Docstoc

mitchell_AMWG

Document Sample
mitchell_AMWG Powered By Docstoc
					Sensitivity of the CAM to Small Ice Crystals
          As Measured by the FSSP

          David L. Mitchell

  Desert Research Institute, Reno, Nevada

             Philip Rasch

         NCAR, Boulder, Colorado




                                              Particle Size, Water Path, and Photon
                                                 Tunneling in Water and Ice Clouds
                               -1-          ARM STM Albuquerque 22-26 Mar 2004
Small mode enhancement with decreasing T for tropical anvil cirrus;
Opposite for mid-latitude cirrus.


                                              Particle Size, Water Path, and Photon
                                                 Tunneling in Water and Ice Clouds
                                   -2-      ARM STM Albuquerque 22-26 Mar 2004
Albedo and emissivity differences between mid-latitude and tropical anvil
cirrus clouds having the same ice water path (IWP).

Also, note the contribution of the small crystal mode to the albedo and
emissivity (dashed curves are for the large mode only).
                                                Particle Size, Water Path, and Photon
                                                   Tunneling in Water and Ice Clouds
                                    -3-       ARM STM Albuquerque 22-26 Mar 2004
A = fraction of the “original” concentration to total concentration of ice particles.
From Field et al. 2003, J. Atmos. Ocean. Tech., 20, 249-261.




                                                        Particle Size, Water Path, and Photon
                                                           Tunneling in Water and Ice Clouds
                                           -4-        ARM STM Albuquerque 22-26 Mar 2004
Corrected for Ice Particle Shattering Effects




                             Particle Size, Water Path, and Photon
                                Tunneling in Water and Ice Clouds
                -5-        ARM STM Albuquerque 22-26 Mar 2004
CAM RESULTS: Tropical vs. mid-latitude size distributions




                            ML PSD

                            Tropical




  SW cloud forcing                       LW cloud forcing



                                         Particle Size, Water Path, and Photon
                                            Tunneling in Water and Ice Clouds
                                -6-    ARM STM Albuquerque 22-26 Mar 2004
     Tropical minus mid-latitude PSD simulations: Cloud forcing




Tropical – ML SWCF differences




Tropical – ML LWCF differences




                                              Particle Size, Water Path, and Photon
                                                 Tunneling in Water and Ice Clouds
                                   -7-      ARM STM Albuquerque 22-26 Mar 2004
PSD median mass flux fall velocities




                            Particle Size, Water Path, and Photon
                               Tunneling in Water and Ice Clouds
                 -8-      ARM STM Albuquerque 22-26 Mar 2004
                    Impact of ice particle fall velocities


  IWP zonal means for
  in-cloud conditions,
  tropical & mid-latitude
  PSD schemes
                                                                   M L PSD
                                                                  Tropical




% difference in high-level
cloud coverage,
tropical – ML PSD scheme



                                                  Particle Size, Water Path, and Photon
                                                     Tunneling in Water and Ice Clouds
                                       -9-      ARM STM Albuquerque 22-26 Mar 2004
Heating rates and tropical cold bias




                              Particle Size, Water Path, and Photon
                    - 10         Tunneling in Water and Ice Clouds
                      -     ARM STM Albuquerque 22-26 Mar 2004
        Cloud Parcel Model Formulation
dSw           dXl     dXi
—— = A1w – A2 —— – A3 ——
dt            dt      dt
Sw = supersaturation with respect to water
w = updraft
dXl/dt = condensation rate of liquid phase
dXi/dt = condensation rate of ice phase
A terms = f(T,P,supersaturation ratio)

Fick’s 1st Law if Diffusion for ice crystal growth

Δm        Δρv
—— = A Dv ——
Δt        Δx
                                          Particle Size, Water Path, and Photon
                               - 11          Tunneling in Water and Ice Clouds
                                 -      ARM STM Albuquerque 22-26 Mar 2004
Nucleation clue: Small mode crystals are quasi-spherical
                                            CPI Images
                                            from SPEC




                                      Particle Size, Water Path, and Photon
                            - 12         Tunneling in Water and Ice Clouds
                              -     ARM STM Albuquerque 22-26 Mar 2004
              Ice Crystal Nucleation Assumptions

Small Mode: Inside-out contact nucleation (Shaw and Durrant 2005, J.
Phys. Chem. B, Letters).
Cloud droplets evaporate via Bergeron-Findeisen process and some percent
freeze as ice nuclei within contacts the droplet interface. We assume this
occurs for D < 2 µm about.

                                          Frozen droplet


Large Mode: Based on Huffman (1973); NIN = C SK. C modified to give
typical N as measured by the 2DC probe (i.e. 10-20 l-1).




                                                   Particle Size, Water Path, and Photon
                                      - 13            Tunneling in Water and Ice Clouds
                                        -        ARM STM Albuquerque 22-26 Mar 2004
Criteria for testing ice crystal nucleation process:
 Small crystal mode,         Large crystal mode,
 mid-latitude ice clouds:    mid-latitude ice clouds:

 Mean length: 27 ± 3 μm      Mean length: > 100 μm,
              (for cirrus)   up to 1-2 mm in frontal clouds
 Typical N: 500 – 5000 l-1   Typical N: 10-20 l-1
 IWC: 10 – 40% of total      IWC: 60-90% of total




                                          Particle Size, Water Path, and Photon
                               - 14          Tunneling in Water and Ice Clouds
                                 -      ARM STM Albuquerque 22-26 Mar 2004
Different growth rates produce bimodal spectra, but small mode
IWC >> large mode IWC. Simultaneous nucleation.




                                           Particle Size, Water Path, and Photon
                                - 15          Tunneling in Water and Ice Clouds
                                  -      ARM STM Albuquerque 22-26 Mar 2004
    Propose 2-stage ice crystal nucleation process to satisfy
        observations of ice cloud bimodal size spectra



                                         +
Large mode crystals    Vapor pressure drops below water saturation
form from ice nuclei   as droplet surface area decreases. Fraction of
and grow at expense    smallest evaporating droplets freeze (2%) to
of cloud droplets      form small crystal mode.




                                                  Particle Size, Water Path, and Photon
                                     - 16            Tunneling in Water and Ice Clouds
                                       -        ARM STM Albuquerque 22-26 Mar 2004
2-stage nucleation process in an updraft




                               Particle Size, Water Path, and Photon
                    - 17          Tunneling in Water and Ice Clouds
                      -      ARM STM Albuquerque 22-26 Mar 2004
2-stage nucleation process with negligible vertical motion.
Initial droplet diameter = 10 μm, LWC = 0.10 g m-3.




                                          Particle Size, Water Path, and Photon
                               - 18          Tunneling in Water and Ice Clouds
                                 -      ARM STM Albuquerque 22-26 Mar 2004
         Particle Size, Water Path, and Photon
- 19        Tunneling in Water and Ice Clouds
  -    ARM STM Albuquerque 22-26 Mar 2004
2-stage process for droplet diameter = 5 μm, LWC = 0.10 g m-3




                                          Particle Size, Water Path, and Photon
                                - 20         Tunneling in Water and Ice Clouds
                                  -     ARM STM Albuquerque 22-26 Mar 2004
         Particle Size, Water Path, and Photon
- 21        Tunneling in Water and Ice Clouds
  -    ARM STM Albuquerque 22-26 Mar 2004
                          Conclusions – Parcel Model
1. Predicted size spectra in ice clouds were bimodal due to different crystal shapes
and their corresponding growth rates.

2. To approximate the observed properties of ice cloud bimodal size spectra, a 2-stage
process of ice crystal production appears necessary.

3. This 2-stage process may account for the stable observed properties of the small
crystal mode and observed ice supersaturations (Korolev and Isaac 2005).

4. This 2-stage process indicates that the small crystal mode in ice clouds should be
affected by changes in aerosol concentration similar to liquid water clouds. Since the
small mode may potentially dominate cloud radiative properties and heating rates
for polluted atmospheres, aerosol particles may play a critical role in determining the
radiative properties of mid-latitude cirrus clouds.

5. An aerosol-cirrus IOP should relate aerosol concentrations to the small crystal
mode of the cirrus size distribution.


                                                           Particle Size, Water Path, and Photon
                                              - 22            Tunneling in Water and Ice Clouds
                                                -        ARM STM Albuquerque 22-26 Mar 2004

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:3
posted:12/19/2011
language:
pages:22