GCSS WGII
(Cirrus) Parcel Model Comparison (PMC) Project
The GeWex Cloud System Study (GCSS) Working Group on Cirrus Clouds (WGII) is coordinating a series of model intercomparisons involving volunteer groups from throughout the world. The parcel model comparison project is the most recent undertaking. Dr. Ruei-Fong Lin (NASA-GSFC) is coordinating this project. The purpose is to explore our understanding of cirrus cloud formation processes and determine how specification of microphysics affects the cirrus cloud properties. The goal is to assist in better simulations of clouds on larger scales. Our group at CSU is participating in this effort using the parcel model described below. Initial simulation conditions and results are described.
The DMKY
(DeMott-modified Ken Young) Microphysical Parcel Model
General description: This adiabatic parcel model evolved from the two dimensional microphysical model of Young (1974). The microphysical framework is essentially the same as described in that paper and in the first "parcel" version of the model described by Rokicki and Young (1978). No dynamics is prescribed beyond a simple updraft. Precipitation does not fall into or out of the parcel. With few exceptions, modifications made by the author have only been in descriptions of ice nucleation (DeMott et al., 1994; DeMott et al., 1997; DeMott et al., 1998).
Summary details in PMC format:
|
item |
description |
|
Hydrometeor categories included |
liquid, spherical ice crystals, variable-included axis (a and c) ice crystals |
|
Hydrometeor distribution |
explicit with continuous bin structure |
|
Diffusional growth |
diffusional growth equation, kinetic effects and ventilation effect included, option for direct radiation effect |
|
Time step |
User-specified, typically = 10/updraft (cm/s) in cirrus regime |
|
Collection |
quasi-stochastic (see Young, 1974) |
|
Nucleation modes included |
homogeneous freezing: explicit, 100 size bins of dry CCN particles that grow but do not deplete vapor until activation or ice formation, water activity and freezing conditions depend on composition chosen(sulfuric acid, ammonium sulfate, or ammonium bisulfate). Heterogeneous freezing: explicit options for deposition/condensation freezing (Meyers/DeMott/Cotton form) and immersion freezing; immersion freezing-only used for CPMC following DeMott et al. (1998) (CCN above 0.1 micron diameter contain 50% mass fractions of insoluble particles that act as heterogeneous freezing nuclei with the temperature spectrum observed by DeMott et al. in upper tropospheric sampling). |
Special Notes:
1. ice crystal shapes: Ice crystals grow as spherical ice particles with a density of 0.9 g/cc until particles exceed a size of 20 microns radius (12 bins). Then particles enter a 36x36 bin array describing growth along a- and c-axes. The ice crystal axial ratio and density relationships at temperatures warmer than -40 degrees C were given by Young (1974). These were based on observations. Below -25 degrees C, the relative axial growth ratio (dc/da) is currently prescribed to depend on modified (for ventilation and kinetic effects) vapor density excess (DELR) in the form gamma = 42.5*DELR +0.35. Gamma is restricted to > 1.2 (columnar habit). The diffusionally-grown ice density is taken as 0.9 g/cc. Further development is expected in this area.
2. Ambient temperature: determined by the adiabatic temperature change and particle temperatures.
References:
DeMott, P.J., M.P. Meyers, and W.R. Cotton, 1994: Numerical model simulations of cirrus clouds including homogeneous and heterogeneous ice nucleation. J. Atmos. Sci., 51, 77-90.
DeMott, P.J., D.C. Rogers, and S.M. Kreidenweis, 1997: The susceptibility of ice formation in upper tropspheric clouds to insoluble aerosol components. J. Geophys. Res., 102, 19575-19584.
DeMott, P.J., D.C. Rogers, S.M. Kreidenweis, Y. Chen, C.H. Twohy, D. Baumgardner, A.J. Heymsfield, and K.R. Chan, The role of heterogeneous freezing nucleation in upper tropospheric clouds: Inferences from SUCCESS. Geophys. Res. Lett., 25, 1387-1390, 1998.
Young, K.C., 1974: A numerical simulation of wintertime, orographic precipitation: Part I. Description of model microphysics and numerical techniques. J. Atmos. Sci., 31, 1735-1748.
Rokicki, M.L. and K.C. Young, 1978: The initiation of precipitation in updrafts. J. Appl. Meteor., 17, 745-754.
The
Phase I Simulations
Two sets of simulations have been performed initially. These represent warm and cold idealized cirrus clouds forming in an environment with an ice-psuedoadiabatic lapse rate. The simulations begin with the initial conditions listed in the tables below. Parcels rise for 800 meters. The direct radiation effect on ice crystal growth is not included in these cases. Also, ice crystals are "collapsed" into unit mass categories for comparison to other models. So preferential growth along a or c axes is not indicated in the ice crystal size distributions. Aerosols are assumed to be sulfuric acid with total concentration equal to 200 per cc and lognormally distributed with a dry mode radius rg of 0.02 micron, sigma = 2.3 (from Jensen et al. [1994], J. Geophys. Res., 10,421-10,442), i.e.
N(r)*dr = Nt /[sqrt(2*pi) * ln sigma]* exp {-(1/2) * [(ln r - ln rg))/ln sigma]2}*d(ln r).
The freezing conditions of the sulfuric acid follow recent results obtained in the laboratory at CSU.
|
simulation title |
Height |
pressure |
temp |
Updraft |
RH ice |
Lapse rate |
|
Wa004 |
8.3 |
340 |
-40 |
0.04 |
100 |
0.0093 |
|
Ca004 |
13.4 |
170 |
-60 |
0.04 |
100 |
0.0097 |
|
simulation title |
Height |
pressure |
temp |
Updraft |
RH ice |
Lapse rate |
|
Wa004 |
8.3 |
340 |
-40 |
0.04 |
100 |
0.0093 |
|
Ca004 |
13.4 |
170 |
-60 |
0.04 |
100 |
0.0097 |
Phase I
Simulation Results
Cases: Wh Cases: Wa Cases: Ch Cases: Ca
Cases: Wh Cases: Wa Cases: Ch Cases: Ca
Cases: Wh Cases: Wa Cases: Ch Cases: Ca
Cases: Wh and Wa
Some Special
Test Results (under construction)
