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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/amt-2017-167
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
22 Jun 2017
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Measurement Techniques (AMT).
Retrieval of an Ice Water Path over the Ocean from ISMAR and MARSS millimeter/submillimeter brightness temperatures
Manfred Brath1, Stuart Fox2, Patrick Eriksson3, R. Chawn Harlow2, Martin Burgdorf1, and Stefan A. Buehler1 1Meteorologisches Institut, Fachbereich Geowissenschaften, Centrum für Erdsystem- und Nachhaltigkeitsforschung (CEN), Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
2Met Office, FitzRoy Road, Exeter, UK, EX1 3PB
3Department of Space, Earth and Environment, Chalmers University of Technology, 41296 Gothenburg, Sweden
Abstract. A neural network (NN)-based retrieval method to determine the snow ice water path (SIWP), liquid water path (LWP), and integrated water vapor (IWV) from millimeter and sub-millimeter brightness temperatures, measured by using airborne radiometers (ISMAR and MARSS), is presented. The NNs were trained by using atmospheric profiles from the ICON numerical weather prediction (NWP) model and by radiative transfer simulations using the Atmospheric Radiative Transfer Simulator (ARTS). The basic performance of the retrieval method was analyzed in terms of offset (bias) and the median fractional error (MFE), and the benefit of using submillimeter channels was studied in comparison to pure microwave retrievals. The retrieval is offset free for SIWP > 0.01 kg/m2, LWP > 0.1 kg/m2 and IWV > 3 kg/m2. The MFE of SIWP decreases from 100 % at SIWP = 0.01 kg/m2 to 20 % at SIWP = 1 kg/m2 and the MFE of LWP from 100 % at LWP = 0.05 kg/m2 to 30 % at LWP = 1 kg/m2. The MFE of IWV for IWV > 3 kg/m2 is 5 % to 8 %. The SIWP retrieval strongly benefits from sub-millimeter channels, which reduce the MFE by a factor of two, compared to pure microwave retrievals. The IWV and the LWP retrieval also benefit from sub-millimeter channels, albeit to a lesser degree. The retrieval was applied to ISMAR and MARSS brightness temperatures from FAAM flight B897 on 18 March 2015 of a precipitating frontal system west of the coast of Iceland. Considering the given uncertainties, the retrieval is in reasonable agreement with the SIWP, LWP, and IWV values simulated by the ICON NWP model for that flight. A comparison of the retrieved IWV with IWV from 12 dropsonde 15 measurements shows an offset of 0.5 kg/m2 and an rms difference of 0.8 kg/m2, showing that the retrieval of IWV is highly effective even under cloudy conditions.

Citation: Brath, M., Fox, S., Eriksson, P., Harlow, R. C., Burgdorf, M., and Buehler, S. A.: Retrieval of an Ice Water Path over the Ocean from ISMAR and MARSS millimeter/submillimeter brightness temperatures, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-167, in review, 2017.
Manfred Brath et al.
Manfred Brath et al.
Manfred Brath et al.

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Short summary
A method to estimate the amounts of ice, liquid water, and water vapor from aircraft radiation measurements at wavelengths just over and under 1mm is presented and its performance is estimated. The method uses an ensemble of artificial neural networks. It strongly benefits from the sub-millimeter frequencies reducing the error for the estimated amount of ice by a factor two compared to a traditional microwave method. The method was applied to measurement of a precipitating frontal system.
A method to estimate the amounts of ice, liquid water, and water vapor from aircraft radiation...
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