Systematic Forecast Errors Research Articles

Jet Stream Winds: Comparisons of Analyses with Independent Aircraft Data over Southwest Asia

Cruise-level wind data from commercial aircraft are obtained, and these data are compared with operational jet stream analyses over southwest Asia, an area of limited conventional data. Results from an ensemble of 11 cases during January 1989 and individual cases during December 1988-March 1989 are presented. The key results are: (1) European Centre for Medium-Range Weather Forecasts (ECMWF), National Meteorological Center, and United Kingdom Meteorological Office analyses of the subtropical jet in southwest Asia are 11 percent, 17 percent, and 17 percent weaker, respectively, than aircraft observations; (2) analyzed poleward shears range up to 1 f (0.00007/s) compared with up to 3 f (0.00021/s) in the aircraft observations where f is the local Coriolis parameters; (3) the ECMWF errors are largest at the base of the jet; (4) the mean ECMWF core location is latitudinally correct but has an rms latitude variance of 1.5 deg; (5) isolated erroneous radiosondes produce unmeteorological structures in the analyzed subtropical jet stream; and (6) the increased utilization of automated aircraft reports is likely to produce a spurious secular increase in the apparent strength of the jets. The magnitude and spatial extent of the errors seen are near limits of current GCM resolution (100 km) but should be resolvable. The results imply that studies of GCM systematic jet stream wind errors in weather and climate forecasts must be interpreted with caution in this region.

Short-term systematic errors in global forecasts: their estimation and removal

Short-term systematic errors in global forecasts: their estimation and removal

Short-term systematic errors in global forecasts: their estimation and removal

We have found spatial and temporal coherencies in the departures of short-range-forecasts from observed and analysed geopotential fields, which can be used diagnostically and as the basis for adjustment of forecast products. We identify characteristics of the differences between forecast and observed, and forecast and analysed values, and we demonstrate the significant impact on forecast accuracy which may be gained through the routine estimation and removal of these discrepancies. The removal of these discrepancies in the data assimilation step of the forecast cycle was found to have little impact, in contrast with the effect of direct correction of forecast products in conjunction with post processing. DOI: 10.1034/j.1600-0870.1990.00001.x

Relationships between Systematic Errors in Medium Range Numerical Forecasts and Some of the Principal Modes of Low-Frequency Variability of the Northern Hemisphere 700 mb Circulation

Abstract The relationship between the existence of low-frequency 700 mb height anomalies in the initial conditions of NMC's MRF global spectral model and subsequent 5-, 7-, and 10-day forecasts of 700 mb height from 1982 to 1988 is explored. Low-frequency 700 mb flow regimes are specified in each of four two-month seasons by performing a rotated principal component analysis (RPCA) on 38 or 39 year time series of daily, low-pass filtered 700 mb height analyses. In a given season, the amplitude time series (ATS) for each mode is used to decide which MRF forecast error maps should be used in forming a composite map corresponding to either the “+” phase or the “−” phase of the given mode. Several methods, including Monte Carlo simulations, are used to evaluate the statistical significance of the composite maps. Many modes, including the Pacific North American (PNA) mode in winter and the leading summer mode, are found to be related to either unusually strong or unusually weak systematic error signature. Two d...

Systematic Surface Cyclone Errors in NMC's Nested Grid Model November 1988–January 1989

Abstract Preliminary results from a study examining the performance of the nested grid model (NGM) in predicting cyclones covering a period from 13 November 1988 to 31 Match 1989 reveal that the NGM tends to overdevelop surface cyclones over continental regimes and underdevelop surface cyclones in oceanic regimes. The results also indicate an overall cold bias in the model forecasts of thickness and 850 mb temperatures over the cyclone center. Displacement error data indicate the NGM tends to move cyclones too slowly in the southern half of the forecast domain. A semiautomated method has been developed at the National Meteorological Center (NMC) to track and verify sea level pressure features in the NGM. The method allows the user to interactively track a system, store its coordinates, and then retrieve information about the system from selected model forecast and analysis grids. This information can then be used to determine systematic forecast errors, compare past forecasts with the most recent forecast...

Occupational Choice under Uncertainty

An econometric problem in estimating models of occupational choice is that the agents' forecasts of future wages and occupational tenure are unobservable. This paper solves the problem by assuming that agents have rational expectations and by considering the effects of arbitrage both within and between cohorts. The solution consists of two time series regressions of the demand and supply functions of entrants into an occupation. From these regressions one obtains estimates of the rate of return to education, the direct cost of education, and other parameters that influence the market. The model was estimated with data from the market for lawyers. 1. MANY OCCUPATIONS REQUIRE the agent to acquire some schooling before he can practice. How does the agent choose between two occupations that require different schooling periods? Since Adam Smith [17, Book I: Chapter 10], economists have assumed that the agent decides by comparing the expected present value of incomes between the two occupations (e.g., Friedman and Kuznets [7], Schultz [15], Becker [1], and Mincer [10]; see [13] for a survey of the literature). While the decision rule is well known, the problem of uncertain wages plagues empirical applications of models embodying the decision rule. Most empirical works rely on cross-section data or short panel data sets. Thus the total wages over the lifetime of the agent are not observed. "The question of ex ante expectations cannot of course be known. Short term macroeconomic factors tend to confound estimates of costs and returns to education" (Rosen [13]). Rosen also raised the question of dropouts. That is, agents may leave an occupation in which they practiced. Therefore the question of uncertain wages is complicated by the additional uncertainty of occupational tenure. There was an important attempt to solve the problem of uncertain wages by Richard Freeman [3, 4, 5]. Freeman recognized that demand conditions in the market affected the supply of new students. He argued that new entrants made their decisions to enter by comparing current wages in the relevant occupations. Since future conditions may change, current wages may not be good predictors of future wages. These systematic forecast errors lead to cycles in the supply of new entrants; these cyclical models are known as Cobweb models. This paper also considers the problem of occupational choice under uncertainty. An agent's decision problem based on maximizing expected discounted value of income is explicitly solved. The result is a generalization of Mincer's schooling model [10]. I also close the model by positing a demand curve for the

Total and Planetary-Scale Systematic Errors in Recent NMC Operational Model Forecasts

The total and systematic errors in the 500 mb geopotential height forecasts from the NMC grid-point and spectral operational models are compared and contrasted for two recent winters. The spectral model is shown to be an improvement in the forecasts through a more skillful prediction of the planetary-scale (zonal wavenumbers 1-2) quasi-stationary wave amplitudes, and through the elimination of the grid-point model's large systematic error at low latitudes. In agreement with estimates from related studies, the systematic error in the NMC spectral model accounts for 15-20% of the total error variance. Approximately one-half of the total systematic error resides in the planetary scales.

A Systematic Error Comparison Between the ECMWF and NMC Prediction Models

Abstract The 24 systematic forecast errors in the 50 mb geopotential height from the ECMWF and NMC operational prediction models are compared and analyzed. During the winter of 198 the error patterns from the two models over the Northern Hemisphere were remarkably similar. The error signatures within composites of selected dates, which were chosen according to the character of the flow regime over the Rocky Mountains, were also very similar, indicating that improper orographic forcing may represent one deficiency common to both models. The general similarity of the patterns in each composite, however, does not rule out the existence of yet another important common deficiency.

Reduction of systematic forecast errors in the ECMWF model through the introduction of an envelope orography

AbstractDay 1 forecast errors in the ECMWF model geopotential height fields during wintertime show a distinctive and highly reproducible signature with negative biases over the major mountain barriers. These biases are largest on days when the 500mb flow over the mountains is strong and they tend to shift position from day to day so as to remain close to the region where the jet stream crosses the mountain barrier. These systematic errors evolve through the forecast interval until by day 10 they assume an equivalent barotropic structure which strongly resembles the upper‐level stationary wave pattern, but has the opposite sign, which indicates that the model does not have sufficient (presumably orographic) forcing to maintain the stationary waves.The time evolution of the systematic error pattern is investigated by means of a series of experiments with a barotropic model, in which the observed mean wintertime 300 mb flow is perturbed by a steady forcing derived from the day 1 forecast error pattern. The day 10 error patterns, as simulated by the barotropic model, bear a strong similarity to the observed ones. The forcing in the vicinity of the northern Rockies makes a particularly large contribution to the simulated day 10 error pattern whereas that in the Himalaya region appears to be relatively less important.The impact of an enhanced orography upon the climate of the ECMWF model is investigated by carrying out a pair of extended integrations out to 50 days. The control run is based on the conventional average‐type orography used for operational forecasting; the other run is based on an ‘envelope orography’ constructed by adding to the conventional, grid‐square averaged, orography an increment proportional to the standard deviation of the sub‐grid scale variance. Results for this one, rather short pair of integrations suggest that the envelope orography may be capable of yielding a more realistic simulation of the observed wintertime flow pattern, particularly with respect to features in the Pacific and western North American sectors. Certain aspects of the zonally averaged circulation are also more realistic in this envelope simulation.A series of 21 successive ten‐day forecast integrations has also been carried out with this envelope orography from February 1982 data. Results indicate that the introduction of the envelope orography results in a slight degradation of the short‐range forecasts together with a distinct improvement of the forecast beyond four days. The beneficial impact towards the end of the forecast interval appears to be large enough to increase the forecast usefulness by perhaps as much as half a day.

Reduction of systematic forecast errors in the ECMWF model through the introduction of an envelope orography

Reduction of systematic forecast errors in the ECMWF model through the introduction of an envelope orography

Identification of Systematic Errors in a Numerical Weather Forecast

Many numerical model verification schemes are handicapped by their inability to separate non-systematic errors and systematic errors. In this study, for a specific synoptic event, a statistical method is described to determine a minimum number of cases which can be averaged to represent numerical forecast errors which are truly systematic and not smoothed fields of rapidly varying non-systematic errors. Error patterns derived from forecasts and observations stored at Fleet Numerical Oceanography Center are used to compare a systematic error pattern, defined by the total number of available cases with subset error patterns to determine the minimum number of cases needed to filter out the unwanted non-systematic error components. The analysis indicates that a minimum of 8 cases must be averaged to adequately identify systematic errors in a 24 h forecast of a Shanghai Low. A minimum of 5 cases are needed for a 72 h forecast of the same event. Error patterns are identified by contours of the Student's t statistic calculated at each grid point. This contour pattern objectively determines the significance of the forecast errors and is shown to be a very useful method of portraying, systematic forecast errors.

An Investigation of Systematic Errors in the Barotropic Forecasts

Forecast-errors in the 24- and 48-hour barotropic 500-mb prognostic charts prepared by JNWPU for the winter of 1957 were investigated. Certain large-scale forecast-errors were found to persist from day-to-day in fairly localized geographical areas. In general, the numerical prognoses exhibited a tendency to forecast excessively high values off the southeastern coasts and excessively low values off the northwestern coasts of the continents. Little forecast-error was observed in the continental interiors.
 A Fourier analysis revealed that the forecast-errors were largely due to incorrect phase forecasts for wave numbers 1 through 4.
 The forecast-errors associated with wave number 1 usually “positioned” the large-scale systematic forecast-errors near Japan and Scandinavia and contributed materially to their intensities.
 Relationships between forecast-error fields and topography, geography, synoptic situation, and “non-adiabatic” heating are discussed.
 Hemispheric fields of “non-adiabatic” heating, computed using a two-level graphical model, are shown.
 Charts illustrating the dependence of “non-adiabatic” heating on the general circulation are also shown.