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Charles Warner

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Charles Warner

Abstract

A new method is described for photogrammetry of clouds from aircraft side camera movies, to take into account cloud motion. In extreme circumstances of measurement, this leads to improvements of 500 m in cloud height. General use of the method is desirable when cloud measurements are related to other data.

On 10 December 1978 over the South China Sea, cumulus fractus were of depth ∼0.2 km, and updrafts were of width ∼0.25 km. For humilis the corresponding numbers were 0.7 and 0.4 km. Mediocris with tops at height (z) 2.2 km had updrafts of width (w) ∼1.3 km. Updrafts in congestus towers increased with height, with dw/dz ∼0.17, a figure similar to that found for day 245 of the GARP Atlantic Tropical Experiment.

Cloud number densities on 10 December varied over an order of magnitude over distances of 100 km, even in circumstances relatively uniform on the large scale. Order of magnitude estimates of percent area coverage by updrafts yield values up to 10% for fractus and humilis combined, up to a few percent for mediocris, and less than 1% for congestus in the mid-troposphere. An area measuring 1° latitude × 2° longitude was found to contain nine cumulonimbi, yielding ≈0.1% coverage by updrafts in the mid-troposphere.

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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

Equations for photogrammetry from an aircraft nose camera movie are based on angles from a fixed point on the horizon toward which the aircraft is flying. The positions and dimensions of an arc of clouds occurring on day 261 of GATE are presented as an example. The accuracy of such measurements is a few kilometers in the horizontal and about 500 m in the vertical.

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Charles Warner

A technique for viewing stereo-pairs of photographs is described. Stereo-pairs are used to illustrate the slender structure of cumulus clouds generally observed during the Winter and Summer Monsoon Experiments; the visual appearance of “hot towers”; the prevalence of thin stratus layers, including the existence of discontinuities in the vertical through them and the growth of altocumulus from them; and the delicate structure and considerable depth of elements of cirrus.

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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

Abstract

Summer MONEX aircraft flight level and dropwindsonde data have been used to examine the central core structure of a mature Bay of Bengal monsoon depression on 7 July 1979. Continuous aircraft data including cloud photographs were obtained at three flight levels.

The depression sloped toward the southwest with height, with buoyant cloudy ascent within cold air to the south and west of the axis and warm subsidence of clear air to the northeast of the axis. Marked departures from gradient balance were found. In the northeast subgradient subsiding flow was found throughout the lower troposphere. In the southeast, new cloud base, supergradient flow accompanied areas of convergence; near 700 hPa, the overlying flow was subgradient and subsident; near 400 hPa, marked variations in flow were related to clouds above flight level.

Near cloud base, convergence reached magnitudes of approximately -2 × 10-4 s-l in areas of ∼300 km2 to the south and west of the center (ascent reaching roughly -1 Pa s-1) with divergence and subsidence of similar magnitude in and to the northeast of the center. These two regimes were separated by cloud arcs in the boundary layer below a subsidence inversion.

Northward out of the cold region of cloudy ascent in the westerlies south of the depression center, flow at 700 hPa subsided at ∼0.1 Pa s-1 around the cast side of the depression.

A cumulonimbus anvil spread at 150 hPa over the area of convergence and ascent west of the centers of circulation at low levels. Airborne radar showed ∼1% area coverage by echoes of reflectivity ≳ 35 dB(Z) (rainfall rate ≳ 6 mm h-1), with average rainfall ∼0.5 mm h-1. The flow configuration was consistent with ascent in cumulus convection, coupled with compensating dry adiabatic subsidence.

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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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Charles Warner and Richard H. Grumm

Abstract

A monsoon depression over the Bay of Bengal on 7 July 1979 has been studied using a variety of observations, in particular, cloud photographs from aircraft. Ascent in the lower troposphere was concentrated in mesoscale features of cumulus clouds covering ∼1% of the inner area (3 × 105 km2) of the depression. Across these mesoscale features discontinuities in thermal fields were found along with abrupt wind shifts.

Much of the volume of the depression featured thin fragmentary layers of stratus, implying an absence of strong vertical motion. Observed by photography, individual rising cumulus towers were of width up to a few kilometers, increasing with height; rise rates of towers reached 9 m s−1. Measured with aircraft instruments, mean updrafts in cumulus clouds were ∼2.5 m s−1. In cumulus populations scattered throughout the storm, number densities of cumulus ranged from ∼1 km−2 for fractus to ∼1 per 1500 km2 for Congestus. Congestus penetrating the 500 (300) hPa level were ∼1 per 3200 (13 000) km2. Fractional area coverage by cumulus updrafts was ∼0.5% in humilis, less in other categories. Coverage by cumulus updrafts was roughly 20 times less than coverage by inert remnants of cumulus. Cloudy ascending motion in populations of cumulus was generally on the order of hundredths of Pascals per second. It appeared to be mostly compensated by local subsidence. Great number densities of humilis were found moistening the central area following subsidence and drying.

Total cloud cover was dominated by mid-level thin fragmentary status layers and cumulus debris. There was extensive anvil cloud based at ∼400 hPa, apparently arising from cumulus.

Detailed observations were made of a cloud line growing out of the southwesterly flow south of the center of the depression. The line was followed for 3 h on GOES-1 visible imagery. It propagated faster than the low-level winds. Aircraft altimetry showed an abrupt height drop from 6097 to 6090 m at 483 hPa, over a distance of 50 km from southeast to northwest through the line. Southwesterly momentum was lifted from 900 to 600 hPa and from southeast to northwest through the line. Other colocated singularities in convection and wind fields were found.

Ascent in the lower troposphere over the depression as a whole (1066 km2) was assessed from aircraft and dropwindsonde data to be approximately −0.3 Pa s−1.

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Charles Warner and K. L. S. Gunn

Abstract

A transmissometer has been used to provide a continuous record with good time resolution of falling snow. The pulsed light, of wavelength 0.45μ, traversed a path 71 m long about 20 m above ground level A total snow amount of 160 millimeters of water (mmw) from 20 storms through the 1966-67 winter season was recorded and analyzed, Attenuation by snow was found to be proportional to rate of snowfall, with the constant of proportionality 11 (db km-1) / (mmw hr-1). A previous experiment by Lillesaeter yielded 18 for this constant.

The attenuation from precipitation-free air was found to increase as the relative humidity increased above about 60%. Relative humidity increase between the beginning and end of a storm could lead to an increase in "clean air" attenuation of 4-5 db km-1. Lack of correction for this together with the effect of thermal fluctuations on Lillesaeter's narrower transmitted beam probably account for his higher value of the constant of proportionality.

Snow amounts for individual storms deduced from the attenuation record agreed with amounts measured by standard instruments to within a factor of 2. When depths on the ground were compared, agreement was within a factor of 1.5. Over the 20 principal storms of the season, the total snow amount from the attenuation records agreed to within 2% with the accumulation in a standard Nipher gage.

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