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H. Hersbach
,
R. Mureau
,
J. D. Opsteegh
, and
J. Barkmeijer

Abstract

An ensemble prediction system, especially designed for the short to early-medium range for the European domain, is presented. The initial perturbations of each ensemble are based on singular vectors that maximize the 3-day total energy error growth above the European area and the northern Atlantic. In total, a set of 51 ensembles, each consisting of 51 members, has been integrated, comprising 28 winter cases in 1996 and 1997, 2 spring, and 21 autumn cases in 1997. The impact on performance, relative to the operational ensemble prediction system at ECMWF, appears to be modest for the winter set. However, for the combined spring and autumn set, the impact is significantly positive, especially for rare (extreme) events of large-scale precipitation, 2-m temperature, 10-m wind speed, and the pressure at mean sea level. For the targeted ensembles, the situation that a complete ensemble misses an extreme event occurs much less than for the operational ensemble system. As a result, the range of cost–loss ratios for which a user has benefit is larger. The benefit of the targeted ensembles is maximal between days 2 and 3. For this range in spring–autumn, an ensemble system consisting of a subset of 21 targeted members performs comparable to the 51-member operational ensemble prediction system (EPS), when evaluated for the same area of interest. The spread within a targeted ensemble prediction system (TEPS) is somewhat larger than within the EPS ensembles. However, both systems appear to be underdispersive for large-scale precipitation, 2-m temperature, and 10-m wind speed. Only for mean sea level pressure is the spread of the TEPS ensemble, on average, comparable to the magnitude of the forecast error of the ensemble mean between days 2 and 3.5.

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R. Mureau
,
J. D. Opsteegh
, and
J. S. Winston

Abstract

For 35 seasons in the years 1974–84, the importance of seasonal anomalies in tropical diabatic heating was investigated for the circulation in the tropics and in the extratropics. The heating was estimated from anomalies in outgoing longwave radiation as measured by satellite and was prescribed as a forcing in a linear study state model. With this model a small part of the observed spatial variance in the streamfunction anomalies in the tropics and lower midlatitudes in the northern hemispheric winter and autumn (5–10%) could be explained. In the tropics, in particular in the central Pacific at 700 mb, the explained variance was largest (10–25%). When the beating was exceptionally large, as during El Niño 1982–83, the similarity between observed and simulated streamfunction anomalies was much better than average not only directly over the major heat sources in the tropics but also in midlatitudes. In spite of the simplicity of the model and the neglect of the other forcing terms, the explained variance was between 25 and 60% in these regions.

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C. J. Kok
,
J. D. Opsteegh
, and
H. M. van den Dool

Abstract

Using a two-level linear, steady state model, we diagnose the 40-day mean response of a GCM to a tropical sea surface temperature (SST) anomaly. The time-mean anomalies produced by the GCM are simulated as linear response to the anomalous hemispheric distributions of latent heating, sensible heating and transient eddy forcing. Also, the anomalous effect of mountains, caused by anomalies in the zonal mean surface wind is taken into account. All anomalies are defined as the difference between perturbation and control runs. For our analysis, we have taken the tropical Atlantic SST anomaly experiment performed by Rowntree.

We have compared the linear model's response in temperature at 600 mb and winds at 400 mb with the same anomalous quantities produced by the GCM. The similarity between the time-mean anomalies of the GCM experiment and the linear model's response is very high. The pattern correlation coefficients are between 0.6 and 0.7 in the region between 30°N and 60°N. The response to each of the anomalous forcings separately is positively correlated with the GCM anomaly pattern. The amplitude of the response to anomalous forcing by transient eddies is a factor of two or three larger than the effects of anomalous sensible and latent heating. The anomalous effect of the orography is negligible.

Although intended to be a tropical SST anomaly GCM experiment, the difference between control and perturbation runs does not seem to be directly related to tropical heating near the SST anomaly. Instead, most of the forcing of anomalies in the midlatitudes took place in the midlatitudes itself and, in particular, the remote effects of forcing by tropical latent heat sources were minor.

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