FAAM Facility for Airborne Atmospheric Measurements FAAM Aircraft

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FAAM Facility for Airborne Atmospheric Measurements FAAM Aircraft Water Vapour Measurements Woolley, A M

FAAM Facility for Airborne Atmospheric Measurements FAAM Aircraft Water Vapour Measurements Woolley, A M FAAM operates a BAe 146 -301 aircraft, deployed globally, making measurements to support the UK atmospheric science community. The aircraft operates at altitudes between 15 m and 11 km and therefore experiences a wide range of humidities, from moist air at low levels in the tropics to dry air within the lower stratosphere at higher latitudes in winter. High quality water vapour data is required throughout this range to help study atmospheric dynamics and to help improve key composition measurements. FAAM’s installation of a Buck Research CR 2 hygrometer (CR 2) seeks to provide an accurate reference measurement of water vapour in the dew/frostpoint range +20°C to -80°C. A Reference Instrument Comparison and Data Analysis Chilled Mirror dew/frostpoint measurement: ±Pass ambient air over a cooled mirror ±Maintain a constant layer of ice/water using optical measurement and feedback control ±Traceable directly to the ITS-90 - An absolute technique Data have been studied for a number of flights showing a wide range of atmospheric humidity, comparing the performance of the CR 2 with the existing techniques on the aircraft. Prior to this, both GE and CR 2 instruments were traceably calibrated in the laboratory, with 2σ uncertainties of better than ± 1 K (GE) and ± 0. 2 K (CR 2). FAAM’s core capability: ±General Eastern 1011 B chilled mirror hygrometer (GE) measuring water vapour – slow below -30°C frostpoint, insufficient cooling ±Lyman-alpha absorption instrument measuring total water (TWC), derives absolute calibration from GE. Measures humidity in clear air ±GE and CR 2 show excellent agreement when humidity gradients are low. Flight B 433 ±GE shows increasing positive bias in dry air, due to a lack of cooling power or other physical limitations. Buck Research CR 2: ±Increased cooling power gives access to lower ambient frostpoints ±More accurate reference measurement for other faster but less absolute techniques flown on the platform, such as the TWC. Installation and Inlet Measurement of water vapour at frost-points below -40°C is affected by the adsorption of water vapour onto parts of the sampling system / aircraft skin. Mitigation strategies were: ± Shortest possible sample line (60 cm, constrained by fit location) ± Heated line to +50°C, maximising H 2 O mobility on the surface ± Line composed of electro-polished stainless steel, coated with Silconert 2000® treatment for reduced H 2 O molecule adherence ± Minimised surface area and maximised flow – residence time 0. 4 s at sea level, falling to 0. 1 s at 35 kft (max alt). ± Shortest inlet at minimum boundary-layer thickness – 2. 6 cm The pumped reverse-facing inlet design prevents cloud-water ingress and external icing. ±CR 2 mirror instability during fast changes can be flagged. ±No detectable CR 2 sample line hysteresis caused by water vapour adsorption at these frostpoints. ±Both chilled mirror devices lag the fast Ly-α TWC by an amount dependent on mixing ratio and phase of water on mirror. Flight B 414 ±Phase change implied here from CR 2/TWC bias change associated with different vapour pressures of water and ice. Flights B 412, B 414, B 433 ±CR 2 response lag dependent upon mixing ratio - upper limit of the response time is around 30 seconds. GE unable to cope with dry air – response lags over 90 s at frostpoints below -45°C, not shown above. ±CR 2 measurements in cloud consistent with temperature – sample line does not admit liquid water. Temperature ≈ Dewpoint in SCu. ±FAAM GE samples liquid water in cloud, measuring even higher water concentrations than TWC under circumstances shown. Suspect inlet, which is widely employed as standard on other aircraft. Conclusions CR 2 instrument augments FAAMs existing capability, providing a better absolute reference for faster measurement techniques. Flight B 414 Inlet demonstrated to handle ambient water vapour with minimal ad/de-sorption effects, and without introducing response time lags. Chilled mirror instrument response time dependent on phase of mirror condensate as well as ambient water vapour mixing ratio. Standard GE instrument inlet used widely in the community shown to introduce measurement bias in cloud owing to water ingress. Thanks to: FAAM Working Groups, Avalon Aero, BAES, Geraint Vaughan (Manchester University), Mark Stevens (NPL)