Performance of the Meteolabor Snow White chilled-mirror hygrometer. Masatomo Fujiwara Hokkaido University, Japan

Transcription

1 Performance of the Meteolabor Snow White chilled-mirror hygrometer Masatomo Fujiwara Hokkaido University, Japan

2 Coauthors and Collaborators F. Hasebe, M. Shiotani, H. Voemel, S. J. Oltmans, H. Takashima, N. Nishi (SOWER core members) P. Ruppert (Meteolabor) P. Fortuin, G. Verver, H. Kelder (KNMI), C. Becker (MD Surinam) T. Shibata (Nagoya U.), T. Horinouchi, T. Tsuda (Kyoto U.), R. Neuber (AWI) M. Agama, F. Paredes, J. Cornejo, L. Poveda, H. Enriquez (INAMHI) S. Saraspriya, N. Komala, A. Suripto (LAPAN) Y. Inai, H. Kubokawa, T. Uetake, E. Tanaka (Hokkaido U.)

3 Measurements of Atmospheric Water Vapor 1/2 Instruments and platforms suitable for sciences in the tropical tropopause region? Instruments: RS humidity sensors Various optical sensors Dew/frost point sensors Remote sensing sensors Platforms: Balloons Aircraft Ground-based remote Satellite SPARC [2000]

4 Measurements of Atmospheric Water Vapor 2/2 Instruments and platforms suitable for sciences in the tropical tropopause region? 1. Extremely cold temperatures and low water vapor pressures; measurements with high accuracy/precision Hygrometers with reference/standard principles 2. Process studies on small-scale and large-scale disturbances; climatology and trend studies low-cost hygrometers and platforms Balloon-borne chilled-mirror hygrometers, e.g., Snow White

5 Meteolabor Snow White Chilled-mirror Hygrometer Meteolabor AG, Switzerland Established in the 1960s Two branches: Meteorology & Lightning Protection Chilled-mirror hygrometers for surface, aircraft, laboratory, and radiosonde (e.g., THYGAN, Snow White) Snow White Dew/frost point (chilled-mirror) hygrometer for radiosondes Low cost, light weight, easy to operate (a plug-in instrument) Thermoelectric Peltier device to cool the mirror in production since 1996 (The sky over Bandung, Indonesia)

6 Condensation Dew/Frost Point (or Chilled-mirror) Hygrometry & Snow White Hygrometer

7 Condensation Dew/Frost Point (or Chilled-mirror) Hygrometry Based on the physical (thermodynamical) theory, i.e., Clausius- Clapeyron equation (relationship between dew/frost point temperature and saturation vapor pressure) Secondary or reference standards or laboratory transfer standard cf. Categories for hygrometers for industry and/or meteorology by Sonntag, 1994 : Primary standards : gravimetric hygrometer, various precision humidity generators Secondary standards : psychrometer (wet-and-dry bulb thermometer), D/FPH,etc. Reference standards: same methods as secondary, but less accurate instruments Working instruments : radiosonde humidity sensors (capacitance, resistance, hair), optical sensors (IR/UV absorption methods), etc. The system works as follows: 1. Mirror (or other material) is cooled with refrigerating device/material 2. Condensate on the mirror is detected by optical (or other) methods 3. Electronic controller maintains a constant condensate thickness 4. Mirror temperature is monitored as dew/frost point

8 1. Mirror (or other material) is cooled with refrigerating device/material Air flow lamp Peltier cooler Mirror optical fiber Mirror (3 mm x 3 mm) : gold-plated constantane sheet (with copper sheet behind, as a thermocouple thermometer) Thermoelectric Peltier cooler : activated with 1.5V dry cell battery, produces temperature difference between the two sides, and hot side is cooled by air with the radiator 3 cm Peltier (vs. cryogenic materials) : easy to operate tough for extremely low RH Night-type Snow White

9 2. Condensate on the mirror is detected by optical (or other) methods Air flow Mirror is illuminated by a lamp, and the reflectance (or condensate thickness ) is measured by a photo-transistor. ( Condensates will lower the reflectance. ) Peltier cooler lamp Mirror optical fiber ( to photo-transistor & controller ) ~80% of the dry reflectance is often used as the appropriate condensate thickness on the mirror. Feedback controller works as to maintain the constant condensate by changing the Peltier strength, so that the mirror temperature equals to dew/frost point.

10 Condensates on the Mirror Dew size depends on the thickness setting Below 0 C, supercooled dew changes to ice particles Pragnell [1993]

11 3. Electronic controller maintains a constant condensate thickness Day-type Snow White The key is the optimum feedback gain (i.e., controller response/sensitivity to the small condensate change) following the changing ambient water vapor. Too strong gain produces artificial oscillation; too weak gain produces meandering. This greatly depends on the development of electronics (vacuum tube; analog circuit; microprocessor), and on the controlling logics (relay (on/off) vs. constant thickness; PID, FUZZY,?)

12 4. Mirror temperature is monitored as dew/frost point The mirror of Snow White is a thermocouple thermometer. ( Most dew point hygrometers measures mirror temperature with an independent thermometer mounted on the backside of the mirror. ) [ Principle of Thermocouple Thermometry ] Snow White mirror Reference thermometer in the circuit Output voltage is proportional to the temperature difference.

13 Characteristics and Problems for Atmospheric Measurements Supercooled dew : water does not freeze at 0 deg. C Ambiguity in vapor pressure calculation House-keeping data indicates the dew to frost conversion for Snow White Atmospheric P-T profile, with 5m/s ascent, causes difficulties 30 deg. C to -85 deg. C in air temperature 42 hpa to hpa in (saturation) water vapor pressure 1000 hpa to 100 hpa in air pressure (reduced mass flow) in 50 minutes! Response time problems : changes of the condensate become very slow at TP, LS optimization of the feedback gain needed; more airflow needed Contamination (de-gassing) problems in the upper atmosphere Clouds (contamination / sciences)

14 Observations with Snow White (The sky over Christmas Is., Kiribati)

15 TMAX system = TMAX-C interface board + Vaisala RS80 + ECC ozonesonde + Snow White ( SW & Humicap comparisons) Night-type Snow White Flown with NOAA FPH (Indonesia)

16 Day-type Snow White ( sensor head & radiator inside the styrofoam box to avoid solar radiation ) Modification trials (Sapporo)

17 Observations with Snow White ( ) Tropics SOWER Soundings of Ozone and Water in the Equatorial Region ( Surinam: Collaboration with KNMI and MDS ) Sapporo 21st COE project by Hokkaido Univ. EES & Lowtem Polar regions NA: SWAN project, Syowa: 43rd JARE Ny Aalesund Thailand Indonesia Sapporo R/V Mirai Christmas Is Tarawa Surinam Galapagos Is Syowa

18 Results and Experiences with Snow White 1. Dew to frost conversion and house-keeping data 2. Comparisons with Vaisala RS80A and RS80H 3. Comparison with Vaisala RS90 4. Cloud water estimation 5. Comparisons with NOAA FPH 6. Stratospheric behaviors (The sky over Sapporo, Japan)

19 1. Dew to frost conversion & house-keeping data (example of SW profile in Indonesia) - Vapor calculations depend on dew/frost. - Supercooled dew often down to -20 to -30 C. - Peltier current : cooling strength. - Photo. voltage : condensate thickness

20 2. Comparisons with Vaisala RS80A & RS80H H-Humicap : since early 1990s A-Humicap : since 1980 ( Fujiwara et al., JTECH, 2003 ) - 47 soundings in the tropical Pacific in different seasons and longitudes - SW and RS80H agree well in the lower and middle troposphere - SW and RS80A do not agree in the wet lower troposphere, (RS80A) ~ 0.9 x (SW) (new) dry bias for recent RS80A (at least for products)! working instruments should be monitored/calibrated continuously

21 3. Comparisons with Vaisala RS90 Project in Paramaribo, Surinam (MD. Surinam & KNMI & SOWER) 16 simultaneous soundings of SW, RS80, and RS90 RS90 : ( Humidity sensor basically same as RS92 ) RS90 humicap sensor shows improved time response in the upper troposphere compared to RS80H.

22 4. Cloud Water Estimation In clouds, a heater on the housing evaporates cloud droplets, and the dew/frost point reading shows apparent supersaturation. Cloud water (or total water) amount may be estimated. Not yet validated (J.Wang et al., GRL, 2003)

23 5. Comparisons with NOAA FPH - NOAA/CMDL cryogenic frost point hygrometer (more than 20 years of stratospheric water vapor monitoring, etc.) - Two sensors sometimes agree well and sometimes not in the tropopause region ( Voemel et al., JTECH, 2003 )

24 6. Stratospheric Behaviors Peltier coolers are not capable of cooling to midstratospheric frost points because of the very large temperature difference needed Snow White house-keeping data shows that the mirror does not dry up until the balloon burst/rapid falling, even with too high mirror temperature (confirmed also by some night-time soundings with exposed mirror) This may indicate a general feature of very slow frost change in the stratosphere due to small amount of water molecules (including the effect of reduced massflow) Optimization of the feedback controller may be the only solution?

25 Summary Meteolabor Snow White is a low-cost, balloon-borne chilled-mirror hygrometer for radiosondes Its design, principle, and platform should be suitable for tropical UT/tropopause sciences Validity of the measurement in the tropical tropopause region still needs careful examination for each sounding Further considerations for controller optimization and for contamination problems might be necessary Good for validation/continuous-monitoring of various radiosonde humidity sensors in the troposphere Potential for cloud sciences

26 ECC-SW launch this morning

27

28