Long-range weather forecasts through numerical and empirical methods

There has been much improvement in numerical weather prediction since L.F. Richardson (1922, Weather Prediction by Numerical Process, Cambridge University Press, Cambridge, p. 236) wrote his famous book. NWP has primarily been successful in improving day-by-day forecasts starting from an observed detailed Initial Condition (IC) out to about a week. The purpose of this paper is to discuss first the state of the art in long-range NWP by presenting results of a new large numerical experiment (named DERF90; from Dynamical Extended Range Forecasting in 1990 out to 90 days) conducted at the National Meteorological Center (NMC) during the summer and autumn of 1990 (Section 2). One hundred and twenty eight 90-day global forecasts were made from successive daily initial conditions (IC), thus giving us ample opportunity to assess skill of forecasts at lead times beyond 1 week.We then move on to define the notion of a limit of predictability (LOP), and following a procedure by Lorenz (1982), give a numerical estimate of the LOP using the DERF90 data set. We then produce a list of reasons, as to why this estimate (LOP = 18 days) should not be taken too literally. In particular, we argue that the LOP varies as a function of the flow itself, and it would be (much) larger if we had, as we will ultimately, a coupled ocean-atmosphere model for making long-lead forecasts. Last, but not least, we present results of empirical forecasts that point to modest but significant skill well beyond the traditional LOP (a few weeks).A specific recent example of empirical forecasting is discussed. Through Canonical Correlation Analysis (CCA), experimental forecasts are being made for the United States surface temperatures at lead times of several seasons. While modest, the skill is significant in that it defies the existence or a 3-week LOP, and so demonstrates the potential for model improvements.     

On the yearly phase delay of winter intraseasonal mode in the western United States

In the western United States, persistent and recurrent flow patterns not only modulate precipitation events but also result in prolonged surface inversion episodes. In this region, the frequency of persistent ridge/trough events ranges between 20 and 40 days, well within the intraseasonal timescale. Based on NCEP reanalysis data starting at 1949, with a focus on the interior West, we observed that episodes of prolonged ridge/trough events appear to occur about a week later every year and resets every 5–7 years—a previously undocumented phenomenon examined herein. Diagnostic analyses indicate that the interplay between regional intraseasonal flow patterns and the North Atlantic Oscillation (NAO) alternates the preferred timeframe for the persistent ridge/trough events to occur. This may result from different phases of the NAO shifting the winter mean ridge and such shifts modulate the occurrence and timing of persistent ridge/trough events. When the timing changes evolve around the quasi-6 years cycle of the NAO, the resultant evolution forms what appears to be a steady phase delay in the ridge/trough events year after year. These results are a further step in disclosing the multiple-scale interaction between intraseasonal and interannual modes and its regional climate/weather impact.     

Average winter temperatures at De Bilt (the Netherlands): 1634–1977

A very long series of average winter temperatures in the Netherlands is presented. The series is based on direct observations (1735–1977) and administrative data concerning Dutch canals (1634–1734). The sources and the reliability of the data are discussed. Some characteristics of the series are shown.     

Investigations of the 500mb level in relation to the general circulation

Summary The redistribution of kinetic energy in the 500mbar level is studied for 1966 as well as for 1967. The total kinetic energy is split up in three parts: a) the energy due to the mean flow, b) the energy due to eddies with a period of time longer than a month and c) the energy due to eddies with a period of time less than a month but longer than one day.The transport of these energies takes place in formally three different ways, namely by the mean flow, by longperiod and shortperiod eddies (or monthly and daily eddies).Both the transports in zonal and meridional direction are studied. As will be shown in section 2, this yields 16 transportterms. It turns out that for the 500mbar level in the region from 100°W to 30°E and from 30°N to 65°N and based on geostrophic approximated winds: a) the longperiod eddies are important, b) the role of the longperiod eddies as well as the role of the shortperiod eddies decreases with increasing latitude and c) the zonal transports and the divergence of those transports are mostly larger than the meridional transports, and the divergence of the meridional transports.

---

Untersuchungen der 500 mbar-Fläche in bezug auf die allgemeine Zirkulation,

Zusammenfassung Die Verteilung der kinetischen Energie im 500mb-Niveau wird für die Jahre 1966 und 1967 untersucht. Die gesamte kinetische Energie wird dabei in drei Teile zergliedert: a) die kinetische Energie der mittleren Strömung, b) die Energie aus länger als einen Monat andauernden Wirbeln und c) die Energie aus kürzer als einen Monat, aber länger als einen Tag andauernden Wirbelelementen.Der Transport dieser Energien erfolgt formal auf drei verschiedenen Wegen, nämlich durch die mittlere Strömung, durch lang- und kurzperiodige Wirbelelemente (Turbulenzelemente von Monatsdauer bzw. von Tagesdauer). Es werden die Transporte in zonaler und in meridionaler Richtung untersucht; daraus ergeben sich 16 Transportterme. Für die Region von 100°W bis 30°E und 30°N bis 65°N erweist sich unter Zugrundelegung geostrophisch approximierter Winde, a) daß langperiodige Wirbel von Bedeutung sind, b) daß die Bedeutung langperiodiger wie auch kurzperiodiger Wirbelelemente mit zunehmender geographischer Breite abnimmt und c) daß die zonalen Transporte und ihre Divergenz meistens größer sind als die meridionalen Transporte und die Divergenz meridionaler Transporte.

---

---

With 25 Figures     

CFSv2 prediction skill of stratospheric temperature anomalies

This study evaluates the prediction skill of stratospheric temperature anomalies by the Climate Forecast System version 2 (CFSv2) reforecasts for the 12-year period from January 1, 1999 to December 2010. The goal is to explore if the CFSv2 forecasts for the stratosphere would remain skillful beyond the inherent tropospheric predictability time scale of at most 2 weeks. The anomaly correlation between observations and forecasts for temperature field at 50 hPa (T50) in winter seasons remains above 0.3 over the polar stratosphere out to a lead time of 28 days whereas its counterpart in the troposphere at 500 hPa drops more quickly and falls below the 0.3 level after 12 days. We further show that the CFSv2 has a high prediction skill in the stratosphere both in an absolute sense and in terms of gain over persistence except in the equatorial region where the skill would mainly come from persistence of the quasi-biennial oscillation signal. We present evidence showing that the CFSv2 forecasts can capture both timing and amplitude of wave activities in the extratropical stratosphere at a lead time longer than 30 days. Based on the mass circulation theory, we conjecture that as long as the westward tilting of planetary waves in the stratosphere and their overall amplitude can be captured, the CFSv2 forecasts is still very skillful in predicting zonal mean anomalies even though it cannot predict the exact locations of planetary waves and their spatial scales. This explains why the CFSv2 has a high skill for the first EOF mode of T50, the intraseasonal variability of the annular mode while its skill degrades rapidly for higher EOF modes associated with stationary waves. This also explains why the CFSv2’s skill closely follows the seasonality and its interannual variability of the meridional mass circulation and stratosphere polar vortex. In particular, the CFSv2 is capable of predicting mid-winter polar stratosphere warming events in the Northern Hemisphere and the timing of the final polar stratosphere warming in spring in both hemispheres 3–4 weeks in advance.     

Northern Hemisphere atmospheric blocking as simulated by 15 atmospheric general circulation models in the period 1979–1988

 As a part of the Atmospheric Model Intercomparison Project (AMIP), the behaviour of 15 general circulation models has been analysed in order to diagnose and compare the ability of the different models in simulating Northern Hemisphere midlatitude atmospheric blocking. In accordance with the established AMIP procedure, the 10-year model integrations were performed using prescribed, time-evolving monthly mean observed SSTs spanning the period January 1979–December 1988. Atmospheric observational data (ECMWF analyses) over the same period have been also used to verify the models results. The models involved in this comparison represent a wide spectrum of model complexity, with different horizontal and vertical resolution, numerical techniques and physical parametrizations, and exhibit large differences in blocking behaviour. Nevertheless, a few common features can be found, such as the general tendency to underestimate both blocking frequency and the average duration of blocks. The problem of the possible relationship between model blocking and model systematic errors has also been assessed, although without resorting to ad-hoc numerical experimentation it is impossible to relate with certainty particular model deficiencies in representing blocking to precise parts of the model formulation. Received: 16 July 1997/Accepted: 20 October 1997