CFA rainband was initiated by a Pacific cold front that occluded with the warm-frontal surface. This mesoscale occlusion process produced a narrow region of enhanced ascent at the dryline, which resulted in the lifting of the western edge of an air mass with high convective available potential energy. The lower-tropospheric mesoscale occlusion process was not resolved by a quasigeostrophic vertical velocity diagnosis. Also, although an upper-level front and tropopause fold were present, the CFA was separate from that feature.

Full access
Jonathan E. Martin, John D. Locatelli, and Peter V. Hobbs

Abstract

The origins of a rainband of moderate intensity that occurred over the eastern Carolinas is investigated. It is concluded that the band formed in the updraft portion of a thermodynamically direct vertical circulation about an upper-level frontal zone in a region of conditional symmetrical instability (CSI). The release of CSI is presumed to have been responsible for the dimensions of the band and its orientation relative to the shear vector. An adiabatic mechanism for destabilization of the environment of the upper-level front to CSI was explored but found to be insignificant in this case.

Full access
Mark T. Stoelinga, John D. Locatelli, Ralph D. Schwartz, and Peter V. Hobbs
Full access
Mark T. Stoelinga, John D. Locatelli, Ralph D. Schwartz, and Peter V. Hobbs

Abstract

On 19–20 June 1979 a cyclone moved through the central United States. This cyclone contained a squall line associated with a cold front aloft (CFA), which caused significant damage. The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) was used to diagnose the role of the cold pool in the maintenance of the squall line. A control simulation with “full physics” was run, at 18- and 3.6-km grid spacing. Both simulations produced a squall line that was similar in location, orientation, and speed to the observed squall line, and displayed several characteristics that differed from the “leading line–trailing stratiform” paradigm for midlatitude squall lines. Sensitivity test simulations were run for both grid spacings, with diabatic cooling due to evaporation and melting of precipitation withheld to prevent the formation of a cold pool. These simulations produced a squall line similar to that in the control simulation, in terms of the location, orientation, and movement of the squall line. The simulations showed that the CFA provided the primary lifting responsible for the maintenance and movement of the simulated squall line by means of the hydrostatic surface pressure pattern it induced. The cold pool did not play a critical role in the maintenance of the simulated CFA squall line, but it did retard the progression of the synoptic-scale trough that trailed the simulated squall line, thereby increasing the forward tilt of the Pacific cold front.

Full access
John D. Locatelli, Mark T. Stoelinga, Ralph D. Schwartz, and Peter V. Hobbs

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.

Full access
John D. Locatelli, Ralph D. Schwartz, Mark T. Stoelinga, and Peter V. Hobbs

Abstract

Conventional data and mesoscale model simulations are used to analyze two cyclones that developed east of the Rocky Mountains in June and November 1998. Both cyclones formed when a Pacific cold front overtook a lee trough/dryline east of the Rockies. In one case the leading edge of the Pacific cold front was on the surface, as depicted in the classic Norwegian model of a cyclone. In the other case, which is referred to as a cold front aloft (CFA) cyclone, the leading edge of the Pacific cold front was aloft and in advance of the lee trough. The lifting and severe weather associated with the Pacific cold front was along the leading edge of this front in both the Norwegian-type and CFA-type cyclones.

To obtain an estimate of how often CFA cyclones, with a coincident CFA rainband, occurred in the central United States during the period 15 September 1994 through 15 September 1995, 70 cyclones that maintained closed surface low pressure centers for at least 24 h, and produced precipitation in the United States east of the Rockies, were analyzed. Analysis of these cyclones revealed that 46% were CFA type, 23% were Norwegian type, and the remaining 31% could not be readily classified into either type. Lee troughs were common features east of the Rocky Mountains during this period. They were present in 62% of the 70 cyclones analyzed.

The 1000–500-hPa thickness field is suggested as a useful tool in locating the leading edge of a Pacific cold front, and in determining whether a cyclone is a Norwegian type or CFA type. The issue of how the frontal structures in CFA-type cyclones should be analyzed on surface weather charts is discussed, and some suggestions offered.

Full access
Peng-Yun Wang, Jonathan E. Martin, John D. Locatelli, and Peter V. Hobbs

Abstract

The structure and evolution of a shallow but intense cold front (commonly referred to as an arctic front) and its associated precipitation features that passed through the central United States from 0000 UTC 9 March to 0000 UTC 10 March 1992 are studied with the aid of observations and outputs from a numerical simulation using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model MM4.

Located above the arctic front was a region of midtropospheric, frontogenetical confluence that was attended by a thermally direct vertical circulation. A large banded precipitation feature, for the most part located behind the arctic front, was produced by ice crystals from upper-level clouds (formed by the frontogenetical confluence) falling into low-level stratocumulus associated with the arctic front. The arctic front at the surface separated a region where the precipitation reaching the ground was solid from an adjacent region where the precipitation was liquid. A westward-moving, low-level jet behind the arctic front produced upslope flow over the high terrain of the northern Great Plains, which contributed to heavy snowfalls in this region.

A portion of the arctic front that moved southward, west of a low pressure center, was characterized by sharp drops in temperature and dewpoint and an increase in wind speed. However, the arctic front was not associated with either a pressure trough or much change in wind direction. The proximity of arctic fronts to such nonfrontal features as lee troughs and/or drylines often leads to the latter being misanalyzed as cold fronts.

Full access
Joseph M. Sienkiewicz, John D. Locatelli, Peter V. Hobbs, and Bart Geerts

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 were convective and developed at different times parallel to and just east of the warm-water core of the Gulf Stream. Each of these rainbands, in turn, migrated to the east. Coincident with the dissipation of the wide cold-frontal rainband offshore, a convective rainband developed behind the leading edge of the cold, dry air aloft and a third rainband intensified over the Gulf Stream in advance of the cold, dry air at midlevels. Mechanisms for the development of the Gulf Stream rainbands and for the dissipation of the wide cold-frontal rainband are discussed.

Full access