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Last modified
7/28/2009 2:35:20 PM
Creation date
3/11/2008 11:30:28 AM
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Weather Modification
Title
Harold D. Orville Symposium - Forty Years of Modeling Clouds and Weather Modification
Date
4/26/2003
Weather Modification - Doc Type
Report
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />For the sake of brevity, I will not list the major <br />contributions made by Cotton and colleagues at <br />Colorado State University (e.g., Flatau et al., <br />1989), L. Koenig and F. Murray, and many others, <br />even though they are acknowledged to be <br />significant. <br /> <br />5. TWO-MOMENT BULK MICROPHYSICS <br /> <br />In the early 1990s, I tried to make numerous <br />changes to the basic LFO formulation by <br />expanding the system of prognostic equations to <br />include calculating the number concentrations of <br />various ice species, as well as calculating mixing <br />ratios of graupel and hail. There were three <br />motivating factors for doing this. <br /> <br />i. A general rule had developed at GSFC to use <br />RH for oceanic convective systems and LFO <br />for continental systems based on the results <br />of McCumber et al. (1991). We were <br />uncomfortable with running two different <br />microphysics packages based on geographic <br />location. This approach also lacked any <br />certainty that there was consistency between <br />the microphysics and the dynamics, such as <br />representing dense, newly frozen drops that <br />form in strong updrafts as moderate-density <br />graupel. <br /> <br />ii. Cloud model' simulations were used to <br />develop multichannel passive and active <br />(radar) microwave retrievals for TRMM. <br />Output from the GCE model was used as <br />input into radiative transfer models, from <br />which surface rainfall and crude vertical <br />profiling of hydrometeors would be extracted <br />from a set of satellite-observed brightness <br />temperatures. The idea was great, except <br />that the retrievals were sensitive to the <br />profiles of large ice and their parameterized <br />properties (i.e., their size and density; e.,g., <br />Yeh et al., 1990; 1990; Adler et al., 1991). <br />Over water the lower frequency channels <br />respond well to the presence of liquid water <br />because of its strong microwave emission <br />(looks "black") over weak ("gray") emission <br />from the ocean surface. But high microwave <br />emission over land renders the lower- <br />frequency channels of little use. Instead, <br />scattering of hIgher frequency microwave <br />radiation by ice reduces brightness <br />temperatures. Retrievals over land must rely <br /> <br />more on the less-direct relationship between <br />column-integrated ice contents and surface <br />rainfall, complicated further by the fact that the <br />size and density of the ice affect the amount of <br />scattering. <br /> <br />iii. Difficulty in simulating the salient observed <br />radar reflectivity structures from either the RH <br />or the LFO versions of the GeE microphysics. <br />In 1989 or 1990 I had devised a quick, version <br />of a four-class ice scheme that predicted the <br />mixing ratios of cloud ice, snow, graupel, and <br />hail (in addition to the cloud water and rain), <br />which was tested in a one-dimensional time- <br />dependent cloud model (Ferrier and Houze, <br />1989). Unfortunately the simulated radar <br />reflectivities varied considerably for different <br />values of the assumed intercept of the particle <br />distributions. There were more adjustable <br />coefficients to tune, and there was also little <br />guidance on what these values should be for <br />tropical convection. In addition, the <br />reflectivities in the convective updraft cores <br />were sensitive to the details of how wet growth <br />and melting of graupel and hail were <br />represented. <br /> <br />For these reasons came the development of <br />the two-moment four-class ice scheme (Ferrier, <br />1994; "4ice"), in which the number concentrations <br />and mixing ratios of small ice crystals, snow, <br />graupel, and frozen drops/hail are calculated. <br />Rapid riming of snow or frozen drops forms <br />graupel, the criterion being that a sufficient amount <br />of rime must be collected on the ice particles to <br />change their density. Extensive lookup tables for <br />calculating rime conversion between different ice <br />species were formulated from Heymsfield and <br />Pflaum (1985) and Rasmussen and Heymsfield <br />(1985). The freezing of rain produces high-density, <br />fast-falling ice particles like sleet. The problem <br />with RH was that both riming of snow to graupel <br />and freezing of raindrops produced the same type <br />of "graupel" particle with no distinction for their <br />fundamentally different properties. The mixing <br />ratio of liquid water on wet ice was also calculated <br />for improved radar reflectivities, although I now <br />realize that the computational resources may not <br />justify the limited benefits. Small ice crystals were <br />allowed to fall based on observations in cirrus <br />clouds and the parameterization of cirrus by Starr <br />and Cox (1985). Some detail was spent on <br />accurate calculation of the collection kernels, <br />including the use of accurate and computationally <br /> <br />7 <br />
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