David P. Baumhefner

Predictability of mesoscale atmospheric motions

Publisher Summary This chapter describes the predictability of mesoscale atmospheric motions. The advances in scientific understanding and technology have made it possible to envisage significant improvements in predicting mesoscale atmospheric events. Two properties of the atmosphere are considered important in limiting atmospheric predictability. First is the existence of instabilities that cause neighboring trajectories in phase space to diverge. Surface inhomogeneities including elevation and surface characteristics generate many mesoscale phenomena and modulate their behavior. When global models are used in classic predictability studies, the likelihood of possible errors in the overall estimate of synoptic-scale predictability is minimized by the consideration of the entire earth, which is likely to have a full complement of representative weather patterns over the period of the predictability experiment.

Monte Carlo simulations of explosive cyclogenesis

Abstract The impact of initial condition uncertainty on short-range (0–48 h) simulations of explosive surface cyclogenesis is examined within the context of a perfect model environment. Eleven Monte Carlo simulations are performed on 10 cases of rapid oceanic cyclogenesis that occurred in a long-term, perpetual January integration of a global spectral model. The perturbations used to represent the initial condition error have a magnitude and spatial decomposition that closely matches estimates of global analysis error. Large variability characterizes the error growth rates, both among the individual Monte Carlo simulations and among the case-average values. Some individual simulations display error growth doubling times as fast as approximately 12 h during the 24-h period of most rapid intensification, while others exhibit virtually no error growth. The variability is also reflected in the wide 90% confidence bounds for many surface weather elements such as the cyclone position and central pressure. Howev...

Identification of Highly Predictable Flow Elements for Spatial Filtering of Medium- and Extended-Range Numerical Forecasts

Abstract To determine if some flow components are systematically forecast more accurately than others, 990 wintertime medium-range forecasts made at the European Centre for Medium-Range Weather Forecasts (ECMWF) are examined. It is found that forecasts skill of 500-mb extratropical large-scale heights tends to be a function of empirical orthogonal function (EOF) index, with those components that project onto the leading EOFs being markedly better forecast than the components that project onto the hailing EOFS. This is true for instantaneous forecasts of as long as 10 days’ duration. Furthermore, by answering the question, Of all possible structures which structure on average is most accurately forecast? the potential for constructing a basis that is even more adept than EOFs at distinguishing well-forecast from poorly forecast flow elements is shown. Similarly, it is found that 10-day average ECMWF forecasts, as well as 29-day average forecasts produced by a general circulation model at the National Cente...

Numerical Prediction of the Onset of Blocking: A Case Study with Forecast Ensembles

Abstract The forecastability of a blocking episode during January 1985 over Europe and the eastern Atlantic Ocean is studied with forecast ensembles. Ten-member ensembles from version CCM2 (at T42 resolution) of the Community Climate Model of the National Center for Atmospheric Research are initialized at various lead times prior to the analyzed block onset and run out to 14 days. Particular attention is focused on the ensemble initialized five days prior to block onset since, of all the ensembles, this one was characterized by the greatest variability concerning the block-onset prediction. Two of the 10 members of this particular ensemble predicted a transition from unblocked to blocked flow over the Atlantic–Europe half of the Northern Hemisphere during the 14-day forecast range, but not without error; details regarding the timing and/or location of the block were misforecast. A comparison of these ensemble members, plus one other that did not predict a transition to blocking, with the corresponding ana...

Principal oscillation pattern analysis of the tropical 30- to 60-day oscillation: Part 11: The prediction of equatorial velocity potential and its skill

In Part I (Storch and Xu 1990) the principal oscillation pattern (POP) analysis of 200 mb equatorial velocity potential leads to the definition of a bivariate (POP-) index of the tropical 30- to 60-day oscillation. Using the POP prediction scheme this index is predictable for a few days in advance. In Part 11, the prediction of the equatorial velocity potential field, made by the POP method and made by two GCMs, is investigated. The POP index forecast can incorporate skillful forecasts of the equatorial velocity potential (χ) field. Its ensemble correlation skill score passes the 0.50 level at 7 days, whereas persistence passes after 3 days. If there is a strong 30- to 60-day oscillation signal in the initial state, useful forecasts of more than 20 days are sometimes possible; if the initial signal is weak, the POP forecast fails. Also, the forecast skill of two GCMs is considered. The NCAR T31 GCM appears to be quite skillful in predicting the equatorial χ-field, and in particular the 30- to 60-day oscillation. Its skill, however, is less than that of the POP scheme. The CNRM T42 GCM seems not to be able to predict the regular development associated with the tropical 30- to 60-day oscillation. The power of the POP index in explaining the equatorial x-field is a measure of the strength and dominance of the 30- to 60-day oscillation. This measure at day 0 is an a priori indicator of the NCAR T31 GCMs skill in predicting the equatorial velocity potential field.

APPLICATION OF A DIAGNOSTIC NUMERICAL MODEL TO THE TROPICAL ATMOSPHERE1

Abstract A diagnostic, nonlinear balanced model is applied in order to describe numerically the three dimensional structure of the tropical atmosphere. Several comparisons and experiments are made to gain insight into the physical processes and reliability of the model. These include different types of stream functions and temperature analyses, and the addition of surface friction and latent heat. A comparison between the kinematic vertical motion and the final numerical result is performed. Obtained by using the complete form of the balance model, the derived vertical motion for August 12–14, 1961, in the Caribbean is presented in the form of cross sections. The vertical velocity fields, which are displayed in partitioned form, are compared with the analyzed moisture distribution. The validity of the computed vertical motion is discussed along with its possible influence on the tropical weather.

The Impact of Initial Condition Uncertainty on Numerical Simulations of Blocking

Abstract The impact of initial condition uncertainty (ICU) on the onset and maintenance of eastern North Pacific blocking is examined within the framework of a general circulation model (GCM) and the perfect model assumption. Comparisons are made with the contrasting zonal flow regime. Twenty-member ensembles of perturbed simulations are run out to 15 days for the zonal flow, and for blocking at lead times of 8, 4, 2, and 0 days. Blocking occurs in 95% of the 0-day lead simulations and declines monotonically to 65% for the 8-day lead simulations. The uncertainty in the exact time of onset among those simulations that form blocks also increases with lead time. The synoptic-scale features in both the blocking and zonal ensembles saturate, relative to climatological variance, and decorrelate (anomaly correlation coefficient < 0.5) by 6 days. The planetary-scale features, however, maintain skill relative to climatology beyond 10 days. The zonal simulations are generally the first to saturate and decorrelate, ...

Ensemble Simulations of Explosive Cyclogenesis at Ranges of 2-5 Days

Ensemble simulations of explosive cyclogenesis are examined in a lengthy run of a global general circulation model with the perfect ensemble context. Attention is focused on the day when the deepest low appeared. An ensemble of 31 members is obtained by integrating 30 additional runs starting from slightly perturbed initial conditions. The perturbations are randomly selected to represent equal approximations to the truth, given typical analysis differences between major centers. Ensembles are generated starting two, three, four, and five days prior to maximum depth. Two lows are contrasted, the deepest low near Kamchatka and a marginally explosive low over the central Pacific. The early development of both systems was suppressed by their presence in the confluent entrance region of the Pacific winter jet. An intense low near Kamchatka eventually developed in each member of the ensemble at all projections, but the details of development varied from member to member and were related to the involvement of a surface perturbation coming up into the system from low latitudes. In contrast, cyclogenesis over the central Pacific occurred in some members of the ensemble but not at all in others. The difference in behavior of the two systems is reflected in a localized enhancement of the error growth of the planetary and synoptic scales for the central Pacific low and is related to the smaller horizontal scale of the central Pacific low. Probabilistic estimates of precipitation quantity and surface wind speeds produced by the ensemble showed moderate skill at day 5 with respect to climatology, mainly away from the regions of most vigorous synoptic activity, when verified against individual ensemble members. Skill would be reduced if the ensemble mean proved to be more seriously in error as is the case for a forecast verified against observations.

Initial weather regimes as predictors of numerical 30-day mean forecast accuracy

Abstract A set of thirty 30-day mean 500-mb height anomaly forecasts run from National Meteorological Center initial analyses by the NCAR Community Climate Model is examined in order to learn if the forecast accuracy can be estimated with the initial conditions. Defining initial weather regimes by a 500-mb geostrophic zonal index anomaly difference between 50°W and 10°E discriminates between the best and worst 30-day mean forecasts in the sample. Initial regimes characterized by anomalously high zonal index (500-mb geostrophic westerlies) at 50°W and low index at 10°E yield on average lower 30-day mean forecast-observed anomaly correlation than initial regimes with opposite conditions (anomalously low zonal index at 50°N and high index at 10°E). It is suggested that initial regimes with abnormally fast geostrophic 500-mb westerlies at 50°W are followed in time by intense and poorly forecast synoptic-scale cyclones over the Atlantic Ocean. It is shown in a case study that the local synoptic- to planetary-s...

Forecast Intercomparisons from Three Numerical Weather Prediction Models

Abstract A forecast intercomparison study using six different initial states was carried out with forecasts made by the 2.5°, six-layer, second-generation National Center for Atmospheric Research (NCAR) general circulation model (GCM); the 4° × 5°, nine-layer Goddard Institute for Space Studies (GISS) GCM; and the 4° (at 50°N), six-layer National Meteorological Center (NMC) operational model. The initial conditions for geopotential and velocity were obtained from the NMC operational analyses during December 1972 and January 1973 for the Northern Hemisphere. The operational NMC analyses were used for verification of the forecast 1000 and 500 mb heights. Forecast projections to either three or five days were examined using conventional skill scores, wavenumber analysis of longitude-time plots constructed from the forecasts, and maps of the actual difference (forecast minus verification). The forecast errors were defined for each model and the common errors in all three forecasts were identified. The GISS an...