Middle-atmospheric zonal and meridional wind profiles from polar, tropical and midlatitudes with the ground-based microwave Doppler wind radiometer WIRA

Abstract

Abstract. WIRA is a ground-based microwave Doppler spectroradiometer specifically designed for the measurement of profiles of horizontal wind in the upper stratosphere and lower mesosphere region where no other continuously running measurement technique exists. A proof of principle has been delivered in a previous publication. A technical upgrade including a new high-frequency amplifier and sideband filter has improved the signal to noise ratio by a factor of 2.4. Since this upgrade the full horizontal wind field comprising zonal and meridional wind profiles is continuously measured. A completely new retrieval based on optimal estimation has been set up. Its characteristics are detailed in the present paper.Since the start of the routine operation of the first prototype in September 2010, WIRA has been measuring at four different locations at polar, mid-and tropical latitudes (67 • 22 N/26 • 38 E, 46 • 57 N/7 • 26 E, 43 • 56 N/5 • 43 E and 21 • 04 S/55 • 23 E) for time periods between 5.5 and 11 months. The data presented in this paper are daily average wind profiles with typical uncertainties and resolutions of 10 to 20 m s −1 and 10 to 16 km, respectively. A comparison between the data series from WIRA and European Centre for Medium-Range Weather Forecasts (ECMWF) model data revealed agreement within 10 % in the stratospheric zonal wind. The meridional wind profiles agree within their error bars over the entire sensitive altitude range of WIRA. However, significant differences in the mesospheric zonal wind speed of up to 50 % have been found.

Simulation study for measurement of horizontal wind profiles in the polar stratosphere and mesosphere using ground-based observations of ozone and carbon monoxide lines in the 230–250 GHz region

“…Measurements by such a system achieve a single-sideband noise temperature of 200 K and a total system temperature of 1400 K. We also assess how wind measurements would be improved by using radiometers with system temperatures of 700 and 1000 K. Spectral analysis of the down-converted signal over a 300 MHz bandwidth, centred on the emission line(s) of interest, and with channel widths ≥10 kHz is based on the specifications of commercially available chirp transform and fast Fourier transform (FFT) spectrometers for radiofrequency (RF) analysis. Rüfenacht et al (2014) showed for the WIRA instrument that frequency errors arising from reference oscillator instabilities and spectrum baseline artefacts such as standing waves are either small or can be adequately characterised to minimise their impact on the wind retrievals. However for other wind radiometers these effects could make a larger contribution to the measurement uncertainty that is not considered in the simulations here.…”

“…Comparison of their measured time series from four locations at polar, mid-, and tropical latitudes with ECMWF model data showed that meridional and zonal stratospheric winds agreed to within 10 %. However in the mesosphere, above 40-50 km, observed and modelled zonal wind speeds differ significantly by up to 50 % (Rüfenacht et al, 2014 (Hibbins et al, 2006(Hibbins et al, , 2009Nielsen et al, 2012). These observations have been limited to the troposphere, lower stratosphere, and upper mesosphere; co-located radiometric measurements could provide complementary observations filling the gap in altitudes at 25-75 km.…”

“…Wind speeds at 70-85 km above the South Pole were determined with uncertainty ±5 m s −1 by observing the carbon monoxide (CO) line at 461 GHz (Burrows et al, 2007). Rüfenacht et al (2012Rüfenacht et al ( , 2014 used observations of the O 3 microwave emission line at 142.18 GHz by the ground-based wind radiometer (WIRA), and optimal estimation retrieval, to measure daily mean zonal and meridional wind profiles. Comparison of their measured time series from four locations at polar, mid-, and tropical latitudes with ECMWF model data showed that meridional and zonal stratospheric winds agreed to within 10 %.…”

Validation of middle-atmospheric wind in observations and models

“…For the determination of wind profiles the Doppler shifts of signals from opposite azimuths in 68 • off-zenith direction are compared. The retrieval algorithm which is based on the optimal estimation 15 method (Rodgers, 2000) is similar to the version in Rüfenacht et al (2014) with a constant zero wind a priori profile. In this study we use identical a priori standard deviations for both horizontal wind components equivalent to 4 times the standard deviation of 6 years of zonal wind data from ECMWF.…”

“…The trustworthy altitude range, i.e. where the measurement response is > 0.8, the altitude resolution is < 20 km and the altitude accuracy is < 4 km (for details see Rüfenacht et al, 2014), typically extends 25 from 7 to 0.03 hPa (∼ 35 to 70 km). Microwave radiometers can be highly automatised and have the ability to pursue the measurements during overcast conditions what leads to near-continuous time series of observations, a characteristic that will be exploited for the model validations during almost a full annual cycle presented in Sect.…”