Emeis, S.

Foreman, R.

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Estimation of the humidity flux at FINO 1(North Sea)Richard Foreman, Stefan EmeisInstitute for Meteorology and Climate Research.Karlsruhe Institute of Technology (KIT).Garmisch-Partenkirchen, GermanyOutlineMotivationHumidity effects at the North Sea - FINO 1Estimate humidity fluxes - original ideaEstimate humidity fluxes - new ideaConclusionsImportance of humidity for marine conditionsSempreviva & Gryning (1996) - Humidity Fluctuations in theMarine Boundary Layer measured at a coastal site...I Bowen Ratio =cp<wT>Lv<wq>I Marine Conditions: (Bowen Ratio ≈ 0.1);Motivation - Importance of humidity for stabilityI Monin-Obukhov stability parameter: zL =zkg<wTv>Tvu3∗I Virtual potential heat flux can be estimated from:< wTv >=< wT > +0.61T < wq >I An ultrasonic anemometer measures:< wTs >=< wT > +0.51T < wq >I In dry and/or cold conditions, < wq > = humidity flux andcan be neglected: hence < wTv >≈< wTs >.I For example, Andreas et al. (2006) - Evaluations of the vonKarman constant in the atmospheric surface layer, assume< wTv >=< wTs > in arctic conditions.I What then is the magnitude of < wq > at FINO 1?FINO 1 - Measurement platform in the North SeaI In operation since 2003, approx. 45km North of BorkumIsland.I Cup anemometers, wind vanes, temperature sensors,ultrasonic anemometers, hygrometers at multiple levels.I Data presented here from 2005.Humidity at FINO 1 - hygrometers?I Above 50m – Negative humidity flux for stable conditions,positive humidity flux for neutral & unstable conditions.I It is unclear whether fluxes driven by local gradients above50m, or by non-local gradients? Here, sea surface relativehumidity assumed = 100%.Estimate humidity fluxes - original idea:I Assume < wTs >=< wT > +0.51T < wq >.I There, use Ri = zL , valid in unstable conditions, whereRi ≈ −κgz<wT>Tu3∗I Rearranging gives: < wT >= −RiTu3∗kzgI Use humidity dependent sonic measurement to estimate thehumidity flux: < wq >= 10.51T(< wTs > − < wT >).I The humidity flux can then supposedly be estimated as aresidual between the sonic measured heat flux and the heatflux based on a local gradient.Results:I Average humidity flux:< wq > = 10.51T(< wTs > − < wT >) ≈ −2x10−4.I Typical humidity flux (Sempreviva & Gryning, 1996):+3x10−5.Heat Flux - Profile RelationshipI Local heat fluxes are poorly correlated with local temperaturegradients. (Correlation coefficient: -0.06)I Using local gradients to infer fluxes not valid, hence Ri 6= zL .Heat Flux - Profile RelationshipI Heat fluxes are driven by non-local temperature gradients.(Correlation coefficient: -0.47)Example: Temperature profileI At 60m: Locally stable, non-locally unstable.I Heat flux, < wTs >= +0.013 (m/s.K)Estimation of temperature and humidity fluxes - new ideaestimatedhumidity fluxI If humidity flux were zero, averaged heat flux would passthrough the origin when plotted against the potentialtemperature.I Therefore, since < wTs >=< wT > +0.51T < wq >, then< wTs > (∆θ = 0) = 0.51T < wq >.Estimated humidity fluxesI Estimated humidity fluxes very weakly correlated with bothfine humidity gradients and (shown here) bulk differences.I Humidity flux a function of larger atmospheric scales? (ref?)Annual variation of humidity flux estimate:I October-February: < wq > ≈ 0I April-September: < wq > ≈ +1x10−5. (Sempreviva &Gryning, 1996, June, < wq >= +3x10−5)Estimate humidity fluxes - Anholt (Sempreviva & Gryning,1996)I Estimation possibly overestimates humidity flux by factor of 2?ConclusionsI Precise calculation of stability of the marine boundary layerrequires direct measurement of the humidity flux since it maynot be governed by local gradients.I Currently, there is no way of directly measuring humidityfluxes at FINO 1.I Estimates indicate that humidity flux is roughly 1x10−5(m/s.kg/kg) during the middle of the year, but could beneglected in colder months.AcknowledgementsI We acknowledge the reception of valuable data and advicefrom Dr. Anna-Maria Sempreviva which has aided this workgreatly.I The work was done in the project VERITAS (work package 5of OWEA) which is funded by the German Ministry of theEnvironment (BMU) via the PTJ (FKZ 0325060).

Estimation of offshore humidity fluxes from sonic and mean temperature profile data

https://publikationen.bibliothek.kit.edu/230076943/3815546

Estimation of offshore humidity fluxes from sonic and mean temperature profile data

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