Frontal system

In the mid-latitude region depicted in Fig. 7-5, the motion is characterized by large-scale eddy transport." Here the "eddies" are recognizable as ordinary high- and low-pressure weather systems, typically about 10 km in horizontal dimension. These eddies actually mix air from the polar regions with air from nearer the equator. At times, air parcels with different water content, different chemical composition and different thermodynamic characteristics are brought into contact. When cold dry air is mixed with warm moist air, clouds and precipitation occur. A frontal system is said to exist. Two such frontal systems are depicted in Fig. 7-5 (heavy lines in the midwest and southeast). 

The associated frontal system moved across the Pacific northwest states into northern California and became quasi-stationary in that position on November 15. The positions of the surface front taken from the surface charts for Nov. 15-17, 1966, are shown in Figure 2. Subsequent wave developments on this front resulted in heavy precipitation in northern California throughout November 15, and into the morning of November 16 before the front started to push southward along the California coast. 

The 6-hourly surface positions of the frontal system are shown in Figure 10. The front weakened south of Monterey, with only light precipitation reported during the frontal passage at Paso Robles and Van-denberg Air Force Base. The 0400-PST upper circulation patterns at the 500-mbar level for the period January 9-11 are shown in Figure 11. 

Table VIII shows the total storm rainfall and radionuclide depositions measured at each collection site. The storm description given earlier noted the diminishing rainfall as the frontal system moved down the coast. The total collected rainfall decreased from 86 mm. at Crescent City almost smoothly to the low of only 3 mm. at Piedras Blancas. The deposition of the three radionuclides follow a similar pattern the con-...

Convective activity at the meso scale and large scale, associated with the penetration of frontal systems in south/ southeast Brazil that can reach western southern Amazonia. 

Radon ( Rn) measurements made by the ARCAS provide a simple, reliable, real-time indicator of the relative maritime or continental nature of the air over coastal or oceanic areas (8). With a half-life of 3.8 days, Rn originates from the decay of Ra, a member of the decay chain. At least 98% of Rn originates from land masses (9). The radon flux at the surface depends on the radium content of the soils and rocks, the permeability of the source materials, atmospheric pressure, soil moisture, and vegetative cover (10). Relatively abrupt changes in the radon concentration over the ocean usually indicate changes in air masses and the passage of frontal systems. 

Cloud cover, surface wind direction and speed, temperature, dew point, and precipitation are indicated by symbols at each weather reporting station. Interpretation of the weather in a region is facilitated by the identification of high-pressure and low-pressure areas, the delineation of frontal systems, and the construction of isobars. Isobars are lines of constant atmospheric pressure that are interpolated between observation points, and are so named because pressure often is presented using the unit of the bar, which is equivalent to 106 dyn/cm2 or 0.9869 atm. On some maps, such as shown in Fig. 4-21, pressure is given in millibars as a shorthand notation, only the last two digits are presented. The user must add either 900 or 1000 millibars to the value shown, whichever brings the value closest to 1 bar. 

Frontal systems can also develop at shelf breaks, where shallowing and the associated increase in current velocity can result in upwelling and high primary productivity. Smaller frontal zones can be produced in the wake of islands, resulting in upwelling and elevated productivity (e.g. Flawaii and Isles of Scilly). 

Soviet data also demonstrate that the sum of the concentrations of the different ions is independent of sampling location in the case of frontal precipitation systems, and is equal to about 6 mg 1 1 over the whole of the Soviet Union. The pattern shown in Fig. 50 is thus mostly produced by local precipitation systems, which are much more sensitive to local pollution. Petrenchuk and Selezneva (1970) argue that in frontal systems the rain-out of aerosol particles by condensation is the dominant mechanism in the control of precipitation water composition. 

Highly variable habitats such as frontal systems and upwelUng areas lead to smaller size in foraminifers documented in plankton tows (Ortiz et al. 

Fig. 1-6. Mean zonal circulation in the northern hemisphere, 0-20 km. Distribution of wind velocities (in units of m/s) was taken from Labitzke (1980). W, Mean winds from the west E, mean winds from the east the heavy lines indicate the approximate location of the polar front, the broken lines the tropopause. The maximum wind speed coincides approximately with the subtropical jet stream. The location of the polar jet fluctuates considerably and does not show up in the average. The center is to illustrate wind directions near the earth surface (trade winds and westerlies) cyclones (C) and anticyclones (A) imbedded in the westerlies are only sketched the frontal systems associated with cyclones cannot be shown in this extremely simplified diagram.

Fig. 1.19 Distribution of major sediment facies across the frontal system of the Antarctic Circumpolar Current (ACC) between Africa and Antarctica. Numbers are typical sedimentation rates in mm kyh PF = Polar Front, SAF = Sub Antarctic Front, STF = Subtropical Front.

Garbey et al. also predicted that for a descending liquid/liquid front, an instability could arise even though the configuration would be stable for unreactive fluids (29-31). This prediction has yet to be experimentally verified because liquid/liquid frontal polymerization exhibits the Rayleigh-Taylor instability. A thermal frontal system with a product that is less dense than the reactant is required. 

We note a significant difference between the liquid/liquid and the liquid/solid cases. For the liquid/solid case, convection in ascending fronts increases the front velocity but in the liquid/liquid case, convection slows the front. Convection increases the velocity of pH fronts and BZ waves. Why the difference between liquid/liquid frontal polymerization and other frontal systems In liquid/liquid systems the convection also mixes cold monomer into the reaction zone, which lowers the front temperature. The front velocity depends more strongly on the front temperature than on the effective transport coefficient of the autocatalyst. Convection cannot mix monomer into the reaction zone of a front with a solid product but only increases thermal transport so the velocity is increased. 

With that scheme, the analyst identifies first which systems are needed given the PSA scenario. Then from the list of required systems, he determines which actions are plausible that could prevent these systems to work. That method is theoretically the more exhaustive and the more justified. We use it explicitely in EDF s PSA at the step of what we call AQS, in French Analyse Qualitative des s6quences, that means Qualitative Sequence Analysis (Cucciniello Vidal, 2000). As Cucciniello and Vidal said The qualitative analysis of sequences (AQS) aims at characterizing in the accidental situations of reference, defined by the initiators selected and the states of the reactor considered, the missions of the frontal systems, I C and human like their sequence. It is built starting from a detailed structuring of the functions of safety and course of accidental control APE [i.e. French symptom based procedures]. After the characterization of the initiators, the method breaks up into three stages ... 

Texier-Picard et al. analyzed a polymerization front in which the molten polymer was immiscible with the monomer and predicted that the front could exhibit the Marangoni instability even though comparable unreactive fluids would not exhibit the instability [84]. However, no liquid/liquid frontal system with an immiscible product has been identified. Even if such a system could be found, the experiment would have to be performed in weightlessness to prevent buoyancy-induced convection from interfering. 

Fillers are added to frontal systems to prevent convection, to modify the rheology of the unreacted formulation and to affect the mechanical properties of the product. Nason et studied the effect of inorganic fillers on the photoinduced FP of a triacrylate. Not surprisingly, the front velocity decreased with increased loading of calcium carbonate or kaolin clay. 

As an air mass rises and cools, the saturation vapor pres-snre of the atmosphere decreases with deereasing temper-atnre. Eventnally, the air mass reaches the vapor saturation curve and liqnid waterbegins to form. As the air mass cools further, the vapor pressure of water continues to drop and more liqnid or solid water condenses and nltimately falls back to the surface as precipitatioa This process imparts a temperature dependence on the isotopic composition of precipitation that is particularly important for precipitation related to frontal systems that sweep across the continents. 

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