Reconstruction of high-resolution time series from slow-response broadband terrestrial irradiance measurements by deconvolution

Abstract

Abstract. Broadband solar and terrestrial irradiance measurements of high temporal resolution are needed to study inhomogeneous clouds or surfaces and to derive vertical profiles of heating/cooling rates at cloud top. An efficient method to enhance the temporal resolution of slow-response measurements of broadband terrestrial irradiance using pyrgeometer is introduced. It is based on the deconvolution theorem of Fourier transform to restore amplitude and phase shift of high frequent fluctuations. It is shown that the quality of reconstruction depends on the instrument noise, the pyrgeometer response time and the frequency of the oscillations.The method is tested in laboratory measurements for synthetic time series including a boxcar function and periodic oscillations using a CGR-4 pyrgeometer with response time of 3 s. The originally slow-response pyrgeometer data were reconstructed to higher resolution and compared to the predefined synthetic time series. The reconstruction of the time series worked up to oscillations of 0.5 Hz frequency and 2 W m −2 amplitude if the sampling frequency of the data acquisition is 16 kHz or higher. For oscillations faster than 2 Hz, the instrument noise exceeded the reduced amplitude of the oscillations in the measurements and the reconstruction failed.The method was applied to airborne measurements of upward terrestrial irradiance from the VERDI (Vertical Distribution of Ice in Arctic Clouds) field campaign. Pyrgeometer data above open leads in sea ice and a broken cloud field were reconstructed and compared to KT19 infrared thermometer data. The reconstruction of amplitude and phase shift of the deconvoluted data improved the agreement with the KT19 data. Cloud top temperatures were improved by up to 1 K above broken clouds of 80-800 m size (1-10 s flight time) while an underestimation of 2.5 W m −2 was found for the upward irradiance over small leads of about 600 m diameter (10 s flight time) when using the slow-response data. The limitations of the method with respect to instrument noise and digitalization of measurements by the data acquisition are discussed.

Response to reviewer #2

“…Such performances compare well with similar airborne spectroradiometers (e.g. Emery et al, 2014) and satellite sensors (e.g. MODIS).…”

Airborne observations of far-infrared upwelling radiance in the Arctic

“…Close to the surface, measurements show an unexpected peaked minimum. Although the origin of this peak is not fully understood, we believe this is an instrumental artifact resulting from the strong temperature gradient near the surface and the sensor not being at thermal equilibrium (Ehrlich and Wendisch, 2015). This hypothesis is supported by the fact that data taken on the way down just before starting the ascent show a peak in the opposite direction.…”

“…The uncertainty for the measurement of H 2 O is 39.1 ppmv or 2.5 %, whichever is greater. Broadband longwave (LW) radiation was measured with Kipp & Zonen CGR-4 pyrgeometers installed below and above the aircraft (Ehrlich and Wendisch, 2015). These sensors have uncertainties of a few W m −2 .…”

“…The campaign, which involved the two instrumented Basler BT-67 Polar 5 and Polar 6 aircraft (e.g. Ehrlich and Wendisch, 2015), started in Longyearbyen (Spitzbergen) on 5 April with 1-week delay due to harsh weather conditions. There, only one flight dedicated to calibration was performed.…”

Validation of the sea ice surface albedo scheme of the regional climate model HIRHAM–NAOSIM using aircraft measurements during the ACLOUD/PASCAL campaigns

“…The irradiance data were corrected for aircraft attitude (pitch and roll angle) following the techniques described by Bannehr and Schwiesow (1993). A deconvolution method was applied to the pyranometer measurements to enhance the temporal resolution (20 Hz) of the slow-response sensors (Ehrlich and Wendisch, 2015). The daily mean surface albedo along the selected flight sections as derived from the ratio of upward and downward irradiances is shown in Fig.…”