MEASUREMENTS AND MODELLING OF WATER VAPOUR SPECTROSCOPY IN TROPICAL AND SUB-ARCTIC ATMOSPHERES.

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1 MEASUREMENTS AND MODELLING OF WATER VAPOUR SPECTROSCOPY IN TROPICAL AND SUB-ARCTIC ATMOSPHERES. J.P. Taylor, T.J. Hewison, A. McGrath and A. Vance. Airborne Remote Sensing Group, The Met Office, Y70 Building, DERA Farnborough, Hampshire. GU14 0LX. England. ABSTRACT Water vapour plays in important role in the energy budget of the atmosphere. Despite many years of study there are still considerable uncertainties both in the measurement of water vapour and it s representation in radiation models. In an attempt to reduce these uncertainties two field experiments in differing atmospheres were conducted in The results from a tropical atmosphere and a sub-arctic atmosphere show significant differences between differing radio sonde sensors. Microwave radiometer measurements are used to validate radiation models and also to constrain the input profile for thermal infra-red modelling. Results from the GENLN2 v4 line-by-line radiation model with the CKD2.4 water vapour continuum show very good agreement with airborne interferometer measurements in the thermal infra-red. 1. INTRODUCTION In 1999 The Met. Office C130 aircraft was used to study water vapour in tropical and sub-arctic atmospheres as part of a campaign called MOTH - Measurement of Tropospheric Humidity. The field campaigns were based out of Ascension Island in the tropical South Atlantic and Kalmar in the Baltic Sea. upwelling and downwelling radiances between 500 and 3000cm -1. MARSS (Microwave Airborne Radiometer Scanning System) measuring upwelling and downwelling radiances at 89, 157 and 3 channels around 183 GHz. Water vapour - General Eastern chilled mirror dewpoint hygrometer and a fast response Lyman Alpha absorption hygrometer. Chemistry - continuous measurements of ozone and carbon monoxide. 2.2 Radiosondes Radiosondes were launched from Ascension Island in the tropics and Visby in the Baltic in support of the airborne measurements. Balloons carrying multiple sensors were launched on most occasions. The sensors used were Vaisala RS80 and RS90 and Snow White (chilled mirror). 3. COMPARISON OF SONDE MEASUREMENTS Analysis of the 27 dual launches with RS80 and RS90 sensors has shown that they are in reasonable agreement as detailed in figure 1. The aim of the MOTH campaigns was to fully describe the water vapour column and make simultaneous measurements of the radiance using a microwave radiometer and thermal infra-red interferometer. To help in the analysis of these airborne measurements a team from The Met. Office launched a range of radiosondes from islands in the vicinity of the aircraft operations. 2. INSTRUMENTATION 2.1 Aircraft Instruments The Met. Office C130 aircraft was fitted with the following range of instruments: An interferometer (ARIES - Airborne Research Interferometer Evalaution System) measuring Figure 1. The RS80 soundings show a tendency to measure slightly drier for large water vapour amounts. A comparison of the RS90 and Snow White sensors during

2 17 dual launches (Fig. 2) show that the Snow White sensors are moister by approximately 7%. These comparisons show that the best agreement with the aircraft instrumentation is with the RS90 sondes. However the limited sample makes statistically significant conclusions difficult to draw. It should also be noted that there is some evidence that the subset of radiosondes that comprise those when the aircraft flew are not a random selection from the entire data base as they are biased to the times of the aircraft flights and the prevailing weather. Therefore the RS80 vs RS90 and the RS80 vs aircraft and RS90 vs aircraft results are not entirely consistent. 4. MICROWAVE RADIATIVE TRANSFER Figure 2. Where the aircraft was operating in the same air mass as the radiosondes were launched intercomparisons between the aircraft lyman alpha hygrometer and the radiosonde sensors has been carried out. In Fig 3. The RS80 sondes are compared with the aircraft observations this figure includes data from 11 co-located ascents. Figure 4 shows a similar comparison between the RS90 and aircraft lyman alpha hygrometer for 22 co-located ascents. Data gathered with the MARSS radiometer from the tropics and arctic have been used to validate the Liebe 1989 and Liebe 1993 radiative transfer models. The channels that have been studied are those that match the AMSU-B channels 16, 17, 18, 19 and 20. (AMSU-B = Advanced Microwave Sounding Unit -B). Figure 5 shows the microwave absorption spectrum with the opacity due to the oxygen and water vapour lines overlaid with the AMSU channels. The data in this figure are computed for a US Standard Atmosphere. In the sub-arctic atmosphere the contribution from the water vapour and oxygen is similar for channel 16. Figure 3. Figure 5. Figure 6 shows a comparison of the zenith brightness temperature for the AMSU channels modelled using the two versions of the Liebe model (lines) and compared with the aircraft measured brightness temperatures (crosses) in a tropical atmosphere with an integrated water vapour column of 40kg/m 2. Figure 7 shows a similar plot for a sub-arctic atmosphere with an integrated water vapour column of 4.2kg/m 2. The agreement in both cases, particularly for the channels around 183GHz is very good. Figure 4.

3 the Snow White sensors are in poor agreement with the microwave measurements. Figure 6. The microwave brightness temperature observations have been used to build the "best" representation of the temperature and water vapour profiles for one tropical case and one sub-arctic case. The zenith viewing microwave measurements from the highest level aircraft run have been used to select the best radiosonde profile to top up the aircraft measurements by minimising the rms difference between the radiometer observations and the model predictions for channel 18. The best aircraft profile has been constructed by minimising the differences over channels 18, 19 and 20 using data from all heights flown between 30m and around 7-9km. These "best" profiles have then been used to initialise the GENLN2 v4 line by line radiation model and compute the upwelling radiances at around 8km for the two cases. 5. INFRARED RADIATIVE TRANSFER Figure 7. Given the assumption that the 183GHz water vapour line is well modelled by Liebe 89 (as shown in figures 6 and 7) MARSS Channel observations can be used to validate the water vapour profile measurements from the various radiosondes. This has been carried out for 5 cases where the microwave measurements were made in close proximity and time to the sonde launches. The results from this validation exercise are shown in Table 1. Here the average percentage bias (model minus observations) are shown along with the percentage RMS error. These statistics have been computed using data gathered by the aircraft at a range of altitudes and the bias and RMS have been normalised by the average brightness temperature observed. Table 1. A670 A676 A740 A742 A744 Ave Trop Trop Arc Arc Arc RS80 %rms RS90 %rms Snow %rms RS80 %bias RS90 %bias Snow %bias These results clearly show that there is little difference between the RS80 and RS90 sensors but that The GENLN2 model was run with the CKD2.1 and CKD2.4 versions of the water vapour continuum. The results from the model were compared with the radiances observed using the ARIES interferometer looking in the nadir. Figure 8 shows the brightness temperatures from the model and ARIES in the upper panel and the model minus ARIES differences in the lower panel over the infra-red window region ( cm -1 ), the model results shown here use the CKD2.4 water vapour continuum. Figure 8. Between the lines where the only absorption is from the water vapour continuum the agreement between the model and observations is excellent with differences in the region of 0.2K. There are larger differences between the model and observations in the lines. This difference in the lines is observed at higher wavenumbers and is the subject of further study. Reasons for the differences may be in the vertical distribution of the water vapour and the layer widths in

4 the model. Sensitivity studies are being carried out to look at the impact of small changes in the water vapour column (within the accuracy of the observations) on these differences. For this tropical case the GENLN2 model was also run with the CKD2.1 water vapour continuum. The model minus ARIES differences for the two models are shown in figure CONCLUSIONS The measurements from the MOTH field experiments have shown that great caution needs to be taken in the measurement of water vapour in the atmosphere. There are considerable differences between the measurements made with different radio sonde sensors. Our results have shown that the RS90 and RS80 sondes are very similar with the RS90 sondes giving the best agreement with the aircraft data. However further analysis of these measurements are required before firm statements can be made. The microwave radiometry has confirmed our belief that the spectroscopy around the 183GHz water vapour line can be well modelled and this has been utilised in defining the "best" profile for the validation of the infrared models where the spectroscopy is less understood. Figure 9. The lower line in figure 9 is the result from the CKD2.4 continuum run which shows marginally better agreement with the observations than the run with the CKD2.1 continuum. A similar intercomparison between GENLN2 with the CKD2.4 continuum and ARIES has been carried out for a sub-arctic case and the results are shown in figure 10. Once again the agreement between model and observations is very good within +/- 1K. Note that around 1050cm -1 there are larger differences due to poorer representation of the ozone profile. Results using the GENLN2 v4 line-by-line model with the CKD2.4 water vapour continuum have shown very good agreement with in situ measurements made with an airborne interferometer measuring at a resolution of 0.5cm-1 between 800 and 1200cm-1. There are still questions to be answered regarding the line absorption which need to be investigated by a series of sensitivity tests with the model inputs. ACKNOWLEDGMENTS We would like to acknowledge the staff of the Meteorological Research Flight and the Royal Air Force Aircrew who supported us in the MOTH experiment. We would also like to thank the authorities in Ascension Island and Kalmar for their logistical support. Thanks should also go to the radiosonde team from The Met. Office. REFERENCES Figure 10. Liebe, H.J. 1989: MPM - An atmospheric millimeter wave propagation model. Int. J. Infrared and Millimeter Waves, vol.10(6), pp Liebe, H.J., G.A.Hufford and M.G.Cotton, 1993: Propagation modeling of moist air and suspended water/ice particles at frequencies below 1000GHz. AGARD 52nd Specialists Meeting of the Electromagnetic Wave Propagation Panel, Ch3.

5 Keywords: Water vapour, continuum, thermal infrared, spectroscopy, interferometer, microwave, radiosondes.