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Keith F. Brill

Abstract

The gradient wind is defined as a horizontal wind having the same direction as the geostrophic wind but with a magnitude consistent with a balance of three forces: the pressure gradient force, the Coriolis force, and the centrifugal force arising from the curvature of a parcel trajectory. This definition is not sufficient to establish a single way of computing the gradient wind. Different results arise depending upon what is taken to be the parcel trajectory and its curvature. To clarify these distinctions, contour and natural gradient winds are defined and subdivided into steady and nonsteady cases. Contour gradient winds are based only on the geostrophic streamfunction. Natural gradient winds are obtained using the actual wind. Even in cases for which the wind field is available along with the geostrophic streamfunction, it may be useful to obtain the gradient wind for comparison to the existing analyzed or forecast wind or as a force-balanced reference state. It is shown that the nonanomalous (normal) solution in the case of nonsteady natural gradient wind serves as an upper bound for the actual wind speed. Otherwise, supergradient wind speeds are possible, meaning that a contour gradient wind or the steady natural gradient wind used as an approximation for an actual wind may not be capable of representing the full range of actual wind magnitude.

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Jeffrey S. Whitaker
,
Louis W. Uccellini
, and
Keith F. Brill

Abstract

A model simulation of the rapid development phase of the Presidents' Day cyclone of 19 February 1979 is analyzed in an effort to complement and extend a diagnostic analysis based only on 12-h radiosonde data over the contiguous United States, with a large data-void area over the Atlantic Ocean (Uccellini et al. 1985). As indicated by the SLP and 850 mb absolute vorticity tendencies, rapid cyclogenesis commences between 0300 and 0600 UTC 19 February and proceeds through the remaining 18 h of the simulation. This rapid development phase occurs as stratospheric air [marked by high values of potential vorticity (PV) approaches and subsequently overlies a separate, lower-tropospheric PV maximum confined to the Fast Coast, or during the period when the advection of PV increases in the middle to upper troposphere over the East Coast. The onset of rapid deepening is marked by 1) the transition in the mass divergence profiles over the surface low from a diffuse pattern with two or three divergence maxima to a two-layer structure, with maximum divergence located near 500 mb and the level of nondivergence located new 700 mb., 2) the intensification of precipitation just north of the surface low pressure system., and 3) an abrupt increase in the low-level vorticity.

Model trajectories and Eulerian analyses indicate that three airstreams converge into the cyclogenetic region during the rapid development phase. One of these airstreams descends within a tropopause fold on the west side of an upper-level trough over the north-central United States on 18 February and approaches the cyclone from the west-southwest as the rapid development commences. A second airstream originates in a region of lower-tropospheric subsidence within the cold anticyclone north of the storm, follows an anticyclonically curved path at low levels over the ocean, and then ascends as it enters the storm from the east. A third airstream approaches the storm from the south at low levels and also ascends as it enters the storm circulation. All of the airstreams pan through the low-level PV maximum as they approach the storm system, with the PV increase following a parcel related to the vertical distribution of θ due to the release of latent heat near the coastal region.

A vorticity analysis shows that absolute vorticity associated with the simulated storm is realized primarily through vortex stretching associated with the convergence of the airstreams below the 700 mb level. Although the maximum vorticity is initially confined below the 700 mb level, the convergence of the various airstreams is shown to be directly related to dynamic and physical processes that extend throughout the entire troposphere. Finally, the divergence of these airstreams within the 700 to 500 mb layer increases the magnitude of the mass divergence just north and cast of the storm center and thus enhances the rapid deepening of the surface low as measured by the decreasing sea level pressure.

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Mukut B. Mathur
,
Keith F. Brill
, and
Charles J. Seman

Abstract

Numerical forecasts from the National Centers for Environmental Prediction’s mesoscale version of the η coordinate–based model, hereafter referred to as MESO, have been analyzed to study the roles of conditional symmetric instability (CSI) and frontogenesis in copious precipitation events. A grid spacing of 29 km and 50 layers are used in the MESO model. Parameterized convective and resolvable-scale condensation, radiation physics, and many other physical processes are included. Results focus on a 24-h forecast from 1500 UTC 1 February 1996 in the region of a low-level front and associated deep baroclinic zone over the southeastern United States. Predicted precipitation amounts were close to the observed, and the rainfall in the model was mainly associated with the resolvable-scale condensation.

During the forecast deep upward motion amplifies in a band oriented west-southwest to east-northeast, nearly parallel to the mean tropospheric thermal wind. This band develops from a sloping updraft in the low-level nearly saturated frontal zone, which is absolutely stable to upright convection, but susceptible to CSI. The updraft is then nearly vertical in the middle troposphere where there is very weak conditional instability. We regard this occurrence as an example of model-produced deep slantwise convection (SWC). Negative values of moist potential vorticity (MPV) occur over the entire low-level SWC area initially. The vertical extent of SWC increases with the lifting upward of the negative MPV area. Characteristic features of CSI and SWC simulated in some high-resolution nonhydrostatic cloud models also develop within the MESO. As in the nonhydrostatic SWC, the vertical momentum transport in the MESO updraft generates a subgeostrophic momentum anomaly aloft, with negative absolute vorticity on the baroclinically cool side of the momentum anomaly where outflow winds are accelerated to the north.

Contribution of various processes to frontogenesis in the SWC area is investigated. The development of indirect circulation leads to low-level frontogenesis through the tilting term. The axis of frontogenesis nearly coincides with the axis of maximum vertical motion when the SWC is fully developed. Results suggest that strong vertical motions in the case investigated develop due to release of symmetric instability in a moist atmosphere (CSI), and resultant circulations lead to weak frontogenesis in the SWC area.

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Louis W. Uccellini
,
Paul J. Kocin
,
Ralph A. Petersen
,
Carlyle H. Wash
, and
Keith F. Brill

Abstract

The Presidents' Day cyclone of 18–19 February 1979 was an intense and rapidly developing storm which produced heavy snowfall along the East Coast of the United States. An analysis of the cyclone is presented which isolates three jet streaks that appear to have played important roles in the development of two separate areas of heavy snow. One area of heavy snow developed prior to cyclogenesis and is linked, in part, to an increasingly unbalanced subtropical jet streak (STJ) and a noticeably ageostrophic low-level jet. The second area of heavy snow developed in conjunction with the explosive cyclogenesis off the East Coast as a polar jet streak and midtropospheric trough propagated toward the coastal region from the north-central United States.

This paper examines the STJ in detail. The maximum wind speeds associated with the STJ increased by 15 to 20 m s-1 between 1200 GMT 17 and 1200 GMT 18 February 1979 as the jet propagated from the south-central toward the eastern United States. During the 24 h period, the flow in the STJ became increasingly supergeostrophic and apparently unbalanced. Ageostrophic wind speeds increased to greater than 30 m s-1, with a significant cross-contour component directed toward lower values of the Montgomery streamfunction, as the flow along the STJ became increasingly divergent with time. The increased wind speed, ageostrophic flow, and divergence along the axis of the STJ are linked to the increasing confluence in the entrance region of the jet streak and the decreasing wavelength of the trough-ridge system in which the jet streak was embedded. The upper level divergence and upward vertical motion near the axis of the STJ along with the moisture transport associated with the LLJ are found to be important factors in the development of the first area of heavy snow.

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Louis W. Uccellini
,
Ralph A. Petersen
,
Paul J. Kocin
,
Keith F. Brill
, and
James J. Tuccillo

Abstract

A series of numerical simulations is presented for the February 1979 Presidents' Day cyclone in order to understand more fully the roles played by upper-level jet streaks the oceanic planetary boundary layer (PBL), and latent heat release in the development of a low-level jet (LU) and secondary cyclogenesis along the East Coast of the United States. Mesoscale model simulations with and without sensible and latent heating show that the diabatic processes, along with the jet streak circulation patterns, contribute to the enhancement of the low-level winds and the initial development of the coastal cyclone. However. none of the mechanisms acting alone is sufficient to yield a satisfactory simulation of the LIJ and secondary cyclogenesis. Furthermore, the model-based diagnostic analyses indicate that a synergistic interaction must exist among these processes to account for the substantial increase in the magnitude of the low-level winds and the decrease in the sea level pressure that mark the secondary cyclogenesis for this case.

The following sequence is derived from the model diagnostic study: 1) Temporally increasing divergence along the axis of an upper-tropospheric jet streak located near the crest of an upper-level ridge is associated with the development of an indirect circulation that spans the entire depth of the troposphere and is displaced to the anticyclonic side of the jet. The lower branch of the indirect circulation appears to extend northwestward from the oceanic PBL up sloping isentropic surfaces toward 700 mb over the Appalachian Mountains. 2) Sensible heating and associated moisture flux within the oceanic PBL warm and moisten the lower branch of the indirect circulation, enhancing precipitation rates and latent heat release west of the coastline. 3) The combination of a shallow direct circulation associated with a developing coastal front, sloping lower-tropospheric isentropic surfaces just to the west of the coastline, and latent heat release contributes to a vertical displacement of parcels within the lower branch of the indirect circulation as they cross the coastline. 4) The vertical displacement of the parcels in a baroclinic environment (in which the pressure gradient force changes with height) results in the rapid increase in the magnitude of the ageostrophic wind and associated unbalanced flow. This imbalance contributes to parcel acceleration resulting in the formation of a LLJ in the lower branch of the indirect circulation over a 2 to 4 h period. 5) The increasing wind speed associated with the developing LLJ is, in turn, responsible for an increase in the horizontal mass flux divergence in the entrance region of the LLJ. The increase in the mass flux divergence in the lower troposphere just above the boundary layer makes a significant contribution to the decreasing sea-level pressure that constitutes the initial development phase of the secondary cyclone along the coast.

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John Manobianco
,
Louis W. Uccellini
,
Keith F. Brill
, and
Ying-Hwa Kuo

Abstract

The rapid intensification of a surface cyclone that battered the Queen Elizabeth II (QE II) ocean liner in the western Atlantic Ocean during September 1978 has been the focus of several observational and model-based case studies. The storm is considered a classic example of a cyclone that undergoes explosive deepening, marked by a 60-hPa decrease of the central mean sea level pressure (MSLP) in 24 h.

The present study uses a regional-scale numerical model in conjunction with dynamic data assimilation via Newtonian relaxation (or “nudging”) to provide initial conditions for subsequent simulations of the QE II cyclone. The objectives of this paper are 1) to show that the simulations initialized from the results of 12-h precyclogenetic data-assimilation cycles (with and without bogus data) are superior to those initialized statically from the same data and 2) to resolve the evolution of the upper-level trough-jet system in the 24-h period from 0000 UTC 9 September–0000 UTC 10 September using the dynamically consistent four-dimensional (4D) datasets generated by the model.

The 4D model-generated datasets provide the spatial and temporal data resolution not afforded in the observational studies to document the structure and evolution of the dynamical forcing associated with the QE II cyclone. However, the temporal continuity of the cyclone's development, especially the evolution of the upper-level trough-jet system and the associated indirect circulations in the exit region of the upper-tropospheric jet streak, is interrupted at the end of the nudging cycle. This problem poses a limitation for using the 4D datasets for diagnostic studies of the QE II cyclone in the precyclogenetic period during the data-assimilation cycle.

Dynamic data assimilation and the inclusion of supplementary data both have a large positive impact on the simulated position and intensity of the QE II cyclone from 1200 UTC 9 September to 0000 UTC 10 September during the initial phase of rapid cyclone development. These runs capture the developing cyclone and associated rate of MSLP falls at 1200 UTC 9 September, whereas the runs based on static initialization delay the deepening six to nine hours into the model simulation. The diagnostic analyses based on these simulations show that the initial development of the QE II storm between 0000 UTC 9 September and 0000 UTC 10 September was embedded within an indirect circulation of an intense 300-hPa jet streak, was related to baroclinic processes that extended throughout a deep portion of the troposphere and was associated with a classic two-layer mass-divergence profile expected for an extratropical cyclone.

The runs initialized from data-assimilation cycles, including the bogus data, still underestimate the MSLP of the QE II cyclone by 30% at 24 h into the simulations (1200 UTC 10 September). These results provide further supporting evidence that increasing the horizontal model resolution and improving the subgrid-scale physical parameterizations (especially the precipitation schemes) may be required to simulate the most rapid development phase of the QE II cyclone.

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Louis W. Uccellini
,
Daniel Keyser
,
Keith F. Brill
, and
Carlyle H. Wash

Abstract

A diagnostic analysis of an amplifying polar jet-trough system and associated tropopause fold which preceded the 19 February 1979 Presidents' Day cyclone is presented. The analysis is based on conventional radiosonde data, infrared and visible satellite imagery, 6.7 μm water vapor measurements from the Temperature-Humidity Infrared Radiometer (THIR), and ozone measurements from the Total Ozone Mapping Spectrometer (TOMS). The case study indicates that dynamically forced meso-α scale vertical circulations played an important role in the extrusion of stratospheric air along the axis of a polar jet and the subsequent development of the storm system. Specific findings include: 1) Tropopause folding accompanying an amplifying polar jet–trough system occurred along the axis of the intensifying polar jet in response to subsidence forced by geostrophic deformation patterns associated with the jet streak. 2) The folding process extruded dry stratosphere air marked by high values of potential vorticity down toward the 700 mb level, 1500 km upstream of the East Coast, 12 to 24 h prior to the explosive development phase of the cyclone. This result differs from previous case studies which have emphasized the concurrent development of a folded tropopause and cyclogenesis. 3) During the 12 h preceding rapid cyclogenesis, the stratospheric air descended toward the 800 mb level and moved toward the East Coast to a position just upstream of the area in which rapid cyclogenesis occurred. Even though potential vorticity was not strictly conserved, the absolute vorticity increased in the lower to middle troposphere in association with adiabatic mass convergence, vertical stretching and the related decrease of static stability of the air mass originating in the stratosphere. 4) As was inferred from the infrared and visible satellite imagery and the ozone measurements, the stratospheric air mass was nearly colocated with the storm center as explosive deepening and vortex development occurred, suggesting that the explosive development of the cyclone was likely influenced by the stratospheric air mass as it descended toward a deep oceanic planetary boundary layer immediately off the East Coast.

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Steven E. Koch
,
William C. Skillman
,
Paul J. Kocin
,
Peter J. Wetzel
,
Keith F. Brill
,
Dennis A. Keyser
, and
Michael C. McCumber

Abstract

A large number of predictions from a regional numerical weather prediction model known as the Mesoscale Atmospheric Simulation System (MASS 2.0) am verified against routinely collected observations to determine the model's predictive skill and its most important systematic errors at the synoptic scale. The model's forecast fields are smoothed to obtain synoptic-scale fields that can be compared objectively with the observation. A total of 23 (28) separate 12 h (24 h) forecasts of atmospheric flow patterns over the United States are evaluated from real-time simulations made during the period 2 April-2 July 1982. The model's performance is compared to that of the National Meteorological Centers operational Limited-area Fine Mesh (LFM) model for this period. Temporal variations in normalized forecast skill statistics are synthesized with the mean spatial distribution of daily model forecast errors in order to determine synoptic-scale systematic errors.

The mesoscale model produces synoptic-scale forecasts at an overall level of performance equivalent to that of the LFM model. Lower tropospheric mass fields are, for the most part, predicted significantly better by the MASS 2.0 model, but it is outperformed by the LFM at and above 500 mb. The greatest improvement made by the mesoscale model is a 73% reduction of cold bias in LFM forecasts of the 1000–500 mb thickness field, primarily over the western United States. The LFM bias is the combined result of model overforecasts of surface anticyclone intensity and underforecasts of surface cyclone intensity and nearby 500 mb geopotential heights.

The poorer forecasts by the MASS 2.0 model in the middle and upper troposphere result primarily from a systematic mass loss which occurs only under a certain synoptic flow pattern termed the mass loss regime. Problems with specification of the lateral boundary conditions and, to a lesser extent, erroneous computation of the map factor seemed to contribute most to the systematic mass loss. This error is very significant since MASS 2.0 performance either equaled or surpassed that of the LFM model in forecasts of virtually every meteorological field studied when mass loss regime days were excluded from the sample.

Two other important systematic errors in MASS model forecasts are investigated. Underforecasts of moisture over the Gulf Coast states are found to be due in large part to a negative bias in the moisture initialization. Also, overforecasts of surface cyclone intensity and 1000–500 mb thickness values over the Plains states are traced to excessive latent beating resulting from the absence of a cumulus parameterization scheme in the model. Awareness of these synoptic-scale forecasts errors enables more effective use to be made of the (unfiltered) mesoscale forecast fields, which are evaluated in the companion paper by Koch.

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