Retrieval of tropospheric water vapour by using spectra from a microwave spectro-radiometer at 22 GHz

Transcription

1 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

2 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

3 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

4 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)

5 Calibration MIAWARA Calibration Balancing calibration switching between line ) and reference +reference absorber) difference spectra performed continuosly corrected for troposphere by using opacity from tips Tb [K] Balanced spectrum observed fitted Frequency [GHz]

6 Calibration MIAWARA Calibration Balancing calibration switching between line ) and reference +reference absorber) difference spectra performed continuosly corrected for troposphere by using opacity from tips Tb [K] Balanced spectrum observed fitted Frequency [GHz] Tipping calibration Tipping curve opacity, total power spectra each 20 minutes Tb [K] Total power spectrum observed fitted Frequency [GHz]

7 Retrieval Middle atmosphere retrieval Middle atmosphere retrieval MIAWARA H O (48h retrieval) / Meas. resp. > 0.6 (white line) H2O VMR [ppm] Pressure [hpa] difference spectra (center part, full resolution) a-priori: Aura/MLS vertical range: km linear OEM vertical resolution: 10 km temporal resolution: hours-days

8 Retrieval Tropospheric retrieval Tropospheric retrieval Pressure (hpa) H2O VMR [0/00] MIAWARA (4h Trop. retrieval) / Meas. resp. > 0.6 (white line) total power spectra (full bandwidth, 20 Mhz resolution) a-priori: campaign balloon soundings from Thun vertical range: 2-7 km nonlinear OEM vertical resolution: 3-5 km temporal resolution: typically 2 hours

9 Comparison Retrieval Tropospheric retrieval MIAWARA (2h Trop. retrieval) / Meas. resp. > 0.6 (white line) Pressure (hpa) Pressure (hpa) SND Payerne 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%

10 Improving the tropospheric retrieval Use informations from other available instruments Meteo station delivers relative humidity, pressure and temperature vmr at surface

11 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

12 Cloud base Improving the tropospheric retrieval Use informations from other available instruments Ceilometer cloud base altitude

13 Cloud base Improving the tropospheric retrieval Use informations from other available instruments Ceilometer cloud base altitude IR-sensor attached to MIAWARA cloud base temperature

14 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 base 100%)

15 Constraining in OEM Improving the tropospheric retrieval Fixed-point approach Altitude [km] Range of values range x a vmr@cloud base H 2 O vmr [ 0 / 00 ] soft constraining to a range of values, defined by x a and S a ( x a ±std)

16 Fixed-Point approach Improving the tropospheric retrieval Fixed-point approach Altitude [km] Range of values range x a vmr@cloud base "Fixed Point" H 2 O vmr [ 0 / 00 ] hard constraining to a known value ( fixed-point ) set std@fp to a low value (not 0)

17 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

18 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

19 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

20 Questions?