A 3-mode, Variable Velocity Jet Model for HH 34

Variable ejection velocity jet models can qualitatively explain the appearance of successive working surfaces in Herbig-Haro (HH) jets. This paper presents an attempt to explore which features of the HH-34 jet can indeed be reproduced by such a model. From previously published data on this object, we find evidence for the existence of a 3-mode ejection velocity variability, and then explore the implications of such a variability. From simple, analytic considerations it is possible to show that the longer period modes produce a modulation on the shorter period modes, resulting in the formation of ``trains'' of multiple knots. The knots observed close to the source of HH-34 could correspond to such a structure. Finally, a numerical simulation with the ejection velocity variability deduced from the HH-34 data is computed. This numerical simulation shows a quite remarkable resemblance with the observed properties of the HH-34 jet.Comment: 28 pages LaTex, 10 postscript figure

On the parameterization of turbulent fluxes over the tropical Eastern Pacific

We present estimates of turbulent fluxes of heat and momentum derived from low level (~30 m) aircraft measurements over the tropical Eastern Pacific and provide empirical relationships that are valid under high wind speed conditions (up to 25 ms^&minus;1). The estimates of total momentum flux and turbulent kinetic energy can be represented very accurately (<I>r</I>²=0.99, when data are binned every 1 ms^&minus;1) by empirical fits with a linear and a cubic terms of the average horizontal wind speed. The latent heat flux shows a strong quadratic dependence on the horizontal wind speed and a linear relationship with the difference between the air specific humidity and the saturated specific humidity at the sea surface, explaining 96% of the variance. The estimated values were used to evaluate the performance of three currently used parameterizations of turbulence fluxes, varying in complexity and computational requirements. The comparisons with the two more complex parameterizations show good agreement between the observed and parameterized latent heat fluxes, with less agreement in the sensible heat fluxes, and one of them largely overestimating the momentum fluxes. A third, very simple parameterization shows a surprisingly good agreement of the sensible heat flux, while momentum fluxes are again overestimated and a poor agreement was observed for the latent heat flux (<I>r</I>²=0.62). The performance of all three parameterizations deteriorates significantly in the high wind speed regime (above 10&ndash;15 ms^&minus;1). The dataset obtained over the tropical Eastern Pacific allows us to derive empirical functions for the turbulent fluxes that are applicable from 1 to 25 ms^&minus;1, which can be introduced in meteorological models under high wind conditions

Estimating the impact of natural and anthropogenic emissions on cloud chemistry: the influence of organic compounds

International audienceIn order to estimate the anthropogenic influence of gas and aerosol emissions from the Petroleum Industry in maritime zones with clouds of small vertical extent, a numerical 1D Eulerian cloud-chemical model with detailed microphysics (Alfonso and Raga, 2002) is used to simulate the influence of water soluble organic compounds (WSOC) and organic+inorganic gas emissions on cloud development. Following Mircea et al. (2002), we tested the sensitivity of the cloud and precipitation development in the classical inorganic case (CIC) and the inorganic+organic case (IOC) with respect to CCN compositions. The results indicate an increase in the droplet concentration for the IOC, and a delay in the development of precipitation. The pH spectral evolution was studied during both the development and precipitation stages. The influence of the diffusion of formic acid and its generation by oxidation of hydrated formaldehyde in the aqueous phase result in a reduction in the pH of precipitation in the range between 0.05 and 0.15 pH units (from 1 to 3%) for the high ambient SO2 concentration (20 ppb) and between 0.2?0.5 pH units (from 4 to 10%) for the low ambient SO2 concentration (1 ppb) case

The influence of organic compounds on the development of precipitation acidity in maritime clouds

International audienceIn order to estimate the anthropogenic influence of gas and aerosol emissions from the Petroleum Industry in maritime zones with clouds of small vertical extent, a numerical 1-D Eulerian cloud-chemical model with detailed microphysics (Alfonso and Raga, 2002) is used to simulate the influence of water soluble organic compounds (WSOC) and organic+inorganic gas emissions on cloud development. Following Mircea et al. (2002), we tested the sensitivity of the cloud and precipitation development in the classical inorganic case (CIC) and the inorganic+organic case (IOC) with respect to CCN compositions. The results indicate an increase in the droplet concentration for the IOC, and a delay in the development of precipitation. The pH spectral evolution was studied during both the development and precipitation stages. The influence of the diffusion of formic acid and its generation by oxidation of hydrated formaldehyde in the aqueous phase result in a reduction in the pH of precipitation in the range between 0.05 and 0.15 pH units (from 1 to 3%) for the high ambient SO2 concentration (20 ppb) and between 0.2-0.5 pH units (from 4 to 10%) for the low ambient SO2 concentration (1 ppb) case

On the diurnal variability of particle properties related to light absorbing carbon in Mexico City

International audienceThe mass of light absorbing carbon (LAC) in individual, internally mixed aerosol particles was measured with the Single Particle Soot Photometer (SP2) in April of 2003 and 2005 and evaluated with respect to concentrations of carbon monoxide (CO), particle bound polycyclic aromatic hydrocarbons (PPAH) and condensation nuclei (CN). The LAC and CO have matching diurnal trends that are linked to traffic patterns and boundary layer growth. The PPAH reaches a maximum at the same time as CO and LAC but returns rapidly back to nighttime values within three hours of the peak. The number of particles containing LAC ranges between 10% to 40% of all particles between 150 nm and 650 nm and the mass is between 5% and 25% of the total mass in this size range. The average LAC equivalent mass diameter varies between 160 and 230 nm and the thinnest coating of non-light absorbing material is observed during periods of maximum LAC mass. The coating varies between 10 nm and 30 nm during the day, but is a strong function of particle size. The mass absorption cross sections, ?abs, derived from the SP2, are 5.0±0.2 m2g?1 and 4.8±0.2 m2g?1, dependent on the optical model used to describe LAC mixtures. The LAC contributes up to 50% of the total light extinction in the size range from 100 nm to 400 nm. The estimated emission rate of LAC is 1200 metric tons per year in Mexico City, based upon the SP2 measurements and correlations between LAC and CO

On the diurnal variability of particle properties related to black carbon in Mexico City

International audienceThe black carbon mass (BCM) of individual, internally mixed aerosol particles was measured with the Single Particle Soot Photometer (SP2) in April of 2003 and 2005. The average BCM, single particle BC mass fraction and BCM equivalent diameter were evaluated with respect to concentrations of carbon monoxide (CO), particle bound polycyclic aromatic hydrocarbons (PPAH) and condensation nuclei (CN). The BCM and CO have matching diurnal trends that are linked to traffic patterns and boundary layer growth. The PPAH reaches a maximum at the same hour as CO and BCM but returns rapidly back to nighttime values within three hours of the peak. The number of particles containing BCM ranges between 10% to 40% of all particles between 200 nm and 700 nm and the BCM is between 4% and 12% of the total mass in this size range. The average BC equivalent mass diameter varies between 300 and 400 nm and reaches its daily minimum value when BCM is a maximum. The BC particles have the thinnest coating of non-light absorbing material during periods of maximum BCM. The scattering and absorption coefficients, Bscat and Babs , derived from the SP2 measurements were compared with direct measurements from a nephelometer and soot photometer. The measured and derived Babs are in close agreement whereas the Bscat comparisons show larger discrepancies in absolute value and daily trends. Even though approximately 40% of the BCM is in particles with diameters smaller than 200 nm, the extinction coefficient is dominated by the BCM in particles larger than this size. The BCM contributes up to 20% of the total extinction in this size range. BCM is emitted at a rate of 1200 metric tons per year in Mexico City, based upon the SP2 measurements and correlations between BCM and CO

The validity of the kinetic collection equation revisited

The kinetic collection equation (KCE) describes the evolution of the average droplet spectrum due to successive events of collision and coalescence. Fluctuations and non-zero correlations present in the stochastic coalescence process would imply that the size distributions may not be correctly modeled by the KCE. &lt;br&gt;&lt;br&gt; In this study we expand the known analytical studies of the coalescence equation with some numerical tools such as Monte Carlo simulations of the coalescence process. The validity time of the KCE was estimated by calculating the maximum of the ratio of the standard deviation for the largest droplet mass over all the realizations to the averaged value. A good correspondence between the analytical and the numerical approaches was found for all the kernels. The expected values from analytical solutions of the KCE, were compared with true expected values of the stochastic collection equation (SCE) estimated with Gillespie&apos;s Monte Carlo algorithm and analytical solutions of the SCE, after and before the breakdown time. &lt;br&gt;&lt;br&gt; The possible implications for cloud physics are discussed, in particular the possibility of application of these results to kernels modified by turbulence and electrical processes

Aerosol particles in the mexican east pacific part I: processing and vertical redistribution by clouds

International audienceAirborne measurements of aerosol particle size distributions were made in the Mexican Intertropical Convergence Zone. The shape of the spectra at cloud base was compared with those at higher altitudes and near cloud boundaries to identify signatures of cloud processed particles. Of 78 cases analyzed, 71% showed enhancement in volume of super-micron particles, 49% had enhanced volume of sub-micron particles, 28% were homogeneous mixtures with boundary layer air, and 24% had super-micron particles removed by precipitation. Almost 100% of the cases with enhanced volume in sub-micron particles also occurred with enhanced super-micron volume. The enhanced volume in super-micron particles is approximately 10 times larger than the sub-micron enhancement. Cloud processed particles in marine air masses had twice as much enhancement of super-micron mass than found in particles processed by clouds formed from continental sources, likely a result of a more efficient coalescence process in clean, maritime clouds. These results are in qualitative agreement with previous observational and theoretical studies that relate enhancements in particle mass to the uptake by cloud droplets of SO2 and subsequent growth by coalescence

Video data modulation study, volume 1 Final report

Video data modulation technique