ship-board radiometric measurements of the air-sea temperature difference p. j. minnett, a. chambers...

Earth Observation for Ocean-Atmosphere Interactions Science Frascati, October 2014

Ship-board radiometric measurements of the air-sea

temperature difference

P. J. Minnett, A. Chambers & N. Perlin

Rosenstiel School of Marine and Atmospheric Science,

University of Miami, USA


Outline

• Conventional measurements vs radiometric measurements

• Marine-Atmospheric Emitted Radiance Interferometer

• Properties of air-sea ∆T• Representation of air-sea ∆T in models,

ECMWF and WRF• Summary and conclusions


Background

The air-sea temperature difference (ΔT) is important in controlling the stability of the lower atmosphere and the efficiency of transfers of heat, moisture and gases from ocean to atmosphere.


Outline


• Marine - Atmospheric Emitted Radiance Interferometer




Conventional measurements

Thermometers in the air and in the sub-surface ocean.– Different fluids with different thermal capacities.– Different thermometers with different calibration

histories.– Air-temperature thermometer, in particular,

susceptibility to solar heating.• Often necessary to discard daytime measurements.


Radiometric measurements

• Using an infrared spectroradiometer, can measure the skin SST and the air-temperature with a single instrument with real-time internal calibration.

• Skin SST measured at wavelengths where the atmosphere is relatively transparent.

• Air temperatures measurements made where the atmosphere is much less transparent (CO2 emission).


Outline






Marine-Atmospheric Emitted Radiance Interferometer


Measured spectra of atmospheric and sea-surface emission

Skin SSTNear-surface

Air Temperature


Outline





Differences between conventional and radiometric air-sea ∆T


RadiometricConventional

Radiometric air-sea temperature difference K

Con

vent

iona

l air

-sea

tem

pera

ture

dif

fere

nce

K


Long duration cruise data• M-AERIs have been mounted on research ships to

sample a very wide range of conditions.• M-AERIs have been mounted on the Royal Caribbean

Cruise Lines ship Explorer of the Seas from 2000-2006.

• M-AERIs are installed where they have a clear view of the sea-surface ahead of the bow wave, and of the sky.


Histograms of air-sea ∆TExplorer of the Seas


Histograms of air-sea ∆T


R/V Atlantis, 2012


Global distributions


Possible feedbacksNight-time

Initial disturbance warms the skin SST

Initial disturbance cools the

skin SST


Outline






Representation of air-sea ∆T in modelsM-AERI air-sea ∆T measured in equatorial Pacific from R/V Mirai

ECMWF air-sea ∆T in equatorial Pacific interpolated to positions of R/V Mirai

U < 3ms-1

U < 3ms-1

3ms-1 < U < 5ms-1

3ms-1 < U < 5ms-1

5ms-1 < U < 9ms-1

5ms-1 < U

Comparison is best for low winds.

ECMWF distribution is wider, with more large – ve values.

For U > 3ms-1 mean ∆T ~ 1.5 - 2x measured mean.

Cases of ∆T > 0 are absent.


WRF simulations in Agulhas Current area

• SST provided by NOAA 0.25o daily OI.

• July 2002

• Air-sea ∆T follows SST perturbations

• Distribution shows many air-sea ∆T > 0


Measurements in Agulhas Current area

Cruise area and time are different to WRF simulations, but both are in similar conditions.

Histogram of measured air-sea ∆T is similar to “standard” form with very few ∆T > 0.


Outline






Summary and conclusions• Infrared hyperspectral measurements from ships provide information on

important aspects of air-sea ∆T.• Histograms of air-sea ∆T show very similar form irrespective of region,

season and SST.• Form of histograms imply dominance of negative feedbacks controlling air-

sea ∆T.• Very few cases of ∆T > 0; limited to areas downwind of coasts, and associated

with atmospheric fronts and SST fronts.• Some aspects of air-sea ∆T histograms reasonably well represented in models,

but as winds increase, width of modeled air-sea ∆T’s increase cf measurements (ECMWF, Equatorial W. Pacific).

• Air-sea ∆T’s appear to be poorly represented in models of conditions with high SST variability (WRF, Agulhas Current area).

• Infrared hyperspectral measurements have the potential to “recalibrate” the quantitative understanding of air-sea temperature difference and the exchanges that depend on them – implications for remote sensing of air-sea fluxes.


Acknowledgements:

• Many students and colleagues who have facilitated or participated in the ship-board campaigns.

• Captains, officers and crews of many ships.• Funding from NASA, NOAA, NSF.

ship-board radiometric measurements of the air-sea temperature difference p. j. minnett, a. chambers...

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