Performance of the Meteolabor“Snow White” chilled-mirror

hygrometer

Masatomo FujiwaraHokkaido University, Japan

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.)

Measurements of Atmospheric Water Vapor 1/2

• Instruments and platforms suitable for sciences in the tropical tropopause region ?

SPARC [2000]

Instruments: RS humidity sensors Various optical sensorsDew/frost point sensorsRemote sensing sensors

Platforms:BalloonsAircraft Ground-based remote Satellite

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 principles2. 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

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)

Condensation Dew/Frost Point (or Chilled-mirror) Hygrometry

& Snow White Hygrometer

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 generatorsSecondary 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/material2. Condensate on the mirror is detected by optical (or other) methods3. Electronic controller maintains a constant condensate “thickness”4. Mirror temperature is monitored as dew/frost point

1. Mirror (or other material) is cooled with refrigerating device/material

Peltier cooler Mirrorlamp

optical fiber

3 cm

Air

flow 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

Peltier (vs. cryogenic materials) : easy to operate tough for extremely low RH

Night-type Snow White

2. Condensate on the mirror is detected by optical (or other) methods

Peltier cooler Mirrorlamp

optical fiber ( to photo-transistor

& controller )

Air

flow Mirror is illuminated by a lamp,

and the “reflectance” (orcondensate “thickness”) is measured by a photo-transistor. ( Condensates will lower the reflectance. )

~80% of the “dry reflectance” is often used as the appropriate condensate “thickness” on themirror.

Feedback controller works as tomaintain the constant condensate by changing the Peltier strength, so that the mirror temperature equals to dew/frost point.

Condensates on the MirrorDew size depends on the “thickness” setting

Below 0 C, supercooled dew changes to ice particles

Pragnell [1993]

3. Electronic controller maintains a constant condensate “thickness”

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, … ?)

Day-type Snow White

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. )

Reference thermometer in the circuit

Snow White mirror [ Principle of Thermocouple Thermometry ]

Output voltage is proportional to the temperature difference.

Characteristics and Problems for Atmospheric Measurements

• Supercooled dew : water does not freeze at 0 deg. C Ambiguity in vapor pressure calculationHouse-keeping data indicates the dew to frost conversion for Snow White

• Atmospheric P-T profile, with 5m/s ascent, causes difficulties30 deg. C to -85 deg. C in air temperature 42 hPa to 0.00023 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, LSoptimization of the feedback gain needed; more airflow needed

Contamination (de-gassing) problems in the upper atmosphere

• Clouds (contamination / sciences)

Observations with Snow White

(The sky over Christmas Is., Kiribati)

Flown with NOAA FPH (Indonesia)

Night-type Snow White

TMAX system = TMAX-C interface board + Vaisala RS80 + ECC ozonesonde + Snow White( SW & Humicap comparisons)

Modification trials (Sapporo)

Day-type Snow White ( sensor head & radiator

inside the styrofoam box to avoid solar radiation )

Indonesia Galapagos Is

Surinam

Sapporo

Ny Aalesund

Syowa

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ThailandR/V Mirai

Tarawa

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Observations with Snow White (2000-2003)

Christmas Is

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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

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1. Dew to frost conversion and house-keeping data

2. Comparisons with Vaisala RS80A and RS80H3. Comparison with Vaisala RS90

4. Cloud water estimation

5. Comparisons with NOAA FPH6. Stratospheric behaviors

Results and Experiences with Snow White

(The sky over Sapporo, Japan)

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”

2. Comparisons with Vaisala RS80A & RS80HH-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 2000-2001 products) ! “working instruments” should be monitored/calibrated continuously

3. Comparisons with Vaisala RS90

RS90 humicap sensor shows improved time response in the upper troposphere compared to RS80H.

RS90 : 1997-2003( Humidity sensor basically same as RS92 )

Project in Paramaribo, Surinam(MD. Surinam & KNMI & SOWER)16 simultaneous soundings of SW, RS80, and RS90

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.

(J.Wang et al., GRL, 2003)Not yet validated …

5. Comparisons with NOAA FPH

( Voemel et al., JTECH, 2003 )

- 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

6. Stratospheric Behaviors

• Peltier coolers are not capable of cooling to mid-stratospheric 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 mass-flow)

• Optimization of the feedback controller may be the only solution ?

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 tropopauseregion 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

ECC-SW launch this morning