Search Results

You are looking at 51 - 60 of 70 items for

  • Author or Editor: Richard J. Reed x
  • Refine by Access: All Content x
Clear All Modify Search
RICHARD J. REED
,
MICHAEL J. OARD
, and
MARYA SIEMINSKI

Abstract

Lindzen's theoretical predictions of the thermally driven diurnal tide are compared with observations of the diurnal amplitude and phase of the meridional wind component between 30 and 60 km for five groups of stations ranging in latitude from 64° N. to 8° S. In general, good agreement is found between theory and observation. The most important deviation occurs above 55 km where theoretical phases become progressively earlier than observed at lower latitude stations. This behavior suggests that the strength of the trapped modes in Lindzen's solutions is under-estimated relative to that of the propagating modes. Since the trapped modes are driven primarily by ozone heating in the upper troposphere and lower mesosphere, it is suggested that the problem of ozone heating requires further examination.

Full access
James F. Bresch
,
Richard J. Reed
, and
Mark D. Albright

Abstract

A polar low that developed over the western Bering Sea on 7 March 1977 and tracked across St. Paul Island is investigated using observations and the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). A series of fine-mesh (20 km) simulations are performed in order to examine the structure of the cyclone and the airflow within it and to determine the physical processes important for its development. Observations show that the low formed near the ice edge in a region of moderate low-level baroclinicity and cold-air advection when an upper-level trough, or lobe of anomalously large potential vorticity (PV), broke off from a migratory, upper-level cold low over Siberia and advanced into the region.

A full physics model experiment, initialized 24 h prior to the appearance of the polar low, produced a small, intense cyclone having characteristics similar to the observed low. The simulated low more closely resembled an extratropical cyclone than a typical circularly symmetric hurricane, possessing a thermal structure with frontlike features and an asymmetric precipitation shield. Although the simulated low developed southeast of, and earlier than, the observed low, the basic similarity of the observed and modeled systems was revealed by a comparison of the sequence of weather elements at a point in the path of the simulated low with the sequence of observations from nearby St. Paul Island, Alaska.

A series of experiments was performed to test the sensitivity of the simulated polar low development to various physical processes. Four experiments of 48-h duration each were initialized 24 h before the low appeared. In the first experiment, in which surface fluxes were turned off, the low failed to develop. In the second experiment, in which the fluxes were switched on after a 24-h delay, only a weak low formed. In the third experiment, in which the ice edge was shifted a degree of latitude to the north, thus increasing the overwater fetch of the cold air, the low’s evolution was slightly altered but the final outcome was little changed. A fourth high-horizontal resolution experiment (6.67-km spacing) displayed more plentiful and sharper mesoscale features but on the storm scale yielded results that were similar to those of the full-physics run. A full-physics experiment initialized 24 h later, at the time the low first appeared, and run for 24 h, produced a system of similar intensity to that in the 48-h full-physics run but somewhat better positioned. Corresponding sensitivity experiments showed that with both surface fluxes and latent heating omitted, the low weakened and nearly died away. Experiments retaining only surface fluxes in one case and only latent heating in the other, produced similar cyclones of moderate depth.

The results suggest that the development of some, if not most, polar lows can be regarded as fundamentally similar to that of midlatitude ocean cyclones. In both cases a mobile upper-level PV anomaly interacts with a low-level thermal or PV anomaly produced by thermal advection and/or diabatic heating. The polar low lies at the end of the spectrum of extratropical cyclogenesis in which concurrent surface fluxes of sensible and latent heat and the immediately ensuing condensation heating in organized convection dominate the development of the low-level anomaly.

Full access
Richard J. Reed
,
Georg A. Grell
, and
Ying-Hwa Kuo

Abstract

The ERICA IOP 5 storm was the third strongest cyclone observed during the three-month Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) and the least successfully predicted by the operational models. This paper documents the storm development with use of nearly all available observational data and presents the results of a simulation of the storm carried out by the Pennsylvania State University-NCAR mesoscale Model MM4.

The observations reveal that the storm formed in two stages: a first stage in which a weak, eastward-moving upper-level trough over the Gulf states excited the growth of two disturbances over the Gulf Stream, and a second stage in which a rapidly moving, moderately intense short-wave trough from the north-central states interacted with the more northerly of the two disturbances, producing rapid intensification. Maximum deepening rates were 11 mb (6 h)−1 and 33 mb (24 h)−1. At the mature stage a thermal gradient of 7°C (35 km)−1 was observed near the surface by a low-flying research aircraft that traversed the occluded frontal zone.

A full-physics simulation, carried out on a movable 30-km grid embedded within a 90-km fixed grid, closely reproduced the storm development, as verified by surface ship and buoy observations, flight level and dropsonde data from research aircraft, and satellite infrared and microwave imagery. Sensitivity tests reported in a companion paper revealed that the development was highly sensitive to condensation heating and moderately sensitive to surface energy fluxes, grid size, and the location of the Gulf Stream. The companion paper also addresses the question of why in this case the MM4 outperformed the operational models of the National Meteorological Center.

Full access
Richard J. Reed
,
Georg A. Grell
, and
Ying-Hwa Kuo

Abstract

This paper continues the study of the ERICA IOP 5 storm begun in a companion paper. The latter documented the storm development, utilizing both conventional and special observations, and presented the results of a successful simulation of the storm by the Pennsylvania State University-NCAR Mesoscale Model MM4. At 24 h into the simulation, the MM4 predicted a central pressure of 984 mb, close to the observed value, whereas the Nested Grid Model (NGM) of the National Meteorological Center forecasted a depth of only 997 mb for the same hour. Here the results of experiments designed to test the sensitivity of the development to latent heating, surface energy fluxes, Gulf Stream position, and grid size are first presented. A high sensitivity to latent heating and a moderate sensitivity to the other parameters are found. A comparison with other cases in the literature reveals that the sensitivity to latent heating, and to the fluxes, was unusually large. In view of this finding, further diagnosis is made of the behavior of a number of moisture-sensitive parameters in the model, namely, the potential vorticity (PV), the stability of the storm environment to vertical and slantwise ascent, and the surface energy fluxes. The diagnosis revealed that (i) large diabatically produced PV, capable of sub-stantially impacting the storm intensity, appeared in the lower portion of the warm-frontal cloud mass, (ii) the storm environment was neutral or oven unstable to vertical ascent (near and ahead of the cold front) and to slantwise ascent (in and above the warm-frontal zone), and (iii) the movement of the storm near and parallel to the Gulf Stream allowed heated and moistened air to be continuously ingested into the storm.

In seeking clues to the cause of the superior performance of the MM4, additional experiments are carried out in which a Kuo-type convection scheme, such as employed in the NGM, replaces the Grell scheme used in the previous MM4 simulations. It is concluded that approximately 60% of the difference between the MM4 and NGM predictions can be accounted for by the utilization of the Grell scheme and a finer grid (30 km vs 85 km) and that the effects of grid size and convective parameterization are highly coupled in this case. The remaining difference is attributed to other elements of the predictions that are not further investigated. An experiment conducted on a slowly deepening ERICA storm (IOP 7) demonstrates that, in this case at least, the MM4 shows no tendency to produce excessive deepening of ocean storms.

Full access
Richard J. Reed
,
Ying-Hwa Kuo
, and
Simon Low-Nam

Abstract

Numerical experiments with dry, inviscid models started from small normal-mode perturbations in baroclinic jet flows provide examples of ideal baroclinic cyclone development. This paper examines, with use of the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model, cyclone development under conditions that resemble the ideal experiments in lacking initial surface fronts and in neglecting latent heating and surface fluxes but that differ from most ideal experiments in including surface friction and an initial large upper-level disturbance. The initial state of the simulation is that of the intense, explosive Experiment on Rapidly Intensifying Cyclones over the Atlantic intensive observation period 4 storm. Issues highlighted are the timing and rate of deepening, the rapidity and intensity of frontal formation, the frontal structure, the airflow relative to the cyclone and fronts, and the nature of the occlusion and warm-core seclusion processes. Principal findings are as follows:

  1. The deepening rate well exceeded the common criterion for rapid deepening, and the period of most rapid deepening commenced only 3 h after the appearance of the surface low center.

  2. Warm, cold, and occluded fronts formed simultaneously and were already sharp by 9–12 h of the simulation.

  3. The warm front was ill defined above the boundary layer (900 mb).

  4. The thermal gradient in the cold frontal zone reached large values near the surface (10°C in 40 km). A plume of strong updraft (30 cm s−1) appeared above the nose of the ftont. A weakly connected middle- and upper-level frontal zone marked by elevated levels of potential vorticity (PV) also sloped rearward from the surface cold front but with a lesser inclination.

  5. Rising, or risen, warm air with low PV levels and sinking, or sunken, cold air with high PV levels were juxtaposed along the occluded front at the mature stage. The near-surface warm-sector air converged on the triple point and ascended above the occluded front, primarily on the forward side.

  6. The motion relative to the cyclone consisted of two basic flows: an ascending warm flow that, depending on point of origin, spread either anticyclonically downstream or cyclonically upstream, and a corresponding descending cold flow that near the low center intertwined with the cyclonic branch of the warm flow. The flow pattern can be crudely likened to that of two interlocking fans.

  7. On the basis of the fully resolved instantaneous relative motions in the vicinity of the occluded front, the occlusion process, after frontal formation, can be described as a motion of the cold front forward along the warm front with the segment of the warm front adjacent to the triple point being transformed into an occluded front.

  8. The warm-core seclusion formed at the tip of the occluded front shortly after its appearance and was subsequently collocated with a pool of large vorticity that broke off from the strip of intense vorticity that lay along the occluded front. The warm pocket and vorticity maximum were carried along together in the flow for a period of at least 21 h.

  9. Diffusive processes in the model were essential to the maintenance of the observed nearly steady-state frontal structures. At low levels (850 mb), on the poleward edge of the occluded front, the diffusion generated a substantial amount of potential vorticity with a peak value of about 5 PVU.

Full access
Ying-Hwa Kuo
,
Richard J. Reed
, and
Yubao Liu

Abstract

Modeling studies have consistently shown the importance of latent heat release in explosive marine cyclogenesis. However, a systematic evaluation of precipitation parameterization in the simulation of marine cyclones has been rare in the literature. This paper is the third in a series of modeling studies on the ERICA IOP 5 storm. The objective is to assess the performance of various subgrid-scale cumulus parameterization and resolvable-scale microphysics schemes in the simulation of the storm using the Penn State–NCAR mesoscale model MM5 at grid resolutions of 20 and 60 km. Emphasis is placed on the intensity, distribution, and character of precipitation and on the mesoscale low pressure centers embedded within the synoptic-scale cyclone. Principal findings are as follows.

  • The distribution and intensity of precipitation, its partitioning into grid-resolvable and subgrid-scale portions, the atmospheric thermodynamic structure in the precipitation region, and the evolution of mesoscale low pressure centers were extremely sensitive to the choice of cumulus parameterization scheme. This is true for both the 20- and 60-km MM5.

  • The partitioning of precipitation into subgrid scale and resolvable scale for a given convective parameterization is nearly the same for both the 20- and 60-km models.

  • The detailed cold-cloud microphysics did not have a significant impact on cyclone deepening for tests carried out on the 20-km grid.

  • The CAPE-based scheme developed by Kain and Fritsch gave the best simulation of this explosive marine cyclone on both the 60- and 20-km grids. This scheme effectively consumed the convective instability and captured the evolution of two observed mesolows.

  • Analysis of the simulated mesolows showed that rapid intensification took place just in advance of a strong upper-level PV maximum. At low levels, the mesolows were characterized by large, diabatically produced PV maxima and by nearly coincident rainfall maxima.

  • A number of PV-rainfall maxima, less visible in the pressure field, with a spacing of 150 km along the cold front, were also simulated in the 20-km Kain–Fritsch experiment. The realism of these PV-rainfall maxima cannot be confirmed due to the lack of observations. Similar features with enhanced pressure signature were seen in the simulations with the Grell scheme and no-cumulus scheme.

  • The number, intensity, and evolution of mesolows were highly variable in the 20-km simulations of this case with different convective parameterization schemes. The amount of mesoscale perturbation of the pressure fields appears to be inversely proportional to the percentage of convective rainfall, being the least in the Kuo scheme (with 90% convective rainfall) and the most in the scheme without subgrid-scale convective parameterization (0%). No mesolows were simulated in an experiment that excluded latent heat release.

Full access
Richard J. Reed
,
John L. Wolfe
, and
Hiroshi Nishimoto

Abstract

The spectral forms of tile energy equations for zonal and eddy kinetic energies and zonal and eddy available-potential energies are used to measure energy changes and energy conversions at 50 mb during the period 25 January–9 February 1957. The warming, which was of the bipolar type, could he divided into two phases, a first phase in which the meridional temperature gradient had its usual poleward direction at high latitudes and a second phase in which the gradient was reversed.

During the first, or amplifying phase, the eddy energy increased and the energy of the zonal flow decreased, the decrease in zonal kinetic energy outweighing that of zonal available potential energy. The energy flow corresponded to that of a baroclinic instability. Zonal available-potential energy was transformed to eddy available-potential energy, eddy available-potential energy to eddy kinetic energy, and eddy kinetic energy to zonal kinetic energy. Indirect meridional circulations with descending motion in the middle latitude warm belt and ascending motions in the tropics and at high latitudes transformed zonal kinetic energy to zonal available-potential energy. During the second stage the eddy energy diminished and the zonal available potential energy increased. The energy flow reversed except that eddy kinetic energy continued to be converted to zonal kinetic energy. Despite this conversion the zonal kinetic energy continued to decline, presumably because of energy transfer to the troposphere.

An attempt is made to infer the normal energy regime of the winter stratosphere. It is hypothesized that in the lower stratosphere the energy store is maintained against the depleting effect of radiation by an upward flux of ultra-long-wave energy from the troposphere. Warmings of the type studied here may represent a baroclinic amplification of the presumably topographically-induced long waves. However, there are facets of the warming which make it unlikely that it corresponds to previously studied baroclinic instabilities. In the relatively small stratospheric mass remaining above 30 mb, zonal available-potential energy is generated radiatively, and a different energy regime must exist.

Full access
Ying-Hwa Kuo
,
Simon Low-Nam
, and
Richard J. Reed

Abstract

A series of eight numerical experiments were conducted on seven cases of explosive marine cyclogenesis, using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. The main objective was to elucidate the role of surface energy fluxes both during and preceding rapid deepening. Results from the 24-h experiments initialized at the commencement of the period of most rapid deepening showed that the fluxes occurring during this stage had essentially no effect on the deepening rate. However, substantial effects of the fluxes were found in 48-h experiments initialized early in the development. When the fluxes were withheld from the 48-h simulations, the predicted cyclones were weakened on average by 7.1 mb at 24 hours and 13.5 mb at 48 hours. Evidently the fluxes occurring during the 24 hours preceding rapid deepening did affect the storm development throughout its lifetime. Additional 48-h experiments confirmed the relatively small impact of the concurrent fluxes during the rapid development stage, as found in the 24-h experiments.

Detailed analysis was performed on a storm that was representative of the sample as a whole. In the early development stage, when the cyclone was located over the warm ocean in the vicinity of the GulfStream, strong energy fluxes occurred near the low center. The low-level heating and moistening caused by the fluxes contributed to the development of a coastal front and significantly reduced the atmospheric stability in and near the frontal region. The frontogenetical action and stability reduction resulted in greatly increased precipitation amounts near the storm center. The incipient cyclone was 11 mb deeper in a 24-h full-physics simulation than in a simulation without fluxes.

During the later stage of the development, the cyclone crossed the GulfStream and moved over colder waters. The surface energy fluxes in the vicinity of the storm then weakened and even reversed direction, while large upward fluxes persisted well to the rear of the cyclone. This pattern of low-level heating and cooling was not conducive to storm intensification. However, the latent beat released in precipitation of moisture supplied by earlier fluxes did contribute to further deepening. To fully explain the deepening rates at different stages of development, one needs to consider both the effects of the fluxes occurring at a given stage and the delayed effects of earlier fluxes.

Full access
Richard J. Reed
,
Mark T. Stoelinga
, and
Ying-Hwa Kuo

Abstract

High spatial and temporal resolution fields generated by a mesoscale prediction model are used to study a case of rapid marine cyclogenesis (26 mb in 12 h) within the context of potential vorticity (PV) thinking. The case, which occurred on 23 February 1987 near the east coast of the United States, was well simulated by the model, as was verified by ship reports and satellite imagery. Three components of the cyclone were investigated: 1) a surface thermal anomaly (or surrogate PV anomaly), 2) a low-level, diabatically produced PV anomaly in and near the frontal cloud band, and 3) a dry PV anomaly of upper-tropospheric and lower-stratosphoric origin.

Trajectory tracing revealed that the surface thermal anomaly was caused primarily by northward transport of warm air into the central region of the low. Although surface sensible heat flux also contributed to the warm anomaly, a sensitivity experiment, run with the flux withheld, showed that it had little impact on the depth of the storm. Furthermore, it was found from trajectory tracing that the region of positive anomaly in the frontal cloud mass formed with extreme rapidity as air from the boundary layer ascended the warm frontal surface and entered the cloud. Differential condensational heating in the vertical was likely responsible for much of the sudden increase. Some of the diabatically produced PV was evident in the wake of the storm, after emerging from the cloud. Finally, from examination of a large number of trajectories, it was found that the anomalous PV of high-level origin had a complex behavior as it approached the low from upstream. Initially the anomalous PV was contained in a tropopause depression and in a fold that protruded downward along the south flank of the depression. Stratospheric air in the depression subsided to mid-to upper levels and advanced directly on the low center. Some of the air in the tropopause fold followed a cyclonically curved trajectory, first subsiding and then rising to midlevels as it neared the low. Other air in the fold sank to low levels along an anticyclonically curved path and was left behind. At 18 h into the development, the low-level, diabatically produced PV, the midlevel PV from the tropopause fold, and the upper-level PV from the tropopause depression were aligned vertically in a column located 100 km upstream of the surface-low center.

Full access
Ying-Hwa Kuo
,
Richard J. Reed
, and
Simon Low-Nam

Abstract

A very fine mesh model simulation of the Ocean Ranger storm of February 1982 is used to study the thermal structure and airflow in an intense marine cyclone. In particular, the study investigates the structures of the occluded front and the secluded pool of warm air in the simulated cyclone and examines the formation of these structures with the help of a large number of air trajectories. It was found that in the model simulation the largest thermal gradient occurred along the occluded front, not along the warm or cold fronts, and that the intense gradient was a product of strong warm and occludofrontogenesis in the inflowing air. The occluded front was embedded within air that was earlier located within the baroclinic zone ahead of the low center, not air that was in proximity to the historical warm and cold fronts. The warm air in the seclusion likewise originated in the baroclinic zone ahead of the low. The seclusion at low levels resulted from a tongue of slower moving, relatively warm air being pinched off by colder, more rapidly moving air that circulated about the low center from front to rear.

Major features of the simulated airflow were 1) a cloud-producing flow that rose from the downstream boundary layer and spread out in a fan-shaped pattern, forming the familiar book-shaped cloud signature of the mature cyclone, and 2) an upper-level dry airstream that advanced on the system from the west (upstream), producing the dry slot commonly seen in satellite imagery over and ahead of the surface occlusion. Air in the dry slot amended rapidly over the occluded front but remained cloud-free because of earlier upstream subsidence and drying. The seclusion of warm air at middle levels was a consequence of a tongue of warm, but not tropical, air being pinched off by colder air from both the front and rear of the system.

Detailed feature of the simulated storm that differed from the classical occlusion model were 1) the sharpness of the thermal gradient along the inner part of the occluded front, 2) the presence of a secluded pocket of warm air near the tail end of the occluded front, 3) the formation of the occluded front by frontogenesis in the baroclinic air mass ahead of the low rather than by a joining of the initial warm and cold fronts, 4) the trans-formation of the low-level occluded front to a warm front aloft without the existence of an intervening cold front, and 5) the lack of mid- and upper-level cloud above the occluded-warm front along much of its length. The fundamental occlusion process described in the Norwegian model, however, was verified. Warm air of widely ranging temperature was squeezed aloft by the sinking and spreading of colder air that encircled the low center.

Full access