LABORATORY EXPERIMENTS OF MIXED- PHASE CLOUD FORMATION

Transcription

1 LABORATORY EXPERIMENTS OF MIXED- PHASE CLOUD FORMATION Takuya Tajiri 1*, Katsuya Yamashita 1, Masataka Murakami 1, Narihiro Orikasa 1, Atsushi Saito 1, Tomohiro Nagai 1, Tetsu Sakai 1, and Hiroshi Ishimoto 1 1 Meteorological Research Institute, Tsukuba, Japan 1. INTRODUCTION 1 A new cloud simulation chamber was built in 25 at Meteorological Research Institute (MRI), Japan Meteorological Agency. Laboratory experiments of ice formation using the MRI chamber has been conducted to identify where ice crystals originate and to quantify their concentrations under various cloud conditions in the troposphere. Mixed-phase clouds are common features of clouds with top temperatures warmer than -35 C. Mid-latitude precipitating clouds frequently develop as the mixed-phase clouds, which form within preexisting supercooled liquid clouds. Cloud droplets appear first, then ice and snow crystals eventually dominate through ice initiation and subsequent depositional growth at the expense of cloud water until the cloud is completely glaciated. The repartitioning of water (i.e., cloud ice and supercooled cloud water) in such clouds varies throughout the cloud life cycles, and affects cloud radiative properties. In most climate models these treatments are simplified and specified as a function of temperature, although they are essential for proper simulations. Aerosol particles play an important role as ice nuclei, and then determine the microphysical structures of mixed-phase clouds. However, it is not sufficiently understood how aerosols affect ice nucleation processes and cloud properties. A quantitative description of * Corresponding author's address: TAKUYA TAJIRI, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 35-52, Japan; ttajiri@mri-jma.go.jp the relation between physico-chemical properties of aerosol particles and their ice nucleation ability is crucial for an improvement of numerical models. To investigate ice nucleation processes in the mixed-phase clouds, microphysical instruments used in this study are Cloud Aerosol Spectrometer with Particle by Particle measuring function of forward and backward scattering (CAS-PbP), Cloud Particle Imager probe (CPI) and scattered laser detection system (laser system). Theses devices are potential for sensing the onset of cloud formation and measuring size distributions, shapes and asphericity of aerosol and hydrometeors. It is expected that the result from this experimental study can be used not only for parameterization in weather prediction, climate and radiative transfer models, but also for development of cloud microphysical retrieval algorithms for remote sensors. 2. OVERVIEW OF THE MRI CHAMBER MRI chamber was designed to simulate natural processes (adiabatic expansion) in experimental volume (1.4m 3 ) by synchronously controlling air pressure and wall temperature, after the controlled-expansion type chamber (DeMott and Rogers 199). The air pressure in the chamber can be decreased down to below 3hPa with a vacuum rate corresponding to updraft velocities of to 3 m/s and the temperature be cooled down to -1 C by circulating the coolant. Sample air, generated aerosol particles and/or natural outdoor air, can be introduced into

2 Thermocouples wiring View Laser Particle Injection and Air Supply Port Dry Air Supply INNER CHAMBER OUTER CHAMBER Wyoming CCN Counter Chilled Mirror Hygrometers Cloud Droplet and Aerosol Spectrometer Scattered Laser Polarization-resolved Detection System Backward Detector View Port Fan Agitator Forward Detector Cooling Coils (Fluid Circulations) Pressure Transducers Vacuum Pump Wiring Port Mirror Re fle ctor Wiring Port 1D Optical Array type Microscopic Ice crystal Detector air exhaust Cloud Particle Imager FRONT VIEW SIDE VIEW Fig. 1. Schematic cross sections of the MRI cloud simulation chamber. The Ice particle detectors used for the experiments are also indicated (as red line squares). experimental volume from the particle injection port, which is located at the top of the chamber. A small fan stirs the injected air to achieve homogeneity of aerosol concentration inside the volume. Figure 1 shows a schematic view of the MRI chamber s major technical components and scientific instrumentation used for the experiments. Almost all experimental procedures during the period from pre- to after-expansion are programmed and automatically controlled by the data acquisition system (SEA s M3). More information of the MRI chamber for ice nucleation studies is described in detail by Tajiri et al. (26). 3. EXPERIMENTAL SETUP AND METHODS At first, natural outdoor air or test aerosols are introduced into the chamber with air pressure reduced below 3 hpa (i.e., nearly aerosol-free chamber), followed by preconditioning of sample air in terms of temperature and humidity, and then the adiabatic expansions are performed. During the pre-expansion initialization period, the Scanning Mobility Particle Sizer spectrometer (SMPS; MODEL3936, TSI) and the CAS-PbP were used for monitoring aerosol size distributions and number concentrations in the range from.1 to several micrometers. Pre-set parameters such as the initial pressure (1hPa), temperature (15 C) and adiabatic ascent rate (3m/s) are common to all experiments. It is difficult to continue cloud formations for a long time inside the chamber without sedimentation of the hydrometeors to the bottom of the chamber and condensation of water vapor onto the chamber wall. Thus the exact adiabatic expansions are realized

3 for rather short time. To simulate the mixed-phase clouds appropriately in spite of these limitations, the experimental procedures are devised. Vacuum rate was kept the initial value corresponding to 3m/s until cloud droplets were activated and well-grown. After that, it was increased to the values corresponding to 5m/s (or 1m/s) to pass through the phase transition zones quickly. The initial dewpoint temperature is also adjusted to around C to reach the LCL around -5-1 C, although clean dry air supply is need to decrease relative humidity. These operations are supposed to help simulate mixed-phase clouds properly. The dilution ratio of sample air by the addition of dry and/or moist filtered air is estimated from the change in number concentrations of aerosols (CAS data) or water vapor mixing ratios at the beginning of expansions, so that number concentrations of the particles detected by microphysical sensors during expansion experiments can be corrected. 4. ICE PARTICLE MEASUREMENTS Since ice particles have a variety of shapes under the atmospheric conditions, frozen droplets should eventually change their shape from spherical to aspherical. The chamber is equipped with optical and electrical devices for sensing the ice cloud formation and measuring size distributions, shapes and asphericity of cloud particles 4.1 CAS-PbP The chamber CAS (DMT) measures both forward (4 12 ) and backward ( ) scattering caused by individual particles that pass through a focused beam from a diode laser and has a Particle by Particle (PbP) function to record the data. Particles can be measured by drawing air from the chamber volume to the CAS at a known flow rate. The minimum detectable concentration is about.4 particles/cc, and the size range was adjusted to.35 to 3 µm. 4.2 CPI Cloud particles from the experimental volume are measured at the base of the chamber by using CPI probe (SPEC), which measures twodimensional digital images of particles larger than 1 µm up to 2 mm in size (Lawson et al. 21). Individual particle images are identified, sized, categorized, cropped and stored in real time, so that we could yield detailed information on early ice formation; shapes (particle type) and size distributions and concentrations of ice and cloud droplets (>.1 particles/cc). 4.3 laser system Laser radiation (532 nm) is scattered by particles in the centre of the chamber, and depolarization ratios with respect to scattering angles of 176 and 4 are measured by a couple of two independent photomultipliers. We expect that ice particles coexist with supercooled drops was detectable in the chamber by measuring the depolarization ratio of back-scattered laser radiation from the laser system data. 5. RESULTS AND DISCUSSION An example of chamber experiments to simulate mixed-phase cloud formation is depicted in Figure 2. The initial values of pressure, air temperature and dewpoint temperature were set to 1 hpa, 15 C and -5 C, respectively. The adiabatic ascent (expansion) was conducted and temperature went down to -4 C (2nd panel). 13 min. after the commencement of expansion, temperature reached -9 C and the chamber air became approximately water-saturated (3rd panel). The results of measurements imply that some of larger aerosol particles commenced to be activated (4th panel), and grew up to cloud droplets (>1µm) before ice crystals were detected by CPI (5th and 6th panels). Below -3 C, ice crystals more than 3 particles/cc were observed and ice formation became very active

4 around -35 C. Figure 3 shows an initial PSD measured by the SMPS and the CAS in this case. We have carried out more than 1 sensitivity experiments. The initial number concentrations of particles measured by the CAS were roughly a few tens particles/cc in general, even though the initial PSD varied from case to case, depending on weather condition. The maximum concentrations of cloud droplets (1 2/cc) throughout each expansion tended to increase with increasing ascent rate from 3 to 1m/s and increasing temperature at the LCL from -23 to -5 C. In evaluating the occurrence of ice formation, the CPI sometimes misses tiny ice crystals (D <1µm), and has time lag to detect the onset of ice initiation. Likewise note that, under sparse cloud conditions, microphysical measurements are eliminated due to the sensor's susceptibility to noise. Nevertheless, the temperature zones where ice initiation was frequently observed during the experiments were above -3 C (T1), around -33 C (T2) and below -38 C (T3). Zone T1 ranged from -23 to -29 C, depending on the temperature at the LCL or the initial dewpoint temperature, suggesting that the ice nucleation processes are associated with the presence and evolution of the cloud droplets. Zone T2 appears rather often and occasionally along with T1. Below -3 C, relatively large cloud droplets (>2µm) are still alive, but turning toward decline in a short time (e.g., Fig.2). The low concentration of large cloud droplets were quickly replaced with high concentrations of ice crystals. For Zone T3, ice crystals were probably produced through homogeneous freezing of tiny droplets. The peak number concentrations of ice crystals in the mixed-phase region had positive correlation with maximum cloud droplets concentrations, which are affected by ascent rate and the condensation rate of water vapor onto the Pressure (hpa) D>.3um 1 3.3<D<1um 1 2 D>1um Temperature (C o ) RH (%) Conc. (#/cm3) Droplet Conc. (#/cm3) Ice Conc. (#/cm3) D>1um 2<D<25um D>3um 1 1 D>1um 2<D<25um D>3um 3m/s CAS CPI_liquid CPI_Ice 5m/s Time (sec) Fig. 2. Ascent profiles (Time versus Pressure, Temperature and Relative Humidity) and the chamber CAS and CPI measurements during an experiment dn/dlogd (#/cm 3 ) SMPS CAS Diameter (um) Fig. 3. PSD measured during pre-expansion. chamber wall. In the cases of low cloud droplet concentrations and/or lower temperatures at the LCL, the peak of ice crystal concentrations in the Zone T1 tends to be ambiguously or be a part of multi-peak distributions with higher ice crystal concentrations. The ratios of forward and backward scattering intensities measured by CAS-PbP are shown for different temperature ranges (Figure 4). For the mixed-phase and completely glaciated stages, the plotted data corresponding to the regions shown by the rectangles in Twall Tair Tdew

5 Figure 5 are not sufficient. From comparison with the results of theoretical calculation using Finite-difference time domain (FDTD) method (Ishimoto 26) shown in Fig.5, it can be hard to definitely distinguish between the cloud droplets and the ice crystal in this case. To accumulate such data from many experiments will be necessary for developing a sub-1µm ice detection method using the chamber CAS-PbP. Intensity (Backward) Cloud droplets Fig. 4. Scattergrams of the CAS-PbP measurements during the 3 stages of cloud particles formation (see Figure 2). Intensity (backward) mixed-phase Ice crystal T= C T= C T< -4 C Hexagonal column (L/a=4.8) Yang & Liou aggregate Compact aggregate Ice sphere Fig. 5. Scattergram of the relationship between forward and backward light intensity (in arb. unit) scattered by individual particles is evaluated for CAS measurements by using the FDTD method. The laser system was also operated during all the experiments and measured the forward- and backward scattering intensities. When ice crystal concentrations were less than several particles/cc, due to weak signals in both normal and depolarized component, we could not always calculate the depolarization ratios. For the accurate detection of weak signals, the incident power will need to be sufficiently high to overcome the sensor s intrinsic noise, besides the background noise will be suppressed by improving the optics of system further. 6. SUMMARY The critical conditions for ice nucleation of natural background aerosol inside boundary layer were investigated at temperatures between about -1 C and -4 C. The ice initiation was achieved in adiabatic expansion experiments simulating ascent rates between 3m/s and 1m/s and the mixed-phase clouds were observed. Specialized microphysical instruments were used to monitor and evaluate the ice initiation processes under realistic atmospheric conditions. According to a tendency of the experimental results, the optimum zone of ice crystal formation was delineated as a double-peak distribution pattern with the maximum ice crystal concentrations above -3 C and around 33 C. It has been suggested these ice nucleation events were caused by different mechanisms. The formation and sedimentation of cloud droplets affect the water vapor budget and subsequent saturation ratio inside the chamber. An adequate method to estimate the supersaturations with respect to ice is required for detailed investigation of ice initiation processes. The microphysical instruments need to be well-calibrated in order to improve the accuracy of water condensate measurements. The optical sensors will need to obtain more precise techniques of particle detection that lead to better understanding of the aerosol effects on cloud microphysics. REFERENCES DeMott, P.J., and D. C. Rogers, 199: Freezing nucleaaggregates by using finite-difference time domain (FDTD) method. J. Remote. Sensing. Soc. Jpn., 26,

6 Lawson R. P., B. A. Schmitt, and T. L. Jensen, 21: An overview of microphysical properties of Arctic cloud observed in May and July 1988 during FIRE ACE. J. Geophys. Res., 16, D14, Tajiri T., M. Murakami, N. Orikasa, A. Saito, and K. Kusunoki, 26: Laboratory experiments of ice formation in cloud simulation chamber. Proceedings of the 12 th Conference on Cloud Physics of the American Meteorological Society, 1-14 July, 26, Madison WI. P2.53.