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H. Hersbach
and
P. A. E. M. Janssen

Abstract

The physics of wind input in the Wave Ocean Model (WAM) cycle 4 is based on scaling with the friction velocity u∗. This implies that in the case of fetch-limited wind-wave growth, universal scaling laws should follow if fetch and wave variance are scaled by means of u∗. For operational applications, such as at the European Centre for Medium-Range Weather Forecasts, the scaling of the WAM model with u∗ is well satisfied. Recently, however, it was found that this scaling is violated for very short waves at small fetches and durations, for which the model is run with very small grid spacings, a very small time step, and a large cutoff frequency. This violation of u∗ scaling, which is a serious problem for implementation on a small lake, was found to be caused by a too severe limit on the increments of the wave spectrum per time step. In this article, an alternative formulation for the limitation of spectral component growth is suggested, which does not violate u∗ scaling and, in addition, gives rise to excellent results over a large range of scaled quantities. At the same time, growth curves for wave height and peak frequency hardly depend on the time step.

Full access
M. Stiassnie
,
Y. Agnon
, and
P. A. E. M. Janssen

Abstract

A solution of Rayleigh’s instability equation, which circumvents the apparent critical-layer singularity, is provided. The temporal and spatial growth rates of water waves exposed to a logarithmic wind profile are calculated and discussed. The findings are similar to previously published results, except for shear velocity–to–wave celerity ratios larger than 2, where the newly calculated growth rates start to decrease after having reached a distinct maximum. The ratio of the spatial to temporal growth rates is examined. It is shown to deviate by up to 20% from the leading-order value of 2. The implications of the growth rate to the modal distributions of energy input from wind to waves, for young and mature seas, and in temporal/spatial growth scenarios, are analyzed.

Full access
G. Kraan
,
W. A. Oost
, and
P. A. E. M. Janssen

Abstract

With video equipment and image processing software the authors have measured the whitecap coverage of the sea surface near the Dutch coast. Reality proved the initial idea about the simplicity of this way of whitecap measurement to be overly optimistic. A model was developed that produces a theoretical estimate of the whitecap percentage as a function of the wave age, which in turn depends on the wave peak frequency and the friction velocity in the air. The comparison between the measured whitecap coverage and the model results shows an order of magnitude correspondence for most data. A group of outliers appears to be connected to the strength of the current.

Full access
C. Mastenbroek
,
G. Burgers
, and
P. A. E. M. Janssen

Abstract

The effect of a wave-dependent drag coefficient on the generation of storm surge in the North Sea is studied. To this end, a barotropic storm surge model is driven by stress that explicitly depend on the ocean waves. To estimate the effects of waves on the boundary layer the theory of Janssen is used. In this theory the aerodynamic drag depends on the fraction of the stress carried by the waves. For waves limited in growth by time, fetch, or depth this gives an enhancement of the drag. The importance to surges of radiation stress is also investigated.

The coupled wave and storm surge models have been tested for three recent storm periods. The calculations with a Smith and Banke stress relation underestimate the surges by 20%. The calculations with the wave-dependent drag give a significant improvement. When corrected for the effects of an external surge, the storm surge caused by a fast moving depression was overestimated slightly. In this case, the generation of the storm surge was dominated by relatively young waves. In the other two cases, the wave-dependent stress reproduced the overall level of the surges within a few percent. The radiation stress increased the surge some 5% during one storm, but the effect was negligible during the other two storms.

Most of the improvements can be reproduced by assuming an overall increase of the dimensionless roughness parameter. If the Smith and Banke relation is replaced by a Charnock one with a dimensionless constant of α = 0.032, the difference in water level between this formulation and the wave-dependent calculation is smaller than the uncertainty in the observations. Different basins would, however, give rise to different choices of α. Therefore, a wave-dependent drag is to be preferred for storm surge modeling.

Full access
L. Cavaleri
,
A. Benetazzo
,
F. Barbariol
,
J.-R. Bidlot
, and
P. A. E. M. Janssen

Abstract

In a parallel paper mainly focused on the meteorological and oceanographic aspects, the conditions were described for the storm during which the iconic Draupner wave was recorded. Because of increased spatial resolution and improved model physics, the results provided new and previously unrecognized features of the storm, in particular of the wave spectra, features relevant for assessing the wave’s conditions nearby the Draupner platform. Starting from these, and after briefly summarizing the relevant information, the focus of this paper is on the nonlinear analysis of the local situation, with the main purpose of assessing if and how the conditions existed for the possible appearance of very large waves. An intensive analysis of the related probability is carried out, attacking the problem with two different statistical approaches, both briefly described: a completely new one working from the point of view of envelope heights, and a recent, though established, one based on space–time extreme waves. It is remarkable, and certainly supports this line of work, that the two different approaches lead independently to consistent results, supporting the idea, already derived from the meteo-oceanographic hindcast, that the wave conditions were indeed special at the position of the Draupner platform. This is related to a general analysis of high waves showing, also on the basis of 3D (2D space + time) measured wave data at open sea, how, given the severe sea state, the Draupner wave features represent what is expected at certain times and positions as the natural documented temporal evolution of wave groups.

Full access
H. Bonekamp
,
G. J. Komen
,
A. Sterl
,
P. A. E. M. Janssen
,
P. K. Taylor
, and
M. J. Yelland

Abstract

The surface-drag coefficients of two versions of the ECMWF's atmosphere–wave model are compared with those of uncoupled model versions and with those of inertial-dissipation measurements in the open ocean made by the RRS Discovery. It is found that the mean drag resulting from the latest coupled version is on average equal to that of the uncoupled version. However, both have a positive bias when compared with the RRS Discovery observations. This bias is discussed, also in the light of other observational open ocean data. In the second part of the paper, bulk parameterizations with and without parameters of collocated sea-state data are validated against the Discovery observations. Using published estimates of the error in friction velocity and the neutral 10-m winds, all bulk parameterizations score low on goodness-of-fit tests. The lowest scores are obtained for the constant Charnock parameter case, whereas the highest scores are obtained for a wave-age-dependent parameterization. On–off experiments are made for the corrections to the inertial-dissipation data that have been proposed in previous studies. These corrections concern the measurement height and the direct wave-induced turbulence in the lower atmosphere. The first correction results in a slightly better agreement, but the second reduces the goodness-of-fit of the bulk parameterizations.

Full access
G. Janssens-Maenhout
,
B. Pinty
,
M. Dowell
,
H. Zunker
,
E. Andersson
,
G. Balsamo
,
J.-L. Bézy
,
T. Brunhes
,
H. Bösch
,
B. Bojkov
,
D. Brunner
,
M. Buchwitz
,
D. Crisp
,
P. Ciais
,
P. Counet
,
D. Dee
,
H. Denier van der Gon
,
H. Dolman
,
M. R. Drinkwater
,
O. Dubovik
,
R. Engelen
,
T. Fehr
,
V. Fernandez
,
M. Heimann
,
K. Holmlund
,
S. Houweling
,
R. Husband
,
O. Juvyns
,
A. Kentarchos
,
J. Landgraf
,
R. Lang
,
A. Löscher
,
J. Marshall
,
Y. Meijer
,
M. Nakajima
,
P. I. Palmer
,
P. Peylin
,
P. Rayner
,
M. Scholze
,
B. Sierk
,
J. Tamminen
, and
P. Veefkind

Abstract

Under the Paris Agreement (PA), progress of emission reduction efforts is tracked on the basis of regular updates to national greenhouse gas (GHG) inventories, referred to as bottom-up estimates. However, only top-down atmospheric measurements can provide observation-based evidence of emission trends. Today, there is no internationally agreed, operational capacity to monitor anthropogenic GHG emission trends using atmospheric measurements to complement national bottom-up inventories. The European Commission (EC), the European Space Agency, the European Centre for Medium-Range Weather Forecasts, the European Organisation for the Exploitation of Meteorological Satellites, and international experts are joining forces to develop such an operational capacity for monitoring anthropogenic CO2 emissions as a new CO2 service under the EC’s Copernicus program. Design studies have been used to translate identified needs into defined requirements and functionalities of this anthropogenic CO2 emissions Monitoring and Verification Support (CO2MVS) capacity. It adopts a holistic view and includes components such as atmospheric spaceborne and in situ measurements, bottom-up CO2 emission maps, improved modeling of the carbon cycle, an operational data-assimilation system integrating top-down and bottom-up information, and a policy-relevant decision support tool. The CO2MVS capacity with operational capabilities by 2026 is expected to visualize regular updates of global CO2 emissions, likely at 0.05° x 0.05°. This will complement the PA’s enhanced transparency framework, providing actionable information on anthropogenic CO2 emissions that are the main driver of climate change. This information will be available to all stakeholders, including governments and citizens, allowing them to reflect on trends and effectiveness of reduction measures. The new EC gave the green light to pass the CO2MVS from exploratory to implementing phase.

Free access
G. Janssens-Maenhout
,
B. Pinty
,
M. Dowell
,
H. Zunker
,
E. Andersson
,
G. Balsamo
,
J.-L. Bézy
,
T. Brunhes
,
H. Bösch
,
B. Bojkov
,
D. Brunner
,
M. Buchwitz
,
D. Crisp
,
P. Ciais
,
P. Counet
,
D. Dee
,
H. Denier van der Gon
,
H. Dolman
,
M. R. Drinkwater
,
O. Dubovik
,
R. Engelen
,
T. Fehr
,
V. Fernandez
,
M. Heimann
,
K. Holmlund
,
S. Houweling
,
R. Husband
,
O. Juvyns
,
A. Kentarchos
,
J. Landgraf
,
R. Lang
,
A. Löscher
,
J. Marshall
,
Y. Meijer
,
M. Nakajima
,
P. I. Palmer
,
P. Peylin
,
P. Rayner
,
M. Scholze
,
B. Sierk
,
J. Tamminen
, and
P. Veefkind
Full access
J. Vilà-Guerau de Arellano
,
O. K. Hartogensis
,
H. de Boer
,
R. Moonen
,
R. González-Armas
,
M. Janssens
,
G. A. Adnew
,
D. J. Bonell-Fontás
,
S. Botía
,
S. P. Jones
,
H. van Asperen
,
S. Komiya
,
V. S. de Feiter
,
D. Rikkers
,
S. de Haas
,
L. A. T. Machado
,
C. Q. Dias-Junior
,
G. Giovanelli-Haytzmann
,
W. I. D. Valenti
,
R. C. Figueiredo
,
C. S. Farias
,
D. H. Hall
,
A. C. S. Mendonça
,
F. A. G. da Silva
,
J. L. Marton da Silva
,
R. Souza
,
G. Martins
,
J. N. Miller
,
W. B. Mol
,
B. Heusinkveld
,
C. C. van Heerwaarden
,
F. A. F. D’Oliveira
,
R. Rodrigues Ferreira
,
R. Acosta Gotuzzo
,
G. Pugliese
,
J. Williams
,
A. Ringsdorf
,
A. Edtbauer
,
C. A. Quesada
,
B. Takeshi Tanaka Portela
,
E. Gomes Alves
,
C. Pöhlker
,
S. Trumbore
,
J. Lelieveld
, and
T. Röckmann

Abstract

How are rainforest photosynthesis and turbulent fluxes influenced by clouds? To what extent are clouds affected by local processes driven by rainforest energy, water and carbon fluxes? These interrelated questions were the main drivers of the intensive field experiment CloudRoots-Amazon22 which took place at the ATTO/Campina supersites in the Amazon rainforest during the dry season, in August 2022. CloudRoots-Amazon22 collected observational data to derive cause-effect relationships between processes occurring at the leaf-level up to canopy scales in relation to the diurnal evolution of the clear-to-cloudy transition. First, we studied the impact of cloud and canopy radiation perturbations on the sub-diurnal variability of stomatal conductance. Stoma opening is larger in the morning, modulated by the cloud optical thickness. Second, we combined 1 Hz-frequency measurements of the stable isotopologues of carbon dioxide and water vapor with measurements of turbulence to determine carbon dioxide and water vapor sources and sinks within the canopy. Using scintillometer observations, we inferred 1-minute sensible heat flux that responded within minutes to the cloud passages. Third, collocated profiles of state variables and greenhouse gases enabled us to determine the role of clouds in vertical transport. We then inferred, using canopy and upper-atmospheric observations and a parameterization, the cloud cover and cloud mass flux to establish causality between canopy and cloud processes. This shows the need of comprehensive observational set to improve weather and climate model representations. Our findings contribute to advance our knowledge of the coupling between cloudy boundary layers and primary carbon productivity of the Amazon rainforest.

Open access