Retrieval of tropospheric water vapour by using spectra from a microwave spectro-radiometer at 22 GHz René Bleisch, Niklaus Kämpfer, Dominik Scheiben Institute of Applied Physics, University of Bern ISTP, L Aquila 7.9.2012
Outline Outline 1 MIAWARA Instrument Calibration 2 Retrieval Middle atmosphere retrieval Tropospheric retrieval 3 Improving the tropospheric retrieval Use informations from other available instruments Fixed-point approach 4 Summary
MIAWARA Instrument MIAWARA (MIddle Atmosphere WAter vapour RAdiometer) located at Zimmerwald observatory near Bern and designed to measure water vapour emission line at 22 GHz
MIAWARA Instrument MIAWARA (MIddle Atmosphere WAter vapour RAdiometer) primary purpose is to retrieve middle atmospheric water vapour profiles equipped with digital FFT-spectrometer (16000 channels, BW 1 GHz, Res. 61 khz)
Calibration MIAWARA Calibration Balancing calibration switching between line (sky@15-20 ) and reference (sky@85-90 +reference absorber) difference spectra performed continuosly corrected for troposphere by using opacity from tips Tb [K] 2.6 2.5 2.4 2.3 Balanced spectrum observed fitted 2.2 22.15 22.2 22.25 22.3 Frequency [GHz]
Calibration MIAWARA Calibration Balancing calibration switching between line (sky@15-20 ) and reference (sky@85-90 +reference absorber) difference spectra performed continuosly corrected for troposphere by using opacity from tips Tb [K] 2.6 2.5 2.4 2.3 Balanced spectrum observed fitted 2.2 22.15 22.2 22.25 22.3 Frequency [GHz] Tipping calibration Tipping curve opacity, total power spectra each 20 minutes Tb [K] 43 42 41 40 Total power spectrum observed fitted 21.8 22 22.2 22.4 22.6 Frequency [GHz]
Retrieval Middle atmosphere retrieval Middle atmosphere retrieval MIAWARA H O (48h retrieval) / Meas. resp. > 0.6 (white line) 2 8 7 0.1 6 1 5 H2O VMR [ppm] Pressure [hpa] 0.01 10 4 2008 2009 difference spectra (center part, full resolution) a-priori: Aura/MLS 2010 2011 2012 vertical range: 35-80 km linear OEM vertical resolution: 10 km temporal resolution: hours-days
Retrieval Tropospheric retrieval Tropospheric retrieval 20 10 5 Pressure (hpa) 300 400 2 1 0.5 500 600 H2O VMR [0/00] MIAWARA (4h Trop. retrieval) / Meas. resp. > 0.6 (white line) 0.2 0.1 0.05 700 800 2008 2009 total power spectra (full bandwidth, 20 Mhz resolution) a-priori: campaign balloon soundings from Thun 2010 2011 2012 vertical range: 2-7 km nonlinear OEM vertical resolution: 3-5 km temporal resolution: typically 2 hours
Comparison Retrieval Tropospheric retrieval MIAWARA (2h Trop. retrieval) / Meas. resp. > 0.6 (white line) Pressure (hpa) Pressure (hpa) 300 400 500 600 700 800 300 400 500 600 700 800 01.08 06.08 11.08 16.08 21.08 26.08 SND Payerne 01.08 06.08 11.08 16.08 21.08 26.08 20 10 5 2 1 0.5 0.2 0.1 0.05 20 10 5 2 1 0.5 0.2 0.1 0.05 H 2 O VMR [ 0 / 00 ] H 2 O VMR [ 0 / 00 ] corresponds well with operational soundings from Payerne correlation of up to 0.9 wet bias of 10-20%
Improving the tropospheric retrieval Use informations from other available instruments Meteo station delivers relative humidity, pressure and temperature vmr at surface
Improving the tropospheric retrieval Use informations from other available instruments Meteo station delivers relative humidity, pressure and temperature vmr at surface operationally used for a-priori
Cloud base Improving the tropospheric retrieval Use informations from other available instruments Ceilometer cloud base altitude
Cloud base Improving the tropospheric retrieval Use informations from other available instruments Ceilometer cloud base altitude IR-sensor attached to MIAWARA cloud base temperature
Cloud base Improving the tropospheric retrieval Use informations from other available instruments Ceilometer IR-sensor attached to MIAWARA cloud base altitude cloud base temperature vmr at cloud base (assuming RH@cloud base 100%)
Constraining in OEM Improving the tropospheric retrieval Fixed-point approach Altitude [km] 10 9 8 7 6 5 4 3 2 Range of values range x a vmr@cloud base 1 0 5 10 15 20 H 2 O vmr [ 0 / 00 ] soft constraining to a range of values, defined by x a and S a ( x a ±std)
Fixed-Point approach Improving the tropospheric retrieval Fixed-point approach Altitude [km] 10 9 8 7 6 5 4 3 2 Range of values range x a vmr@cloud base "Fixed Point" 1 0 5 10 15 20 H 2 O vmr [ 0 / 00 ] hard constraining to a known value ( fixed-point ) set std@fp to a low value (not 0)
Example Improving the tropospheric retrieval Fixed-point approach MIAWARA standard retrieval (1) and retrieval with fixed-point at cloud base (2), compared with operational sounding data from Payerne
Improving the tropospheric retrieval Fixed-point approach Difficulties delta-peak like features in S a ( adaptations of retrieval grid) cloud base temperature not known with enough precision vmr = RH e sat(t) p +1%RH +1% in vmr but +1 K +5-10% in vmr maximum of RH and maximum of vmr not at the same altitude comparison MIAWARA snd is affected by spatial distance
Summary Summary instrument is considered for middle atmosphere it also delivers information about troposphere however, for tropospheric application it is inferior to conventional 22 GHz radiometers but, the instrument provides measurements for 0-80 km with gap in UTLS performance may be improved by taking into account additional information e.g. from Ceilometer
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