113 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
The International Workshop on Wave Hindcasting and Forecasting and the Coastal Hazards Symposium
Following the 13th International Workshop on Wave Hindcasting and Forecasting
and 4th Coastal Hazards Symposium in October 2013 in Banff, Canada, a topical
collection has appeared in recent issues of Ocean Dynamics. Here we give a
brief overview of the history of the conference since its inception in 1986 and
of the progress made in the fields of wind-generated ocean waves and the
modelling of coastal hazards before we summarize the main results of the papers
that have appeared in the topical collection
Wind-induced drift of objects at sea: the leeway field method
A method for conducting leeway field experiments to establish the drift
properties of small objects (0.1-25 m) is described. The objective is to define
a standardized and unambiguous procedure for condensing the drift properties
down to a set of coefficients that may be incorporated into existing stochastic
trajectory forecast models for drifting objects of concern to search and rescue
operations and other activities involving vessels lost at sea such as
containers with hazardous material.
An operational definition of the slip or wind and wave-induced motion of a
drifting object relative to the ambient current is proposed. This definition
taken together with a strict adherence to 10 m wind speed allows us to refer
unambiguously to the leeway of a drifting object. We recommend that all objects
if possible be studied using what we term the direct method, where the object's
leeway is studied directly using an attached current meter.
We divide drifting objects into four categories, depending on their size. For
the smaller objects (less than 0.5 m), an indirect method of measuring the
object's motion relative to the ambient current must be used. For larger
objects, direct measurement of the motion through the near-surface water masses
is strongly recommended. Larger objects are categorized according to the
ability to attach current meters and wind monitoring systems to them.
The leeway field method proposed here is illustrated with results from field
work where three objects were studied in their distress configuration; a 1:3.3
sized model of a 40-ft Shipping container, a World War II mine and a 220 l
(55-gallon) oil drum.Comment: 33 pages, 12 figures, 3 table
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