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- Author or Editor: Einar L. Hovind x
- Journal of Applied Meteorology and Climatology x
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Abstract
The data source for this study is a collection of five years of serial upper-air soundings taken during storms at five stations in the Southern California coastal and offshore region, along with supporting aerial, radar, and surface precipitation observations.
Detailed analyses of frontal systems revealed mesoscale motions and processes which are an important and integral part of the frontal structure. In particular, the flow pattern within the prefrontal precipitation region is found to be characterized by waves aloft and a matching cell structure below, with wavelength of 200 to 300 km and with crusts oriented parallel to the front. Within these cells are found small convection bands with which are associated sharp peaks in the precipitation distribution. The overall pattern slopes aloft over the front, and this slope, along with horizontal and vertical mixing, is an essential element in the dynamic balance within the frontal zone.
The intensity of the mesoscale vertical motion responsible for clouds and precipitation in the frontal zone appears to have some association with the degree of convective activity. In the stronger fronts, vertical velocity peaks of 20 cm sec−1 or more are found to be the rule.
The thermal balance is dominated largely by vertical differences in horizontal advection which are balanced by convective heat exchange. A strong, low-level current of warm air from the south overridden by cold air from the west determines to varying degrees the convective instability and results in a considerable amount of available potential energy being converted directly into convection-scale kinetic energy, thus by-passing its conversion to the cyclone-scale kinetic energy.
In fronts possessing greater stability, vertical velocities are less and the eastward movement of the front is greater, suggesting that in these cases the broad-scale deformation of the cyclone-scale thermal pattern, which controls the deepening and the occlusion processes, is accelerated.
Abstract
The data source for this study is a collection of five years of serial upper-air soundings taken during storms at five stations in the Southern California coastal and offshore region, along with supporting aerial, radar, and surface precipitation observations.
Detailed analyses of frontal systems revealed mesoscale motions and processes which are an important and integral part of the frontal structure. In particular, the flow pattern within the prefrontal precipitation region is found to be characterized by waves aloft and a matching cell structure below, with wavelength of 200 to 300 km and with crusts oriented parallel to the front. Within these cells are found small convection bands with which are associated sharp peaks in the precipitation distribution. The overall pattern slopes aloft over the front, and this slope, along with horizontal and vertical mixing, is an essential element in the dynamic balance within the frontal zone.
The intensity of the mesoscale vertical motion responsible for clouds and precipitation in the frontal zone appears to have some association with the degree of convective activity. In the stronger fronts, vertical velocity peaks of 20 cm sec−1 or more are found to be the rule.
The thermal balance is dominated largely by vertical differences in horizontal advection which are balanced by convective heat exchange. A strong, low-level current of warm air from the south overridden by cold air from the west determines to varying degrees the convective instability and results in a considerable amount of available potential energy being converted directly into convection-scale kinetic energy, thus by-passing its conversion to the cyclone-scale kinetic energy.
In fronts possessing greater stability, vertical velocities are less and the eastward movement of the front is greater, suggesting that in these cases the broad-scale deformation of the cyclone-scale thermal pattern, which controls the deepening and the occlusion processes, is accelerated.
Abstract
Pacific storms entering Southern California have been intensely sampled and subjected to detailed investigation through a storm study program in the Santa Barbara area during the 1960–63 (inclusive) winter storm seasons.
One result which has emerged from the analyses of precipitation and upper-air data was the discovery that organized convection bands were a common feature within the main precipitation region. These bands were detected from storm precipitation distributions, which, through quasi-objective methods, have been separated into the following three components: storm mean motion precipitation, orographic precipitation, and convection band precipitation.
The typical convection bands appear to be 20 to 40 miles wide, centered some 30 to 60 miles apart, oriented along the upper shear vector (between winds in the convective cloud layers and the adjacent layer above), and moving along a direction of the lower shear vector. There is evidence that the increased convective activity within the bands is associated primarily with the destabilization of the air mass through differential thermal advection.
Abstract
Pacific storms entering Southern California have been intensely sampled and subjected to detailed investigation through a storm study program in the Santa Barbara area during the 1960–63 (inclusive) winter storm seasons.
One result which has emerged from the analyses of precipitation and upper-air data was the discovery that organized convection bands were a common feature within the main precipitation region. These bands were detected from storm precipitation distributions, which, through quasi-objective methods, have been separated into the following three components: storm mean motion precipitation, orographic precipitation, and convection band precipitation.
The typical convection bands appear to be 20 to 40 miles wide, centered some 30 to 60 miles apart, oriented along the upper shear vector (between winds in the convective cloud layers and the adjacent layer above), and moving along a direction of the lower shear vector. There is evidence that the increased convective activity within the bands is associated primarily with the destabilization of the air mass through differential thermal advection.
Abstract
A significant question bearing on the prediction of orographic precipitation and the seeding of orographic clouds is what fraction of the water condensed over an orographic barrier falls on the barrier as precipitation. This has been treated in a rather inadequate manner to date, largely because of lack of basic data.
Through the use of abundant storm-sounding data taken upwind of two Southern California orographic barriers and data from the corresponding mountain recording raingage networks, comparisons of computed condensation and observed precipitation have been made for a number of winter storms over a four-year period. The results indicate that approximately one quarter of the orographically produced condensate fell as precipitation on the watersheds.
A breakdown into air mass stability on the basis of the inflow rawinsonde data showed that, for similar orographic flow conditions, more precipitation was produced by unstable air masses than by stable air masses.
Abstract
A significant question bearing on the prediction of orographic precipitation and the seeding of orographic clouds is what fraction of the water condensed over an orographic barrier falls on the barrier as precipitation. This has been treated in a rather inadequate manner to date, largely because of lack of basic data.
Through the use of abundant storm-sounding data taken upwind of two Southern California orographic barriers and data from the corresponding mountain recording raingage networks, comparisons of computed condensation and observed precipitation have been made for a number of winter storms over a four-year period. The results indicate that approximately one quarter of the orographically produced condensate fell as precipitation on the watersheds.
A breakdown into air mass stability on the basis of the inflow rawinsonde data showed that, for similar orographic flow conditions, more precipitation was produced by unstable air masses than by stable air masses.
