Search Results

You are looking at 11 - 20 of 28 items for

  • Author or Editor: H. M. van den Dool x
  • Refine by Access: All Content x
Clear All Modify Search
J. Oerlemans
and
H. M. Van Den Dool

Abstract

A zonally averaged Climate model of the energy-balance type is examined. Recently published satellite measurements were used to improve existing parameterizations of planetary albedo and outgoing radiation in term of surface and sea level temperature. A realistic constant for the diffusion of energy was found by tuning the model to the present climate. For the actual solar constant both the present climate and an ice-covered earth are solutions of the model. They are extremely stable for temperature perturbations.

The effect of variation of the solar constant was investigated in detail. If the solar constant is decreased by 9–10% the warm solution (partial ice cover) jumps to the cold one (complete ice cover). Transition from the cold to the warm solution requires an increase of the solar constant to 109–110% of its present value. Therefore, we conclude that the model climate is much more stable with regard to variations in the solar input than has been assumed so far. This is caused mainly by our updated formulation of the outgoing radiation. Further experiments showed that our model is much more sensitive to changes in the outgoing radiation than to changes in the diffusivity for energy.

Full access
J. D. Opsteegh
and
H. M. Van Den Dool

Abstract

A linear steady-state primitive equation model has been developed for the computation of stationary atmospheric waves that are forced by anomalies in surface conditions. The model has two levels in the vertical. In the zonal direction the variables are represented by Fourier series, while in the meridional direction a grid-point representation is used. The equations governing atmospheric motion are linearized around a zonally symmetric state which depends on latitude and height according to Oort (1980).

We have studied the amplitude and phase relations of the model response as a function of latitude for a very simple beating, which is sinusoidal in the zonal direction, with zonal wavenumber m (m=1, 10) and constant in the meridional direction, using February mean conditions.

The response of the model indicates that a heating in the tropics can have a substantial influence on the middle and high latitudes, provided that part of the heating is in the westerlies. We have compared the model response for such a heating with the results of similar experiments with GCM and a linear barotropic model and also with mean anomaly patterns at middle and high latitudes derived from observations for Northern Hemispheric winters with a warm equatorial Pacific. In all cases we find strong similarities of hemispheric wave patterns.

We plan to test the model for the prediction of that part of the anomalies in the monthly or seasonal mean circulation that comes from persistent abnormal surface conditions In order to predict more than a persistent atmospheric response, such an anomaly in the surface conditions must have different effects in different months or seasons. We have tested the hypothesis that due to a changing zonally symmetric state, the response to a prescribed beating will be different in the four seasons. This effect is computed for a heating in the tropics and in the middle latitudes. Both in amplitude and phase the response to exactly the same heating can change significantly from one season to the next.

Full access
J. D. Opsteegh
and
H. M. Van Den Dool

Abstract

A diagnostic study has been performed to investigate the prospects for developing a time-averaged statistical-dynamical model for making long-range weather forecasts. Estimates are made of nearly all terms in the equations describing the evolution of the time-mean quantities (ū, v̄, T̄, ω¯) and the horizontal second-order eddy statistics (u2¯, v2¯, uv′¯, uT′¯) and vT′¯. These calculations were performed over northwestern Europe, using radiosonde observations of wind, temperature and height for the winter of 1976/11977. Geostrophic winds were estimated from objective analyses, while vertical velocities were determined with a quasi-geostrophic baroclinic model. For each equation, approximate balances are presented on the basis of these estimates.

In the equations for the mean quantities the time derivatives are more than one order of magnitude smaller than the unknown second-order eddy statistics. The same holds for the time derivatives of second-order eddy statistics compared with the unknown third-order and ageostrophic terms in the equations for these eddy fluxes. We therefore conclude that the system of time-averaged equations has no capability of describing the evolution of the atmosphere from one specific mean state to another mean state in the future–since for this purpose a closure of the system or a parameterization of second- order or third-order terms has to be extremely accurate. Even in the case in which only the stationary waves of the mean flow are treated, a higher order closure scheme does not seem to be feasible, for third-order terms and ageostrophic second-order terms are probably large and very difficult to parameterize. This implies that a preferable approach is to explore in greater depth the possibility of parameterizing the second-order statistics directly.

Full access
Suranjana Saha
and
H. M. van den Dool

Abstract

An objective and practical limit of predictability for NWP models is proposed. The time T 0 is said to be the limit of predictability if model forecast beyond T 0 has no extra skill over persisting the T 0 forecast. The “skill” is measured here in terms of standard rms and anomaly correlation scores. For the NMC medium-range forecast model, T 0 is found to be 5–6 days for 250, 500 and 1000 mb height forecasts for the period 5 May–25 July 1987. The T 0 can also be interpreted as the time at which them is no longer skill in the prediction of the time derivative of the quantity under consideration.

Full access
H. M. van Den Dool
and
Suranjana Saha

Abstract

A method is proposed to calculate measures of forecast skill for high, medium and low temporal frequency variations in the atmosphere. This method is applied to a series of 128 consecutive 1 to 10-day forecasts produced at NMC with their operational global medium-range-forecast model during 1 May–5 September 1988. It is found that over this period, more than 50% of the variance in observed 500 mb height fields is found at periods of 18 days or longer. The. intuitive notion that the predictability time of a phenomenon should be proportional to its lifetime is found to be qualitatively correct; i.e., the low frequencies are predicted (at a given skill level) over a longer time than high frequencies. However, the current prediction skill in low frequencies is far below its potential if one assumes that for any frequency the predictability time scale ought to be equal to the lifetime scale. In the high frequencies, however, the current prediction skill has already reached its potential; i.e., cyclones are being predicted over a time comparable to their lifetime; i.e. 3 to 4 days. We offer some speculations as to why the low frequency variations in the atmosphere are so poorly predicted by our current state-of-the-art models. The conclusions are tested, and found to hold up, on a more recent dataset covering 10 December 1988–16 April 1989.

Full access
H. M. van den Dool
and
J. L. Nap

Abstract

Using a predetermined statistical scheme, forecasts are made of the daily air temperature (AT) at San Diego, starting from local antecedent information concerning AT and sea surface temperature (ST) only. These forecasts are verified by calculating skill scores (S) over 1948–79. in this maritime area such simple schemes turn out to have high S for lead times up to a mouth and small but positive S out to a year. Differences in S of the various one predictor schemes (STAT, STST, ATAT, ATST) are discussed; STST is far superior to any of the other three. For most schemes S is low in late summer, this is attributed to the shallowness of the ocean's mixed layer in that season. The effects of time averaging the predictor and/or predictand are discussed for the STAT scheme. For long enough lead times averaging appears to improve forecast skill. The localness of the prognostic information carried by ST is investigated by comparing S for San Diego and an inland station (Esondido). At a forecast lead time of three days S decreases by 50% over a distance of 25 km. Further analysis shows that this decay is primarily caused by a decrease in skill of daily maximum temperature forecasts.

In view of the similarity of the present results to those obtained at the Dutch coast, we conclude that local information about the state of the surface has probably enough prognostic potential to be incorporated in existing operational schemes of short and long range air temperature forecasts near oceans and lakes.

Full access
H. M. Van Den Dool
and
L. Rukhovets

Abstract

A scheme to optimally weight the members of an ensemble of forecasts is discussed in the framework of calculating an as accurate as possible ensemble average. Results show, relative to a single member, a considerably improved 500-mb height forecast in the 6–10-day range for the Northern Hemisphere. The improvement is nontrivial and cannot be explained from simple smoothing. This method is used in operations at the National Meteorological Center.

Full access
H. M. van den Dool
,
W. H. Klein
, and
J. E. Walsh

Abstract

Eighty years of monthly mean station temperatures are used to evaluate the persistence of monthly air temperature anomalies over the United States. The geographical and seasonal dependence of the monthly persistence are described in term of the day-to-day persistence of temperature anomalies, the influence of the large-scale atmospheric circulation, and inferred associations with the slowly varying properties of the earth's surface.

The monthly persistence is generally smallest in the continental interior and largest in coastal regions. The seasonality of this spatial pattern is quite small, although the continental interior is characterized by a summer maximum. For the country as a whole, persistence is highest (0.30) in winter and summer and least (0.15) in fall and spring. For both raw and detrended data, the anomaly pattern correlations at lags of two and three months are much larger than would be expected from a first-order Markov process.

The pattern of persistences computed using day-to-day autocorrelations shows that the presence of nearby bodies of water increases the month-to-month persistence over that to be expected from daily weather fluctuations. This finding is consistent with the results derived from an intuitive energy balance model in which the soil (or ocean) surface layers and the atmospheric boundary layer respond to prescribed daily fluctuations in the free atmosphere.

Local surface influences are also implied by the fact that the 700-mb circulation-derived anomalies of monthly temperature have fewer spatial degrees of freedom than do the actual anomalies. While the large-scale circulation accounts for about half of the winter temperature persistence, small-scale effects, as well as the effects of the antecedent month's circulation, contribute substantially to the persistence of summer temperatures.

Full access
M. Chelliah
,
J. E. Schemm
, and
H. M. van den Dool

Abstract

The impact of anomalous tropical forcing on the anomalous tropical and extratropical circulation is examined by comparing the response of a global steady state linear primitive equation model with observations for nine (1978/79–1986/87) December–January–February (DJF) seasons. Outgoing longwave radiation (OLR) anomalies are used as proxies for tropical latent heat release. Root mean square amplitude and anomaly pattern correlation (APC) are chosen to be the verification tools.

It is shown that with OLR as the only forcing, the linear model produces a fairly realistic level of tropical interannual variability in the wind (both rotational and divergent) and geopotential height fields. The nine DJF's mean skill score, as measured by APC, is highest in the tropics for u, v and Φ between 600 to 900 mb where the moan APC is 0.5–0.6. There is skill in the simulation of both the rotational and divergent components of the circulation. In fact, the APC for the velocity potential at upper levels over the global domain is higher than that of the stream function. In the nine DJF's mean there is a small (APC 0.1–0.3), but significant skill in the Northern Hemisphere high latitudes in u, v and Φ at low levels. Since the explained variance is small, it leads us to conclude that in the mean, using our linear model, there is no demonstrable impact of much practical value on the extratropical circulation due to anomalous tropical heating.

On a year-by-year basis, however, the years with exceptionally large OLR perturbations in the tropics are also the years with the best model simulations (high APC over the globe as a whole and particularly in the tropics. In the ENSO DJF's of 1982/83 and 1986/87, the APC's are considerably above average in all regions including the high latitudes. The explained variance in the u and Φ fields at low levels is about 56% in the tropics and 30% in high latitudes. In thew years, the impact of the tropical heating anomalies on the remote high latitude circulation is therefore not only significant but also of practical value. The inclusion of the Hadley circulation in the model's basic state enhances the amplitude of the high latitude response, however, without improving the APC.

The model simulates the Southern Oscillation index (as measured by the Darwin–Tahiti 1000 mb height anomaly) with a correlation up to 0.9, as long as them is heating (cooling) in the lowest 300 mb in the prescribed vertical heating profile.

Full access
Brant Liebmann
,
M. Chelliah
, and
H. M. van den Dool

Abstract

We examine the persistence of outgoing longwave radiation (OLR) anomalies in the tropics on many different time scales during 1974–86. We calculate “one-lag autocorrelations” by constructing nonoverlapping 1-, 15-, and 60-day averages and calculating the correlation at every grid point between every time average and the following average for the entire dataset. One-day averages produce the 1argest local autocorrelations everywhere except over the equatorial Pacific. Large autocorrelations using 15-day averages are confined to the equatorial Pacific, but large autocorrelations based on 60-day averages extend eastward from the eastern Indian Ocean through South America We attribute the increase in autocorrelation in some areas as the averaging period increases to the presence of the 30–60 day oscillation in those areas The spatial match between the autocorrelation and the standard deviation of OLR is best for 60-day averages and worst for 15-day averages.

We then calculate pattern correlations over a domain that extends along the equator from the eastern Indian Ocean through the central Pacific. When plotted as a time series the one-lag pattern correlations for two-month means are seen to vary wildly, although they are generally positive. There are some extended periods, however, during which the pattern correlation remains 1arge, most notably during the 1982–83 ENSO event.

The average one-lag pattern correlation is plotted for many different time averages. They decrease until a minimum at 20-day averages, beyond which they slowly increase as the averaging 1ength is increased.

The average one-lag pattern correlations using one-day averages are smallest during the mid-year months, but using 60-day averages they are largest during these months. The seasonality, however, is not large.

Finally, we identify eastward propagation of OLR anomalies with at least two distinct phase-speeds in addition to a quasi-persistent signal. It is suggested that forecasts of OLR anomalies might be improved over simple, local persistence by a multiple regression technique.

Full access