4,911 research outputs found
Wave Extremes in the North East Atlantic from Ensemble Forecasts
A method for estimating return values from ensembles of forecasts at advanced
lead times is presented. Return values of significant wave height in the
North-East Atlantic, the Norwegian Sea and the North Sea are computed from
archived +240-h forecasts of the ECMWF ensemble prediction system (EPS) from
1999 to 2009.
We make three assumptions: First, each forecast is representative of a
six-hour interval and collectively the data set is then comparable to a time
period of 226 years. Second, the model climate matches the observed
distribution, which we confirm by comparing with buoy data. Third, the ensemble
members are sufficiently uncorrelated to be considered independent realizations
of the model climate. We find anomaly correlations of 0.20, but peak events
(>P97) are entirely uncorrelated. By comparing return values from individual
members with return values of subsamples of the data set we also find that the
estimates follow the same distribution and appear unaffected by correlations in
the ensemble. The annual mean and variance over the 11-year archived period
exhibit no significant departures from stationarity compared with a recent
reforecast, i.e., there is no spurious trend due to model upgrades.
EPS yields significantly higher return values than ERA-40 and ERA-Interim and
is in good agreement with the high-resolution hindcast NORA10, except in the
lee of unresolved islands where EPS overestimates and in enclosed seas where it
is biased low. Confidence intervals are half the width of those found for
ERA-Interim due to the magnitude of the data set.Comment: 27 pp, 10 figures, J Climate (in press
Nearshore wave forecasting and hindcasting by dynamical and statistical downscaling
A high-resolution nested WAM/SWAN wave model suite aimed at rapidly
establishing nearshore wave forecasts as well as a climatology and return
values of the local wave conditions with Rapid Enviromental Assessment (REA) in
mind is described. The system is targeted at regions where local wave growth
and partial exposure to complex open-ocean wave conditions makes diagnostic
wave modelling difficult.
SWAN is set up on 500 m resolution and is nested in a 10 km version of WAM. A
model integration of more than one year is carried out to map the spatial
distribution of the wave field. The model correlates well with wave buoy
observations (0.96) but overestimates the wave height somewhat (18%, bias 0.29
m).
To estimate wave height return values a much longer time series is required
and running SWAN for such a period is unrealistic in a REA setting. Instead we
establish a direction-dependent transfer function between an already existing
coarse open-ocean hindcast dataset and the high-resolution nested SWAN model.
Return values are estimated using ensemble estimates of two different
extreme-value distributions based on the full 52 years of statistically
downscaled hindcast data. We find good agreement between downscaled wave height
and wave buoy observations. The cost of generating the statistically downscaled
hindcast time series is negligible and can be redone for arbitrary locations
within the SWAN domain, although the sectors must be carefully chosen for each
new location.
The method is found to be well suited to rapidly providing detailed wave
forecasts as well as hindcasts and return values estimates of partly sheltered
coastal regions.Comment: 20 pages, 7 figures and 2 tables, MREA07 special issue on Marine
rapid environmental assessmen
Improving the estimation of zenith dry tropospheric delays using regional surface meteorological data
Global Navigation Satellite Systems (GNSS) are emerging as possible tools for remote sensing high-resolution atmospheric water vapour that improves weather forecasting through numerical weather prediction models. Nowadays, the GNSS-derived tropospheric zenith total delay (ZTD), comprising zenith dry delay (ZDD) and zenith wet delay (ZWD), is achievable with sub-centimetre accuracy. However, if no representative near-site meteorological information is available, the quality of the ZDD derived from tropospheric models is degraded, leading to inaccurate estimation of the water vapour component ZWD as difference between ZTD and ZDD. On the basis of freely accessible regional surface meteorological data, this paper proposes a height-dependent linear correction model for a priori ZDD. By applying the ordinary least-squares estimation (OLSE), bootstrapping (BOOT), and leave-one-out cross-validation (CROS) methods, the model parameters are estimated and analysed with respect to outlier detection. The model validation is carried out using GNSS stations with near-site meteorological measurements. The results verify the efficiency of the proposed ZDD correction model, showing a significant reduction in the mean bias from several centimetres to about 5 mm. The OLSE method enables a fast computation, while the CROS procedure allows for outlier detection. All the three methods produce consistent results after outlier elimination, which improves the regression quality by about 20% and the model accuracy by up to 30%
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Enhanced seasonal forecast skill following stratospheric sudden warmings
Advances in seasonal forecasting have brought widespread
socio-economic benefits. However, seasonal forecast skill
in the extratropics is relatively modest, prompting the
seasonal forecasting community to search for additional
sources of predictability. For over a decade it has been
suggested that knowledge of the state of the stratosphere
can act as a source of enhanced seasonal predictability; long-lived circulation anomalies in the lower stratosphere that follow stratospheric sudden warmings are associated with circulation anomalies in the troposphere that can last up to two months. Here, we show by performing retrospective
ensemble model forecasts that such enhanced predictability
can be realized in a dynamical seasonal forecast system with
a good representation of the stratosphere. When initialized at the onset date of stratospheric sudden warmings, the model forecasts faithfully reproduce the observed mean tropospheric conditions in the months following the stratospheric sudden warmings. Compared with an equivalent set of forecasts that are not initialized during stratospheric sudden warmings, we document enhanced forecast skill for atmospheric circulation patterns, surface temperatures over northern Russia and eastern Canada and North Atlantic precipitation. We suggest
that seasonal forecast systems initialized during stratospheric sudden warmings are likely to yield significantly greater forecast skill in some regions
Bootstrap based uncertainty bands for prediction in functional kriging
The increasing interest in spatially correlated functional data has led to
the development of appropriate geostatistical techniques that allow to predict
a curve at an unmonitored location using a functional kriging with external
drift model that takes into account the effect of exogenous variables (either
scalar or functional). Nevertheless uncertainty evaluation for functional
spatial prediction remains an open issue. We propose a semi-parametric
bootstrap for spatially correlated functional data that allows to evaluate the
uncertainty of a predicted curve, ensuring that the spatial dependence
structure is maintained in the bootstrap samples. The performance of the
proposed methodology is assessed via a simulation study. Moreover, the approach
is illustrated on a well known data set of Canadian temperature and on a real
data set of PM concentration in the Piemonte region, Italy. Based on the
results it can be concluded that the method is computationally feasible and
suitable for quantifying the uncertainty around a predicted curve.
Supplementary material including R code is available upon request
Assessing the skill of precipitation and temperature seasonal forecasts in Spain: windows of opportunity related to ENSO events
1. The skill of state-of-the-art operational seasonal forecast models in extratropical latitudes is assessed using a multimodel ensemble from the Development of a European Multimodel Ensemble System for Seasonalto- Interannual Prediction (DEMETER) project. In particular, probabilistic forecasts of surface precipitation and maximum temperature in Spain are analyzed using a high-resolution observation gridded dataset (Spain02). To this aim, a simple statistical test based on the observed and predicted tercile anomalies is used. First, the whole period 1960–2000 is considered and it is shown that the only significant skill is found for dry events in autumn. Then, the influence of ENSO events as a potential source of conditional predictability is studied and the validation to strong La Niña or El Niño periods is restricted. Skillful seasonal predictions are found in partial agreement with the observed teleconnections derived from the historical records. On the one hand, predictability is found in spring related to El Niño events for dry events over the south and the Mediterranean coast and for hot events in the southeast areas. In contrast, La Niña drives predictability in winter for dry events over the western part and for hot events in summer over the south and the Mediterranean coast. This study considers both the direct model outputs and the postprocessed predictions obtained using a statistical downscaling method based on analogs. In general, the use of the downscaling method outperforms the direct output for precipitation, whereas in the case of the temperature no improvement is obtained
Evaluating the ENVI-met microscale model for suitability in analysis of targeted urban heat mitigation strategies
Microscale atmospheric models are increasingly being used to project the thermal benefits of urban heat mitigation strategies (e.g., tree planting programs or use of high-albedo materials). However, prior to investment in specific mitigation efforts by local governments, it is desirable to test and validate the computational models used to evaluate strategies. While some prior studies have conducted limited evaluations of the ENVI-met microscale climate model for specific case studies, there has been relatively little systematic testing of the model's sensitivity to variations in model input and control parameters. This study builds on the limited foundation of past validation efforts by addressing two questions: (1) is ENVI-met grid independent; and (2) can the model adequately represent the air temperature perturbations associated with heat mitigation strategies? To test grid independence, a “flat” domain is tested with six vertical grid resolutions ranging from 0.75 to 2.0 m. To examine the second question, a control and two mitigation strategy simulations of idealized city blocks are tested. Results show a failure of grid independence in the “flat” domain simulations. Given that the mitigation strategies result in temperature changes that are an order of magnitude larger than the errors introduced by grid dependence for the flat domain, a lack of grid independence itself does not necessarily invalidate the use of ENVI-met for heat mitigation research. However, due to limitations in grid structure of the ENVI-met model, it was not possible to test grid dependence for more complicated simulations involving domains with buildings. Furthermore, it remains unclear whether existing efforts at model validation provide any assurance that the model adequately captures vertical mixing and exchange of heat from the ground to rooftop level. Thus, there remain concerns regarding the usefulness of the model for evaluating heat mitigation strategies, particularly when applied at roof level (e.g. high albedo or vegetated roofs)
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