P14A-01 16:00h
Saharan Air Layer Interaction with Hurricane Claudette (2003)
It has long been observed that the Saharan Air Layer (SAL), a large and seasonally-persistent layer of West African aeolian dust suspended over the Atlantic Ocean, may influence the variability and intensity of easterly waves and tropical cyclones in the Atlantic basin. The radiative and conductive properties of the Saharan aerosols may contribute to warming within the dust layer, creating an anomalous baroclinic zone in the tropical North Atlantic. Environmental baroclinic instability is a mechanism for conversion of potential energy to eddy kinetic energy, facilitating wave growth. However, this same baroclinic mechanism, along with the dry properties of the SAL, could also promote asymmetry in a tropical cyclone, limiting its intensity. Detailed investigations of specific cases are necessary to better understand the radiative heating or cooling impact that the Saharan aerosols cause as well as potential influences on cyclone track and intensity stemming from the aeolian dust cloud. Here, we consider the case of Claudette in 2003. On June 29, 2003, an easterly wave embedded near the southern boundary of a broad Saharan dust layer emerged from the West African Coastal Bend region into the Atlantic Ocean. The wave propagated westward, reaching tropical storm intensity as Claudette in the Caribbean and developing into a hurricane just before making landfall on the southern Texas Gulf of Mexico coast on July 15. The SAL propagated in phase with this system throughout almost its entire evolution. Rapid intensification of Claudette into a hurricane in the last 15 hours prior to landfall was concurrent with a decoupling from the Saharan dust intrusion, with the two following separate tracks into North America at the end of the period. We performed an investigation to understand and diagnose the interaction between the Saharan Air Layer and Claudette. HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory Model) along-trajectory potential temperature plots as well as the MODIS-TERRA (Moderate Imaging Spectroradiometer) aerosol product suggested that the intensity of Saharan dust was well correlated to heating in the environment. NOGAPS (Navy Operational Global Atmospheric Prediction System) model analysis outputs revealed that the mid-level easterly jet along the southern SAL boundary was a source for potential combined barotropic-baroclinic instability, possibly contributing to the growth of the formative easterly wave. The Charney-Stern condition was satisfied for the formative tropical wave throughout most of its evolution, corresponding to the mostly progressive wave growth occurring almost consistently throughout its evolution. The current research suggests that there was a dual-celled set of circulations, forced by the SAL boundaries, but modified by the mid-level easterly jet. The presence of the dust layer appears to have been a factor playing an important role in the life cycle of this tropical cyclone. In this case, the dusty Saharan Air Layer apparently facilitated growth of the formative easterly wave, but later suppressed the intensity of Claudette until shortly before landfall.
P14A-02 16:15h
Monitoring and Assessing Environmental Controls on Blowing Dust in Dryland Central Asia
Mineral aerosols are an important component of the Earth's climate system and potentially a major forcing mechanism for climate change. Monitoring aeolian dust dynamics and improving our understanding of the major environmental controls influencing dust storm occurrence is therefore a key scientific challenge. Meteorological records are regularly used to calculate wind erosion indices for modelling conditions conducive to dust storm occurrence and remote sensing (e.g. TOMS) is increasingly being used as a method for monitoring regional atmospheric aerosol loading. Given that direct measurements of dust concentrations and deposition rates are very limited in number, very few studies have investigated the associations between wind erosion indices, satellite monitoring of aerosol loadings and actual dust flux. In this paper we report the findings of a study aiming to validate correlations between measured dust deposition, meteorological conditions and TOMS aerosol index in the region adjacent to the southern shore of the Aral Sea. Monthly measurements of dust deposition were collected from May 2000 to May 2001 at 7 sites in Karakalpakstan. Daily meteorological data from these sites were used to calculate a climatic index of wind erosion parameterised using a variety of measures of available soil moisture (P-E, mean rainfall, number of rain days) and a range of measures of wind erosivity (mean wind speed, dimensionless vector units, frequency over threshold). Additionally, monthly N7 TOMS and EP TOMS AI data were extracted for a coincident period for 3 areas covering the SW, S and SE regions adjacent to the Aral Sea. Regression analysis was used to identify relationships between direct measurements of dust deposition, various adaptations of the wind erosion index and the TOMS AI data. Findings from the study indicate strong negative (and exponential) relationships between rates of dust deposition flux and various measures of soil moisture availability at the annual and monthly temporal scale. Surprisingly, relationships between dust flux and all measures of wind power were poor. These results suggest that the occurrence of dust storms in the region is strongly controlled by seasonally changing surface erodibility parameters (surface crusting, vegetation growth, damp sediment) rather than erosivity factors. Time lags associated with the dynamics of these erodibility parameters resulted in only weak associations between dust flux and environmental controls over time periods less than one month and discussion highlights the complex role of such surface characteristics in modelling of dust emissions. Particularly strong associations between TOMS AI and dust flux were found suggesting that at temporal scales greater than one month satellite monitoring can provide a useful tool for describing regional dust occurrence.
P14A-03 16:30h
ENSO controls on dust emissions from large ephemeral lakes
Mineral aerosols (desert dust) interact with global climate and biogeochemistry. Recent research has suggested that the most significant sources of mineral aerosols are associated with topographic lows in drylands that contain contemporary or old ephemeral lakes, and that complex relationships may exist between surface water, hydrology and these desert dust sources. Most mineral aerosol emissions are observed in the northern hemisphere. Here, the identification of key processes acting within the dust source areas using remote sensing is far from straightforward; and currently inconclusive. This has implications for the generation and testing of dust emission models, and for estimating the relative impacts of natural and anthropogenic dust emission processes. In the southern hemisphere, dust emissions from a range of relatively discrete dust sources, centred on contemporary ephemeral lake basins, may provide an opportunity to constrain some of the emission processes and forcing mechanisms at regional scales. Research here involves the study of processes occurring within the dust source areas of southern Africa, southern America, and Australia. Using time-series of monthly TOMS, AVHRR, gridded climate data, surface climate data, and occasional gauged hydrological inputs, the relationship between dust emissions and surface processes have been investigated for a 20-year period (1982-2002) for large ephemeral lakes. The sources investigated were: (i) Etosha Pan, Namibia (location: 16$^{o}$E, 18$^{o}$S: area c.4800km$^{2}$), (ii) The Makgadikgadi Pans, Botswana (location: 21$^{o}$S, 26$^{o}$E; area c.4480km$^{2}$ (Ntwetwe Pan) and c.1116 km$^{2}$ (Sua pan), (iii) Lake Eyre, Australia (location 27$^{o}$S 137$^{o}$E, area c.9300km$^{2}$) and (iv) Salar de Uyuni (area c.10580km$^{2}$) and Salar de Coipasa (area c.2220km$^{2}$) located at approx. 20$^{o}$S 67$^{o}$W in Bolivia. For most of these lakes, findings show a significant relationship between anomalies in TOMS AI and the timing/extent of inundation; confirming a link between each ephemeral lake basin and the TOMS dust plumes, and giving a clear indication of dust emission processes acting in each basin. In particular, significant inflows and inundation events in most of these basins are associated with La Nina episodes, and these events lead to significant negative TOMS anomalies over long time-periods; primarily due to raised groundwater levels and increased vegetation abundance. El Nino periods generally equate to periods of low inflows, a reduction in groundwater level, and greater likelihood of positive TOMS anomalies. This research suggests that the large southern hemisphere ephemeral lake basins are significant regional dust sources, and that an appreciation of their hydrological dynamics is essential for understanding/modelling global dust emissions from drylands.
P14A-04 16:45h
Baroclinic storms in the northern hemisphere of Mars
We study Martian baroclinic storms in the northern hemisphere using the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) wide angle global map swaths and Thermal Emission Spectrometer (TES) atmospheric temperature and optical depth data. This study covers the period from the beginning of the mapping phase to the end of the currently released data (from mid northern summer of the first year to mid northern winter of the third year). We present the spatial and temporal distribution of northern hemisphere fronts observed in MOC images, investigate their relationship to the atmospheric thermal state and dust loading, and study the detailed morphology and the thermal structure across the fronts using simultaneous MOC and TES data. We also present the distribution of southward moving dust storms that appear to be related to northern hemisphere baroclinic fronts, and assess their role in dust transport and global circulation.
P14A-05 17:00h
Atmospheric convection and measurements of variability of radiation and surface fluxes of heat, water vapor, and dust.
Convective plumes and vortices produce an efficient vertical transport of dust that supplies significant amounts of aerosols to the atmospheric boundary layer, and consequently might play an important role in aerosol-climate interactions. Our aim is to contribute to the understanding of the direct effects of mineral dust on climate. Our main objective is to quantify the intensity and variability of the surface flux and vertical transport of mineral dust under convectively unstable conditions, and to study their relationship with changes in local and regional atmospheric aerosol content. We will describe the suit of instruments that are being used to measure the variability of atmospheric convection, radiation, and surface fluxes of heat, water vapor, and dust, in addition to their relationship with atmospheric aerosol concentration and the feedbacks between convection and boundary layer aerosol concentration. Sample measurements will be presented and planned future campaigns will be described.
P14A-06 17:15h
Electric Dust Devils and Dust Storms
Electrical fields measurements in terrestrial dust devils show that they maintain tremendous charge separation and that their electric fields exceeds the breakdown potential (~10 kV/m) of the Martian atmosphere (Farrell et al., 2002, 2003; Krauss et al., 2002; Renno et al., 2004). Typical Martian dust devils are be up to 100 times larger and much stronger than the small terrestrial analogues. Martian dust devils have higher dust content and may produce even stronger electrical fields. Indeed, the dust devils observed in the Pathfinder images have about 700 times the dust content of the local background atmosphere (Metzger et al., 1999). Thus, strong charge separations and electric-field breakdown are likely to occur on Martian dust devils and dust storms. Our theory (Renno et al., 2004) and laboratory experiments in a Mars chamber shows that collisions between sand and dust particles produce non-thermal microwave radiation. The non-thermal microwave emission allows not only the indirect detection of electric activity but also the determination of the physical properties of Martian sand and dust by remote sensing. Besides being geologically important, electrically charged Martian dust devils and dust storms are potential hazards to Landers and will be dangerous to future astronauts exploring its surface. Indeed, the design of adequate mechanical and electrical systems for these Landers cannot progress effectively without a better understanding of Martian dust devils and dust storms. Moreover, ancillary phenomena associated with electrically charged vortices can ionize atmospheric gases and might have important implications for atmosphere chemistry and even habitability.
P14A-07 17:30h
MER Atmospheric Results: Pancam and Mini-TES
Although at first glance, the Mars Exploration Rover (MER) payload may be perceived as primarily suited to geological investigation, it is in fact quite well-suited to carry out a robust and dynamic program of atmospheric monitoring and characterization with a particular emphasis on the planetary boundary layer. More to the point, it has been doing so at both the Gusev and Meridiani locations for more than 200 days. Ongoing atmospheric observations include (1) periodic thermal infrared spectra of the Martian sky by the Miniature Thermal Emission Spectrometer (Mini-TES). The actual sequences consist of both standard 200-second integrations and long ``stares'' of up to (almost) an hour. These data are highly diagnostic of vertical thermal structure (from 10 meters to 3-5 kilometers), aerosol optical depth along with particle size, and under the right conditions, the water column. (2) direct solar imaging using the Panoramic Camera (Pancam) and 440/880 nm + neutral density (ND5) filters, providing accurate measurement visible optical depths. (3) near-sun and ``sky-arc'' sequences using the full suite of geological filters, intended to capture the forward-diffraction peak and the phase function characteristics of the aerosol particles. (4) carbon dioxide (15 micrometer band) profiling of the Mini-TES surface observations, providing an average near-surface (1 m) air temperature. The above activities have been (and will continue to be) used to characterize diurnal and secular temporal trends and to examine the spatial variability of such trends. In addition, serendipity has provided the unique opportunities of watching the decay of a moderate dust storm from two widely-separated sites as well as of multiple simultaneous orbiter-rover observing ``campaigns.'' The latter includes thus far the Mars Express and Mars Global Surveyor over-flights. During our presentation, we will summarize the atmospheric results obtained and analyzed through the end of the first 200 days of operations, the unique contributions/capabilities of each instrument, and the synergy which comes from combining the two, e.g., visible-to-infrared optical ratio.
P14A-08 17:45h
Resolving Codependent Processes Within Natural Dust Devil Vortices
The on-going Eldorado Valley Dust Devil Project was expanded this year to include other sites in the Mohave Desert, California, and in northern Nevada on the Black Rock playa. During each field campaign, dust-laden vortices were directly sampled by over 30 instruments on a terrain-following rig that profiled conditions from ground level to over 5m above the surface. On several occasions, dust devils were observed to follow borders between aerodynamically rough (ex. sagebrush or coarse mud-flaked playa surfaces) and smooth terrain (ex. even, undisturbed & undesiccated silt-floored playa surfaces). Regardless of underlying terrain, dust devils rarely formed coherent structure nor could such vortices be sustained when ambient winds dropped below 2 m/s or gusted above 8 m/s (as measured at 2m above ground). Furthermore, vortex formation, rotation, and type were clearly influenced by upstream obstacles, such as range fronts, as much as 10km upwind. Video and high-resolution time-lapse photography are being correlated with the 5 Hz sensor logs to examine the interaction of meteorological conditions (profiled wind speed & direction, pressure, temperature, and humidity) within the vortices. In addition, single-level measurements were recorded for dust impacts and UV occultation (using a sensor suite developed for the Beagle2 Mars Lander), charged particle impacts, and electric fields (horizontal and vertical). These data are still being evaluated. Preliminary results indicate that thermally powered vortices are able to entrain soil material at surprisingly low wind shears and "poorly structured" columns are able to last longer than previously noted. Our research will attempt to decipher the dominant aerodynamic and electric factors that govern dust devil behavior and erosion efficacy.
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