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Climatologically significant effects of some approximations in the bulk parameterizations of turbulent air-sea fluxes
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
2016 (English)In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485Article in journal (Refereed) Epub ahead of print
Abstract [en]

This paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulae on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments are examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the 1982-2014 period are computed using SST and surface meteorological state variables from the ERA-Interim reanalysis. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulae. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically of the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%), but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the salt-related reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity, lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid- and subpolar latitudes.

Place, publisher, year, edition, pages
2016.
Keyword [en]
Turbulent fluxes, parameterisation, general circulation model, error, climatology
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-134784DOI: 10.1175/JPO-D-16-0169.1OAI: oai:DiVA.org:su-134784DiVA: diva2:1038490
Available from: 2016-10-18 Created: 2016-10-18 Last updated: 2016-10-24
In thesis
1. The role of high-latitude circulation and moisture transport in Arctic climate variability and change during winter
Open this publication in new window or tab >>The role of high-latitude circulation and moisture transport in Arctic climate variability and change during winter
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis examines the connections between atmospheric circulation in the high-latitudes, northward moisture transport, and Arctic climate variability and change during winter. An event based approach is taken by objectively defining phenomena termed “moisture intrusions” -- filamentary flows of anomalously moist air which originate at 70°N and cross the entire Arctic basin. They typically emanate from within the poleward advecting branches of mid-latitude cyclones held in place by blocking patterns to the east. Moisture intrusions contribute only a minority of the total northward moisture transport at 70°N, yet drive a significant proportion of the inter annual variability in surface temperature and downward longwave radiation over the entire polar cap. A positive trend in the frequency of these events, in response to a moistening of the atmosphere, is shown to have driven approximately 45% and 35% of the observed warming and sea ice decline in the Barents Sea during Dec-Jan over the past two decades. Moisture intrusions act to erode the temperature inversion and thus contribute to bottom amplified warming even in the absence of sea ice loss. Negative sea ice anomalies induced by intrusions persist for up to weeks at a time -- promoting upward turbulent heat fluxes and further bottom amplified warming. Systematic biases in the statistics of moisture intrusions are discovered in the CMIP5 models. The biases are predominantly a result of misrepresentation of the intense moisture fluxes and are almost entirely due to biases in the meridional velocity. Moisture intrusion biases explain only about 17% of the temperature bias in the Atlantic sector. The predicted biases, while small in amplitude, are very highly correlated with the true biases in the models however, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback. A analysis of the uncertainties in computed turbulent air-sea flux (TASF) climatologies arising due to the parameterisation of bulk formulae is also presented. TASF climatologies are computed over a series of sensitivity experiments using surface state variables from ERA-Interim. The largest source of uncertainty is related to the computation of the transfer coefficients and hence the choice of bulk algorithm itself. The majority of parameter approximations have small impacts when tested individually, but can lead to large disagreements when implemented in tandem.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2016. 38 p.
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-134787 (URN)978-91-7649-575-9 (ISBN)978-91-7649-576-6 (ISBN)
Public defence
2016-12-02, Högbomsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2016-11-09 Created: 2016-10-18 Last updated: 2016-11-16Bibliographically approved

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