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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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Keith F. Brill
,
Louis W. Uccellini
,
Richard P. Burkhart
,
Thomas T. Warner
, and
Richard A. Anthes

Abstract

Uccellini and Johnson present a case study of a severe weather event in Ohio on 10–11 May 1973 to show evidence for coupling between an upper-tropospheric jet streak and a low-level jet within an indirect transverse circulation found in the exit region of the upper-level jet. The differential advection of moisture and temperature created by the shear between the upper- and low-level jets reduced convective stability, thereby enhancing the potential for severe convection.

Two 12 h numerical simulations of the 10–11 May 1973 case are studied to determine 1) if a transverse indirect circulation with a low-level jet imbedded in its lower branch can be diagnosed in the exit region of the upper-level jet and studied using the model output at 3 h intervals and 2) if the initial magnitude and structure of the upper-level jet have a significant effect on the subsequent development of the low-level jet and the decrease in convectivc stability due to differential advection. In an adiabatic model simulation, an indirect transverse circulation having a low-level jet within its lower branch occurs in the exit region of the upper-level jet. The simulated vertical distribution of mass divergence and ageostrophic flow in the exit region agree with the diagnoses of Uccellini and Johnson. At upper levels, mass divergence (convergence) occurs on the cyclonic (anticyclonic) side of the exit region, while the opposite occurs at low levels. The, upper branch of the indirect circulation is dominated by the inertial–advective contribution to the ageostrophic wind which is related to the alongstream isotach gradient in the exit region. The lower branch is dominated by the wind tendency contribution to the ageostrophic wind. Ageostrophic shear associated with this circulation contributes to the development of differential moisture and temperature advection, which act to destabilize the preconvective environment.

A second simulation using a smoothed, nondivergent initialization with a weaker upper-level jet streak and weaker alongside isotach gradient in the exit region of the upper-level jet produces a weaker indirect transverse circulation even though diabatic heating effects are present. The indirect circulation for this simulation is marked by smaller vertical motions, a weaker low-level return branch, and weaker low-level thermal and moisture advection associated with the low-level flow. Comparison of the two simulations suggests that the indirect circulation in the exit region of the upper-level jet is strongly responsive to dynamical processes associated with the initial structure of the jet streak.

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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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Norman W. Junker
,
James E. Hoke
,
Bruce E. Sullivan
,
Keith F. Brill
, and
Francis J. Hughes

Abstract

This paper assesses the performance of the National Meteorological Center (NMC) Nested-Grid Model (NGM) during a period from March 1988 through March 1990, and the NMC medium-range forecast model (MRF) in two 136-day tests, one during summer made up of two 68-day periods (19 July–25 September 1989 and 20 June–28 August 1990) and one during winter and early spring (12 December 1989–26 April 1990). Seasonal and geographical variations of precipitation bias and threat score are discussed for each model. Differences in model performance in predicting various amounts of precipitation are described.

The performance of the NGM and MRF varied by season, geographic area, and precipitation amount. The bias of the models varied significantly during the year. The NGM and MRF overpredicted the frequency of measurable precipitation (≥0.01 in.) across much of the eastern half of the United States during the warm season. Both models, however, underpredicted the frequency of ≥0.50-in. amounts across the South during the cool season.

The smooth orography in both models has a strong impact on the models’ precipitation forecasts. Each model overpredicted the frequency of heavier precipitation over the southern Appalachians, over portions of the Gulf-facing upslope areas east of the Rocky Mountains, and to the lee of the Cascade and Sierra ranges of the West. The NGM underpredicted the frequency of heavier amounts on the Pacific-facing windward side of the Cascade Range of Oregon and Washington.

Model performance also seems to be related to the synoptic situation. Threat scores were higher when the midlevel westerlies were more active, with the highest threat scores found north of the most frequent track of cyclones during the cool season.

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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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David R. Novak
,
Christopher Bailey
,
Keith F. Brill
,
Patrick Burke
,
Wallace A. Hogsett
,
Robert Rausch
, and
Michael Schichtel

Abstract

The role of the human forecaster in improving upon the accuracy of numerical weather prediction is explored using multiyear verification of human-generated short-range precipitation forecasts and medium-range maximum temperature forecasts from the Weather Prediction Center (WPC). Results show that human-generated forecasts improve over raw deterministic model guidance. Over the past two decades, WPC human forecasters achieved a 20%–40% improvement over the North American Mesoscale (NAM) model and the Global Forecast System (GFS) for the 1 in. (25.4 mm) (24 h)−1 threshold for day 1 precipitation forecasts, with a smaller, but statistically significant, 5%–15% improvement over the deterministic ECMWF model. Medium-range maximum temperature forecasts also exhibit statistically significant improvement over GFS model output statistics (MOS), and the improvement has been increasing over the past 5 yr. The quality added by humans for forecasts of high-impact events varies by element and forecast projection, with generally large improvements when the forecaster makes changes ≥8°F (4.4°C) to MOS temperatures. Human improvement over guidance for extreme rainfall events [3 in. (76.2 mm) (24 h)−1] is largest in the short-range forecast. However, human-generated forecasts failed to outperform the most skillful downscaled, bias-corrected ensemble guidance for precipitation and maximum temperature available near the same time as the human-modified forecasts. Thus, as additional downscaled and bias-corrected sensible weather element guidance becomes operationally available, and with the support of near-real-time verification, forecaster training, and tools to guide forecaster interventions, a key test is whether forecasters can learn to make statistically significant improvements over the most skillful of this guidance. Such a test can inform to what degree, and just how quickly, the role of the forecaster changes.

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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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Louis W. Uccellini
,
Keith F. Brill
,
Ralph A. Petersen
,
Daniel Keyser
,
Robert Aune
,
Paul J. Kocin
, and
Mary des Jardins

The synoptic-scale weather conditions preceding and following the ill-fated Space Shuttle Challenger launch are documented, with particular emphasis on the upper-level winds for central and northern Florida. Operational radiosonde data collected by the National Weather Service, visible and infrared imagery from the Geostationary Operational Environmental Satellite, and water-vapor imagery from the VISSR (Visible Infrared Spin Scan Radiometer) Atmospheric Sounder, ozone data collected by the Total Ozone Mapping Spectrometer aboard the Nimbus-7, and soundings collected at Cape Canaveral (XMR) are described. Analyses derived from these data sets point to the juxtaposition of two distinct jet-stream systems (a polar-front jet [PFJ] and a subtropical jet [STJ]) over north-central Florida on the morning of the launch. Both jets were characterized by regions of significant vertical wind shear, which was especially strong above and below the core of the STJ.

Data from a radiosonde released at Cape Canaveral 10 min after the shuttle accident combined with radiosonde and jimsphere wind measurements before the shuttle launch reveal that, over XMR, the magnitude of the maximum wind in the PFJ was increasing with time while the magnitude of the STJ was decreasing. Even with the decreasing magnitude of wind speeds in the core of the STJ over XMR, large vertical wind shears and low Richardson numbers were still diagnosed near the PFJ and beneath the core of the STJ at the time of launch (1639 GMT). The low Richardson numbers associated with the presence of vertical wind shear indicate that conditions were favorable for shear-induced turbulence at the time of the shuttle explosion.

The results from the analyses of the synoptic radiosonde data are inconclusive due to the poor temporal and horizontal spatial resolution of the observational data base and the large number of missing data reports at numerous stations in the southeastern United States (including XMR). In an attempt to overcome this deficiency, numerical simulations of the atmospheric conditions were conducted using a mesoscale numerical model. The simulations initialized at 1200 GMT 28 January confirm the juxtaposition of two distinct jet systems over north-central Florida at the time of the shuttle launch and the presence of large vertical wind shears and low Richardson numbers associated with these jets.

Given the rapid temporal evolution of atmospheric flow regimes which involve strong wind shears, we recommend that consideration should be given to 1) augmenting the observations (both in time and space) upstream and around the Cape Canaveral launch facility, 2) enhancing the analysis and display capabilities of these data, and 3) using numerical-model output to provide the best possible diagnosis and forecast of the meteorological conditions for future shuttle launches.

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