|
Current Research Activities PEARL is currently focusing on the research activities described below, in collaboration with various national and foreign institutions. 1. The thermodynamics of general circulations. Planetary atmospheres absorb energy at higher temperatures and pressures than they emit it back to space. As a result, they are capable of doing the mechanical work that drives motions ranging from small-scale circulations such as convective plumes and vortices, to planetary-scale circulations such as Hadley and Walker cells. In his classical work on the general circulation of the Earth’s atmosphere, Lorenz (1967) stated that the determination and explanation of the thermodynamic efficiency constitute one of the most fundamental observational and theoretical problems of atmospheric energetics. Our group has developed a framework for studies of the basic thermodynamics of these circulations in nature and global climate models (Adams and Renno, 2003). We have been applying this framework to study, among other, the issues outlined below: In particular, our group is collaborating with Japan’s Frontier Research Center for Global Change on the calculation of the thermodynamic efficiency of various versions of their Earth Simulator, the world’s highest resolution global climate model. We are also collaborating with the Data Assimilation Group at NASA’s Goddard Space Flight Center on the calculation of the thermodynamic efficiency of the Earth’s global climate system. The comparison of nature’s thermodynamic efficiency with that of climate models, allow us to use a single fundamental number to test and improve numerical models, key tools for studies of global climate change. 2. Scaling theories for convective circulations. During one cycle of a convective heat engine, heat is taken from the boundary layer, a portion of it is rejected to the troposphere from where it is radiated to space, and the balance is transformed into mechanical work. The mechanical energy is then used to maintain the convective motions against friction. The volume integral of the work produced by the convective heat engine gives a measure of the amount of convective available potential energy (CAPE) that must be present on the planet's atmosphere so that the convective motions can be maintained against dissipation (Renno and Ingersoll, 1996). This integral is a fundamental global number qualifying the state of the planet in quasi-steady state (quasi-equilibrium conditions in the language of the climate research community). For the Earth's present climate, the heat engine framework predicts an equilibrium CAPE value of the order of 1000 J/kg for the tropical atmosphere. This value is in agreement with observations (e.g., Williams and Renno, 1993; Renno and Williams, 1995). It also follows from our results that the total amount of CAPE present in a semi-opaque (_ < 1) convecting atmosphere should increase with increases in the global surface temperature (or the atmosphere's opacity to infrared radiation). The heat engine theory proposed by Renno and Ingersoll (1996) also predicts the vertical velocity and fractional area covered by either dry or moist convection in quasi-steady state. In their theory, Renno and Ingersoll (1996) assumed that, to a first order approximation, moist convection is a reversible heat engine and estimated that their thermodynamic efficiency is of the order of 10 %. Pauluis et al. (2000) used numerical simulations to argue that moist convection is an extremely irreversible heat engine, with thermodynamic efficiency of less than 1 %. Their calculations suggest that frictional dissipation around falling hydrometeors consumes about 1/3 and phase changes and diffusion of water vapor consumes about 2/3 of the work available from the convective heat engine. Renno (2000) generalized the heat engine theory to include any kind of irreversible process and showed that if moist convection where as inefficient as predicted by Pauluis et al. (2000), hurricanes, tornadoes, and the Hadley-Walker circulation would be at least an order-of-magnitude weaker than the observed weather systems. Renno (2000) suggested that numerical diffusion of water vapor makes numerical models of moist circulations unrealistically irreversible when compared to the real world. This is still a controversial issue that our group has been actively working on. 3. Scaling theories for convective vortices. The beauty of convective vortices such as dust devils and waterspouts stimulates the imagination of laymen or scientists who observes them. However, these vortices are not only of exceptional natural beauty, but also of great scientific interest. They are of particular interest to the meteorology community because they are thermodynamically similar to hurricanes and tornadoes, the most destructives storms observed on Earth. Therefore, the theoretical understanding of these smaller and weaker convective vortices has the potential to further our understanding of their larger and violent cousins. Since dust devils and waterspouts are relatively frequent, small, and weak, they allow safer and more efficient in situ measurements. Convective vortices are all low-pressure warm-core vortices with radius of maximum winds varying between less than 1 m (small dust devils) and about 25 km (hurricanes) in the terrestrial vortices. On Mars, dust devils are much bigger and stronger than on Earth. Terrestrial dust devils have typical diameters of less than 10 m and are seldom higher than a few 100 m (Sinclair, 1973). In contrast, dust devils with diameters between 100 m and 1 km, and heights of up to 7 km are frequently observed on Mars (Thomas and Gierasch, 1985; Malin et al., 1998). Martian dust devils also have larger dust content than the terrestrial vortices. The dust devils observed in the images of the Mars Pathfinder (MPF) panoramic camera have about 700 times the dust content of the local background atmosphere (Metzger et al., 1999). Ryan and Lucich (1983) suggest that dust devils play an important role in maintaining the bulk dust content of the Martian atmosphere. Observations indicate that dust devils and dust storms have a higher probability of occurrence and are potentially more intense near the boundary of different air masses and in regions of sloping terrain and large horizontal temperature gradients (Renno et al., 1998; Cantor et al., 2001, 2002), as illustrated in Figure 1. Also, regional and global dust storms frequently form near the edge of the south Martian polar cap during the warm season (Kieffer at al., 1992; Cantor et al., 2001, 2002). We suggest that these storms are broclinic convective heat engines driven by the rising and expansion of the warmer air and the sinking and compression of the colder air. This hypothesis is consistent with theoretical models for dust devils, waterspouts, and hurricanes (Emanuel, 1986; Renno et al., 1998; Renno et al., 2000; Renno and Bluestein, 2001). Our group has been collaborating with the University of Arizona, the University of California at Berkeley, NASA/Goddard Space Flight Center (GSFC), and NASA/Jet Propulsion Laboratory (JPL) on studies of Martian dust devils and dust storms. Researchers and engineers working in these projects have been designing and testing instruments to make in situ measurements in terrestrial and Martian dust devils and dust storms . 4. Fire Vortices. Atmospheric conditions play an important role in the behavior, severity, probability of occurrence, and propagation of forest fires. Fire managers rely on observations and forecast of local, regional, and synoptic atmospheric conditions to plan and to execute fire suppression activities. Thus, it is extremely important to understand the relationship between forest fire and weather on local, regional, and synoptic scales. Forest fires can produce fire vortices of tornadic intensity that can produce fast changes in fire intensity. Observations, numerical modeling and theory suggest that these vortices are not only extremely dangerous themselves, but can also produce anomalous and unexpected propagation of forest fires. Circulations generated by fire vortices can produce "blow-up" conditions similar to those that killed 14 firefighters near Glenwood Springs, Colorado, in 1994. We believe that the intensity of fire vortices and the anomalous propagation of forest fires can be predicted using a simple theoretical framework developed for dust devils and waterspouts (Renno et al., 1998; Renno and Bluestein 2001). Our group has been developing a model for fire vortices. This model will be suitable for calculations of the maximum intensity of fire vortices with fire and atmospheric parameters as input. Our assumptions, theoretical framework and plans for developing the fire vortex model is currently being refined. 5. Aerosol-Climate interactions. The concentration of atmospheric aerosols has increased significantly with human activity. Indeed, there are suggestions that the global aerosol climate forcing might be as large as a factor of two of the direct forcing due to greenhouse gases, and that regionally the aerosol forcing can be even larger. Aerosols produce a direct radiative forcing by scattering and absorbing solar and infrared radiation. They also cause an indirect radiative forcing by altering cloud processes via increases in cloud droplet number and ice particle concentration. This effect increases the cloud albedo (Twomey, 1974) and can decrease the precipitation efficiency of convective clouds (Albrecht, 1989). We have been studying the direct effects of desert dust on climate. Our main objective is to quantify the intensity and variability of the surface flux and vertical transport of desert dust under convectively unstable conditions, and to study their relationship with changes in local and regional atmospheric aerosol content. Besides addressing important scientific issues, we have been assessing the field performance of an instrument capable of directly measuring the surface flux of aerosol. Our goal is to build a physically based parameterization scheme for the surface flux of dust and its vertical transport by convective plumes. The parameterization is based on our scaling theory for convective plumes and vortices and will be tested with data from our field experiments. It will be suitable for application in regions of highly unstable boundary layers such as the mid-latitude deserts during their warm season. An interesting result of the field experiments conducted by our group is that the heat transport by coherent convective plumes and dust devils is respectively more than one and two orders of magnitude larger than the background fair weather value. Figure 2 shows the “eddy-correlation heat flux” measured during the passage of three coherent convective plumes over our sensors. The central plume had a large dust devil (center of the plot) with peak vertical velocity in excess of 5 m/s. This result is not surprising, convective vortices are extremely efficient in transporting heat and tracer species because their intrinsic stability, caused by rotation, prevents mixing. This leads to large buoyancy, strong vertical velocity, and large concentration of tracers inside them. The stability of rotating fluids roots on the conservation of angular momentum. When a unit mass fluid parcel rotating with angular velocity w is perturbed and acquires a velocity v in a direction perpendicular to its axis of rotation, it is deflected by an acceleration of magnitude 2 w v. Thus, the disturbed fluid parcel moves on a circular path of radius r such that v2/r = 2 w v. Then, it goes around a circle of radius r = v /2w that decreases with increases in the fluid’s angular velocity and periodically returns to its original position (twice for every rotation of the fluid). This is the physical reason why rotation inhibits mixing. The transport of dust by convective plumes and vortices might be even stronger than the heat transport, but unfortunately good instruments for directly measuring the surface flux of dust were not available at the time of the Matador field experiments and we only estimated the order-of-magnitude of the dust flux. The dust flux in strong dust plumes and dust devils are, respectively about 0.1 g m-2 s-1 and 1 g m-2 s-1. One of our goals is to refine these numbers and improve the quality of dust flux measurements by field-testing a dust flux sensor and making it available to the aerosol science community. Another interesting result of our field experiments was the discovery of strong oscillations in the surface heat flux and the intensity of boundary layer convection with time-scale of about one-hour (see Figure 3). We hypothesize that these oscillations are due to the interaction between atmospheric convection, airborne dust and solar radiation. Intense boundary layer convection produces an increase in the concentration of atmospheric dust. Then, airborne dust absorbs and scatters solar radiation producing a decrease in surface temperature and stabilizing the atmosphere. This, in turn, produces a decrease in the intensity of atmospheric convection and dust flux. The one-hour time scale is of the order of the convective time-scale. Indeed, it is the time scale that takes boundary layer convection to disperse the dust plume and intensify again. Figure 4 shows typical “dusty” convective plumes and dust devils observed during the Matador field campaigns. It also shows an aerial picture of our field test site during a day of shallow cumulus clouds and light winds (the field site is just below the aircraft’s wingtip). Note that the atmosphere was quite dusty when the picture was taken. The frequency of the “dusty” convective plumes illustrated in Figure 4 is correlated with the oscillations in surface temperature and heat budget shown in Figure 3. 6. Electric activity in convective vortices. Triboelectric charging of colliding sand and dust particles produce strong electric fields in terrestrial dust devils and dust storms (Farrell et al., 2002; Krauss et al., 2002; Towner et al., 2002). The bulk electric fields can be many orders of magnitude stronger than the fair weather field, whereas local electric fields between individual particles can be strong enough to produce corona discharges (Krauss et al., 2003; Renno et al., 2003). Charge accelerations and discharges between individual sand and dust particles generate wideband electromagnetic radiation. On Mars, dust devils and dust storms are much bigger, stronger, and more frequent than on Earth, and there electrical discharges occur at lower electric fields. Therefore, electrical activity in Martian dust events is expected to be ubiquitous (see Figure 5). We have been planning to correlate radio emission data from observations of Mars with the Very Large Array (VLA), with simultaneous Mars Global Surveyor (MGS) images of dust events to search for enhanced microwave emission in the regions of dust activity. Our group has been collaborating with the University of California at Berkeley, Signal Research Corporation, and Rincon Research Corporation on studies of the electrification of dust devils and the design of instruments to make measurements in terrestrial and Martian vortices.
Figure 1. (Left) A MOC WAC, red-filter image showing a group of 6 large dust devils. The dust devils are easily identified by their dust plume and shadow. The image is centered at18∞S, 85∞W and the solar incidence angle is 51.53∞ (from Biener et al., 2002). (Right) A dust storm developing at the edge of the north polar cap during the spring (courtesy of NASA/MSSS).
Figure 2. Surface sensible heat flux (from 10 Hz Eddy-Correlation measurements at 3 m above the surface) for a 30 min interval. The peak heat flux at the center of the plot occurs during the passage of a large dust devil.
Figure 3. Ground temperature (red) and soil heat flux (blue). Note the oscillations with time-scale of about 1hour (the vertical lines are at 30 min interval). These measurements were made during a day in which clouds were not present as illustrated in the next figure.
Figure 4. Typical “dusty” non-rotating convective plume (top left), pair of dust devils at the boundary between dry and irrigated fields (top right), and large dust devil (bottom left) observed during the Matador field campaigns. The big dust devil is about 200 m behind the truck near the center of the image at the bottom left. The Matador field site is located below the wingtip of the aircraft of the image at the bottom right.
Figure 5. Measured Martian disk radio brightness temperatures as a function of the central meridian longitude for 1975 (top) and 1978 (bottom) campaigns. The re-normalized brightness temperature for 1975 is also shown in the bottom plot (after Doherty et al., 1979). Some of the most active dust devil/storm incubator regions are marked in the top plot. They show larger and more variable temperature brightness. |
