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Charles Warner

Abstract

Aircraft data from Winter MONEX have been combined with other data to study mesoscale features, and organization of cumulus clouds, on 10–12 December 1978. A moderate cold surge in the northeasterly monsoon flow, toward cloudiness in an equatorial trough off Borneo, peaked on 11 December.

Clouds in the northeasterly monsoon flow were similar to those in the trades, with variations in convective regime on length scales on the order of 100 km. Marked mid-tropospheric subsidence was accompanied by low-level divergence near 20°N. During 10 December, anvil clouds near Borneo expanded; cumulus congestus and cumulonimbus formed on the periphery of this area. The approach of the low-level northeasterlies to the area of anvils was marked by a diminution of subsidence, conditional instability, and a weak field of low-level convergence, with randomly organized cumulus of increasing height. A low-level easterly jet was found in this transition zone, downstream from cloudiness over the Philippines. South of Vietnam, a clear area was associated with low air temperatures, and not subsidence. Congestus and cumulonimbus clouds formed near the eastern coast of the Malay Peninsula.

Cloud streets were seen from latitude 19°N to the Malaysian coast (with a break south of Vietnam). These clouds were confined below the level of an inflection point in the profile of winds normal to the street direction. Greatest spacings of streets occurred with greatest vertical shears of the cross-winds. Cloud number densities were more closely related to the instability of the vertical stratification than to any other parameter.

Cross-wind organization of clouds occurred in circumstances of unstable, stratification and apparently of net ascent. Alignment of clouds was at an angle to the directions of both winds and vertical wind shears. It is inferred that when convergence was strong, deep clouds occurred along lines of convergence in the surface streamlines.

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Charles Warner

Abstract

Confusion has existed as to sources of entropy due to irreversible processes in the atmosphere, the total of which matches the export of entropy by radiation. What is the mechanical efficiency of convection? For an ideal tropical oceanic system in radiative–convective equilibrium, relative magnitudes of sources of entropy are reviewed—from both observations and numerical model results. Recycling of moisture is shown to be important. Leading terms are those relating to evaporation of precipitation, water loading by falling precipitation, and mixing of unsaturated parcels of air, contributing roughly 37%, 30%, and 15% of the total irreversible production of entropy, respectively. Evaporation from the surface accounts for 11%. The remaining 7% is due to turbulent kinetic energy, generation of gravity waves, and sensible heating at the surface.

A mechanical efficiency of conversion of heat supply at the surface into kinetic energy of the direct circulation, ≈2.0%, is obtained after the budget study. The leading contribution to the conversion is due to the effect of hydrometeors. Drag of hydrometeors is split into two components based on relative contributions of form drag plus water loading (50%) and frictional drag (50%); however, only the former contributes to the direct circulation. The contribution of turbulent kinetic energy is found to be small.

Results from the budget study are found to correspond with the finding of a threshold in values of convective available potential energy by Roff and Yano, and with numerical results from a three-dimensional model of convective equilibrium by Shutts and Gray.

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Charles Warner and Donna P. McNamara

Abstract

Refinements have been made to a standard procedure for calculating vertical air velocities from parameters measured routinely during flights by the Electra aircraft of the National Center for Atmospheric Research. Accuracies generally near one or two meters per second have been attained.

Using this procedure with the Electra aircraft, together with data from one of the WP-3D aircraft of the National Oceanic and Atmospheric Administration, a survey has been made of 99 different updraft cores and 43 downdraft cores encountered during the Winter and Summer Monsoon Experiments, in which vertical air speed exceeded 2 m s−1 for 1 s. Ignoring peripheral drafts of speed less than the threshold 1 m s−1, MONEX updraft cores were of median width 1.4 km, median peak updraft 3.2 m s−1, mean updraft 2.3 m s−1 and log10(mass transport, in kg s−1 m−1 normal to the flight track) equal to 3.4. Downdraft cores were of median width 1.3 km, peak downdraft 2.6 m s−1, mean downdraft 1.9 m s−1 and log(mass transport) equal to 3.2. The greatest 1 s updrafts reached 17 m s−1.

For draft cores other than in vigorous cumulonimbus, an equation was found relating mean draft speed, air density and width. Generally the mean draft speed approximately equals 2.5 m s−1.

Characteristics of draft cores were found to be similar to those found over the tropical Atlantic by LeMone and Zipser.

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