Cloud-Microphysics and Small-Scale Processes
Cloud-microphysical processes and processes smaller than about 1 km have to be parameterized in present cloud-systems resolving models. Improved understanding is needed to improve the parameterizations.
Turbulent Mixing in Clouds
Miroslaw Andrejczuk and Szymon Malinowski (both at Warsaw University, Poland) continued collaborative investigation with W. Grabowski and P. Smolarkiewicz of the small-scale dynamics of decaying moist turbulence reported in FY2003. This problem is important for radiative transfer through clouds, initiation of precipitation in warm (i.e., ice-free) clouds, and parameterization of small-scale and microscale processes in models resolving larger scales. The most recent research focused on the validity of low spatial resolution simulations reported in previous fiscal years and on sensitivity to the initial fraction of the volume occupied by the cloudy air. The latter is important for macroscopic properties of buoyancy-reversing systems and determines the extent of microphysical transformations (i.e., the homogeneous versus inhomogeneous mixing) during the small-scale turbulent mixing and homogenization. A series of model simulations applying detailed microphysics approximation for the moist thermodynamics with variable initial fraction of the volume occupied by the cloudy air and with the variable input of the turbulent kinetic energy (TKE) suggests that the evolution of the mean cloud droplet spectrum during mixing follows a universal path determined primarily by TKE. These results suggest that it is possible to develop a simple parameterization of the cloud droplet evolution resulting from cloud entrainment usable in cloud-resolving models that apply bulk cloud-microphysics parameterization.
Turbulent Mixing, Collision Rate and Collision Efficiency of Droplets
Lian-Ping Wang (University of Delaware) and W. Grabowski (who is an Adjunct Professor at the University of Delaware), with two graduate students Orlando Ayala and Yan Xue (University of Delaware), continued investigations of the effects of turbulence on the geometric collision rate and collision efficiency of cloud droplets when droplet inertia, gravity, and turbulence microstructure are all considered. This is an important problem because the impact of cloud turbulence on microphysical processes (warm rain initiation in particular) remains ambiguous. A significant progress in the last year is the development of a methodology for conducting DNS of hydrodynamically interactive droplets in turbulent flow. The method combines DNS of air turbulence with an improved formulation of the superposition method for treating hydrodynamic interactions. They validated the new methodology against the hydrodynamic-gravitational problem and against mechanics of random suspensions. It allows the problem of collision efficiency of hydrodynamically interactive droplets in a turbulent flow to be addressed. They found that turbulence enhances the collision efficiency because, in turbulent flow, hydrodynamic interactions become less effective in reducing the average relative radial velocity and that the increase in the collision efficiency depends on the flow dissipation rate. Turbulence can increase the net collision rates by a factor of 2 to 3 at dissipation rate of 400 cm s . They plan to develop a parameterization of collision-enhanced efficiency due to turbulence.
Convection Effects and Particle Generation
Effects of Absorbing Aerosols on Subtropical Shallow Convection
Observations during the Indian Ocean Experiment (INDOEX) showed that isolated trade-wind cumuli were embedded in the widespread anthropogenic haze over northern Indian Ocean. Prof. Gregory McFarquhar and Hailong Wang (University of Illinois, Urbana-Champaign), and Wojciech Grabowski examined factors affecting cloud cover, seeking to better characterize the semi-direct and indirect effects of three-dimensional limited-domain simulations of trade wind cumuli in the Indian Ocean region. The Eulerian version of EULAG running with warm-rain bulk microphysics parameterization was used. A control experiment without environmental soot and sensitivity experiments with soot examined the effects of absorption on cloud formation and development. The daytime reduction of cloud fraction and liquid water path by soot was found to be at 0.8% and 5.3gm . They found that at the ocean surface (top-of-model atmosphere) absorbing aerosols exert a total diurnal radiative forcing of -15.1 Wm (-1.0 Wm ) and a semi-direct forcing of 1.4 Wm (1.5 Wm ), respectively. The main conclusion is that downdrafts, updrafts, total water mixing ratio, potential temperature and the diurnal evolution of cloud fraction, liquid water path and cloud heights are sensitive to not only to the concentration and absorption properties of soot, but also to its vertical distribution.
Convectively Generated Tropical Ice Clouds
Andrew Heymsfield and Larry Miloshevich, and Cynthia Twohy Oregon State University), Michael Poellot (University of North Dakota), and Ann Fridlind (NASA Ames Research Center), studied the formation of ice in convective cells and anvil genesis and radiative properties associated with tropical convection. They investigated the conditions facilitating the presence of supercooled liquid water in convective cells at temperatures below –30C, which, if present, will lead to copious small ice crystals to be produced by homogeneous ice nucleation and highly reflective anvils. Observations in deep, low-latitude vigorous convective cells, together with calculations of droplet and ice particle growth and homogeneous ice nucleation were used to investigate the influence of large ice particles lofted in updrafts from lower levels on the homogeneous nucleation process. The pre-existing large ice particles act to suppress homogeneous nucleation through competition via diffusional and accretional growth, mainly when the updrafts are <5 m/s. In deep convective updrafts > 5-10 m/s. The anvil is the depository for the small, radiatively important ice particles (homogeneously nucleated) and the large ice particles from below. This result could and should be included in cloud resolving and general circulation models to more realistically treat the convective cloud-anvil radiative interplay.
Microphysics and Measurement
Ice-Phase Characteristics Derived from Satellite Measurements
John Latham (long-term visitor) and collaborators Hugh Christian, Walt Petersen & Wiebke Deierling (NASA/MSFC), Alan Gadian & Alan Blyth, (University of Leeds, UK), Dr Rumjana Mitzeva (University of Sofia, Bulgaria), Scott Ellis (ATD) and James Dye are continuing their examination of the extent to which it is possible to determine thundercloud ice characteristics from satellite observations of lightning, which are now routinely made on a global scale, using NASA/MSFC devices. A specific goal is to ascertain whether measurements of lightning frequency (f) can yield estimates of precipitating and non-precipitating ice fluxes. Computations – and particularly, recent data analysis - support the hypothesis that f is roughly proportional to the product of the downward flux (fg) of graupel through the body of the thundercloud and the upward flux (fi) of ice crystals into its anvil. This raises the possibility of determining, on a global basis, values of fg and/or fi from lightning measurements. Such information could have considerable climatological and nowcasting importance, particularly with respect to flooding.
In-Situ Measurements of Electric Field and Microphysics in Anvils
The threat of the shuttle or other space vehicles triggering lightning when launched through anvils and other seemingly benign clouds is considered a major hazard at Kennedy Space Center (KSC). As a result of the loss of an Atlas-Centaur rocket in 1987 due to triggered lightning, the present Lightning Launch Commit Criteria for the launch of the shuttle and other vehicles is very restrictive. In order to better understand the conditions in which the threat of triggered lightning might be present and with the hope of increasing launch availability, an airborne project was conducted near KSC during June 2000 and 2001. The project, lead by James Dye, used the University of North Dakota Citation II jet aircraft to obtain in-situ measurements of electric field and microphysics in anvils and other clouds in coordination with radar observations from KSC and the NEXRAD radar at Melborne FL. Collaborating investigators in the effort were Anthony Grainger (University of North Dakota), Hugh Christian, Monte Bateman, and Douglas Mach (all NASA Marshall Space Flight Center), Philip Krider and Natalie Murray (University of Arizona), Paul Willis (NOAA National Hurricane Division), John Willett (Garrett Park, MD), Francis Merceret (NASA KSC) and by Sharon Lewis and William Hall.
Results from the more than 30 anvils studied during the project have now been analyzed and show that there is often an abrupt increase in electric field when the aircraft entered regions of reflectivity of 10 dBZ or greater. Figure 70 presents one such example of the Citation penetration of an anvil from the downwind tip toward the convective core of the storm. Note that the abrupt increase in electric field is not reflected by similar changes in particle concentration. The particle concentrations in all size ranges show gradual increases throughout the penetration. This and similar results of this investigation are now being used to develop new, less restrictive Lightning Launch Commit Criteria for NASA and Air Force.
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