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Paul S. Schopf and Andrew Loughe

Abstract

A global isopycnal ocean model is presented for the study of interannual to interdecadal variability in the global ocean. The model treats the primitive equations on a sphere with a generalized vertical coordinate. This coordinate is designed to represent a turbulent well-mixed surface layer and nearly isopycnal deeper layers. Disappearing isopycnics are treated through the quasi-isopycnal technique, in which the coordinate separates from the isopycnic in order to maintain a minimum layer thickness. A reduced gravity treatment is made, with the deepest interface at a mean depth of 2300 m. Coastal topography is represented, but the reduced gravity treatment precludes the use of variable bottom depth. The model is used for hindcast studies of El Niño during the decade from 1982 through 1991 using a combination of climatological wind forcing and wind anomalies derived from various sources. In order to carry out the hindcast experiments, a technique is developed for constructing a mean climatological surface heat flux using the model, climatological wind forcing, and climatological surface temperatures. In the hincast runs, the climatological winds and heat flux are augmented by the wind anomalies and a weak damping of surface temperature anomalies. A series of tests compares different data products for the wind anomalies. The first product is obtained from the Florida State University (FSU) wind analysis. The second and third wind products are obtained from global climate GCM simulations run over observed sea surface temperatures (SST). Although the wind products appear quite similar, the model results show large differences in hindcast skill, reflecting the fact that subtle features of the winds can have large impacts on ocean simulations and can be seen as a primary cause of wide differences in coupled GCM performance. The model maintains a sharp thermocline and a strong equatorial undercurrent in the center of the ocean basin. The heat flux needed to keep the model near the observed temperatures appears consistent with observational studies of the mean heat flux. When measured in terms of the skill in simulating the Niño-3 SST, the NASA Coupled Climate Dynamics Group (CCDG) model and FSU wind products provide the highest skill.

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Jeffrey S. Whitaker and Andrew F. Loughe

Abstract

Statistical considerations suggest that 1) even for a perfect ensemble (one in which all sources of forecast error are sampled correctly) there need not be a high correlation between spread and skill, 2) the correlation between spread and skill should be larger where the day-to-day variability of spread is large, and 3) the spread is likely to be most useful as a predictor of skill when it is “extreme,” that is, when it is either very large or very small compared to its climatological mean value. The authors investigate the relationship between spread and skill in an operational setting by analyzing ensemble predictions produced by the National Centers for Environmental Prediction. The geographical dependence of the spread–skill relationship is found to be related to the geographical dependence of day-to-day variability of spread. Dynamical mechanisms for spread variability are investigated using a linear quasigeostrophic model. Problems associated with the sample size needed to define what constitutes an extreme value of spread at a given location are discussed.

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Andrew F. Loughe, Chung-Chieng Lai, and Daniel Keyser

Abstract

The methodology developed by Keyser et al. for representing and diagnosing three-dimensional vertical circulations in baroclinic disturbances using a two-dimensional vector streamfunction, referred to as the psi vector, is restricted to f-plane channel-model geometry. The vertical circulation described by the psi vector consists of the irrotational (or divergent) part of the ageostrophic wind and the vertical velocity. A key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations, allowing separation of a three-dimensional vertical circulation into two two-dimensional components, and thus objective assessment of the extent to which a three-dimensional vertical circulation is oriented in a preferred direction. Here the methodology for determining the psi vector is modified to be suitable for real-data applications. The modifications consists of reformulating the diagnostic equations to apply to conformal map projections and to limited-area domains; despite the desirability of incorporating topography, this is deferred to future research. The geostrophic wind is defined in terms of constant Coriolis parameter, rendering it nondivergent and thus confining the horizontal divergence to the ageostrophic wind. The ageostrophic wind is partitioned into harmonic, rotational, and divergent components. This three-field with its counter-part determined from the psi-vector calculation.

The modified psi-vector methodology is illustrated for two well-documented East Coast midlatitude cyclones. The first case (the President's Day storm: 1200 UTC 19 February 1979) considers an interpretation that ascent in the vicinity of a curved upper-level jet-front system may be viewed as a superposition of contributions from cross-stream divergent ageostrophic flow associated wit a jet streak and from alongstream divergent ageostrophic flow associated with a baroclinic wave. The second case (the megalopolitan strom: 1200 UTC 11 February 1983) addresses the hypothesis of Uccellini and Kocin that vertical circulations transverse to meridionally displaced upper-tropospheric jet streaks are coupled in a lateral sense. In both of these cases, the diagnoses reveal that the cross-stream component of the divergent ageostrophic circulations isolates meaningful mesoscale signatures coinciding with regions of precipitation and ascent in the vicinity of upper-level jet-front systems whereas the alongstream component is indicative of synoptic-scale vertical motion. Furthermore, it is found that the cross-contour ageostrophic flow, necessary for a Lagrangian rates of change of kinetic energy in jet entrance and exit regions, is due primarily to the nondivergent (i.e., harmonic plus rotational) ageostrophic wind. This result suggests that the practice of linking cross-contour ageostrophic winds and vertical motions in jet entrance and exit regions in the qualitative assessment of energy tranformations in these regions may be problematic in the case of upper-level jet-front system situated in three-dimensional flows.

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Joseph J. Barsugli, Jeffrey S. Whitaker, Andrew F. Loughe, Prashant D. Sardeshmukh, and Zoltan Toth

Can an individual weather event be attributed to El Niño? This question is addressed quantitatively using ensembles of medium-range weather forecasts made with and without tropical sea surface temperature anomalies. The National Centers for Environmental Prediction (NCEP) operational medium-range forecast model is used. It is found that anomalous tropical forcing affects forecast skill in midlatitudes as early as the fifth day of the forecast. The effect of the anomalous sea surface temperatures in the medium range is defined as the synoptic El Niño signal. The synoptic El Niño signal over North America is found to vary from case to case and sometimes can depart dramatically from the pattern classically associated with El Niño. This method of parallel ensembles of medium-range forecasts provides information about the changing impacts of El Niño on timescales of a week or two that is not available from conventional seasonal forecasts.

Knowledge of the synoptic El Niño signal can be used to attribute aspects of individual weather events to El Niño. Three large-scale weather events are discussed in detail: the January 1998 ice storm in the northeastern United States and southeastern Canada, the February 1998 rains in central and southern California, and the October 1997 blizzard in Colorado. Substantial impacts of El Nino are demonstrated in the first two cases. The third case is inconclusive.

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Jeremy S. Grams, Willam A. Gallus Jr., Steven E. Koch, Linda S. Wharton, Andrew Loughe, and Elizabeth E. Ebert

Abstract

The Ebert–McBride technique (EMT) is an entity-oriented method useful for quantitative precipitation verification. The EMT was modified to optimize its ability to identify contiguous rain areas (CRAs) during the 2002 International H2O Project (IHOP). This technique was then used to identify systematic sources of error as a function of observed convective system morphology in three 12-km model simulations run over the IHOP domain: Eta, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), and the Weather Research and Forecasting (WRF). The EMT was fine-tuned to optimize the pattern matching of forecasts to observations for the scales of precipitation systems observed during IHOP. To investigate several error measures provided by the EMT, a detailed morphological analysis of observed systems was performed using radar data for all CRAs identified in the IHOP domain. The modified EMT suggests that the Eta Model produced average rain rates, peak rainfall amounts, and total rain volumes that were lower than observed for almost all types of convective systems, likely because of its production of overly smoothed and low-variability quantitative precipitation forecasts. The MM5 and WRF typically produced average rain rates and peak rainfall amounts that were larger than observed in most linear convective systems. However, the rain volume for these models was too low for almost all types of convective systems, implying a sizeable underestimate in areal coverage. All three models forecast rainfall too far northwest for linear systems. The results for the WRF and MM5 are consistent with previous observations of mesoscale models run with explicit microphysics and no convective parameterization scheme, suggesting systematic problems with the prediction of mesoscale convective system cold pool dynamics.

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Lígia Bernardet, Louisa Nance, Meral Demirtas, Steve Koch, Edward Szoke, Tressa Fowler, Andrew Loughe, Jennifer Luppens Mahoney, Hui-Ya Chuang, Matthew Pyle, and Robert Gall

The Weather Research and Forecasting (WRF) Developmental Testbed Center (DTC) was formed to promote exchanges between the development and operational communities in the field of Numerical Weather Prediction (NWP). The WRF DTC serves to accelerate the transfer of NWP technology from research to operations and to support a subset of the current WRF operational configurations to the general community. This article describes the mission and recent activities of the WRF DTC, including a detailed discussion about one of its recent projects, the WRF DTC Winter Forecasting Experiment (DWFE).

DWFE was planned and executed by the WRF DTC in collaboration with forecasters and model developers. The real-time phase of the experiment took place in the winter of 2004/05, with two dynamic cores of the WRF model being run once per day out to 48 h. The models were configured with 5-km grid spacing over the entire continental United States to ascertain the value of high-resolution numerical guidance for winter weather prediction. Forecasts were distributed to many National Weather Service Weather Forecast Offices to allow forecasters both to familiarize themselves with WRF capabilities prior to WRF becoming operational at the National Centers for Environmental Prediction (NCEP) in the North American Mesoscale Model (NAM) application, and to provide feedback about the model to its developers. This paper presents the experiment's configuration, the results of objective forecast verification, including uncertainty measures, a case study to illustrate the potential use of DWFE products in the forecasting process, and a discussion about the importance and challenges of real-time experiments involving forecaster participation.

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