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  • Author or Editor: Marcin J. Szumowski x
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Marcin J. Szumowski
,
Robert M. Rauber
, and
Harry T. Ochs III

Abstract

A Lagrangian drop-growth trajectory model, applied within dual-Doppler-derived four-dimensional kinematic fields, is used to test the hypothesis that accretion of cloud water on giant and ultragiant cloud condensation nuclei (CCN) can explain the growth of raindrops in warm maritime convective clouds. Radar data collected within offshore rainbands during the 1990 Hawaiian Rainband Project are used to provide realistic timescales and magnitudes of convective updrafts and to capture the horizontal flow variations responsible for transporting drops into and out of these updrafts. The range of conditions under which cloud droplets can grow to large raindrops during simple up–down trajectories is determined.

The model results show that accretion of cloud water on giant and ultragiant nuclei can account for the formation of rain in observed timescales. Raindrops with diameters of 1–4 mm formed for the entire range of tested conditions. Maximum drop sizes ranged from 3.5 to 8.5 mm. The general tendency in the simulations was for the largest drops to fall within regions of radar reflectivity greater than 35 dBZ, and for the smaller drops to fall in parts of the rainband with weaker reflectivity. In the control runs, which the authors believe represent natural conditions, drops as large as 5 mm formed on cloud droplets whose initial diameters were comparable to deliquesced sea salt particles observed in concentrations of ∼10 to 103 m−3 near the base of Hawaiian clouds. The growth rates and trajectories of 1–5-mm raindrops in these runs agreed well with the observed evolution of the reflectivity fields. The most rapid rate of drop growth in the model occurred during a near-suspension period and early fall through the upper parts of the cloud, which is in good agreement with the sharp reflectivity gradients observed near cloud top by the radars. Despite the time and space constraints placed on the results by the evolution of the updrafts, the calculations showed that the process of rain formation in warm maritime convective clouds is simple and efficient, provided that giant and ultragiant CCN are present near cloud base. While more complex processes leading to drop spectra broadening, such as mixing, stochastic condensation, stochastic coalescence, and breakup also occur in nature, they appear to be unnecessary to explain the rapid formation of rain in these warm maritime convective clouds.

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Marcin J. Szumowski
,
Robert M. Rauber
,
Harry T. Ochs III
, and
L. J. Miller

Abstract

Radar reflectivity factors exceeding 60 dBZ are documented within shallow (<3 km), warm (>0°C), summertime tropical rainbands offshore of the island of Hawaii. Dual-Doppler radar measurements from the Hawaiian Rainband Project are used to document the formation, evolution, and kinematic structure of the high reflectivity cores. The authors show that extremely high radar reflectivities (50–60 dBZ) can develop from echo free regions (−20 dBZ) within approximately 15 min and are preceded by 5–9 m s−1 peak updrafts. High reflectivities (>50 dBZ) typically first formed in the middle or upper part of the clouds. Over the next 10–15 min, the mature high reflectivity cores extended vertically through the cloud depth and then collapsed to the surface as the updrafts weakened. A near-upright orientation of most updrafts producing these high reflectivity cores is conceptually consistent with the idea that large raindrops grow in the highest liquid water content while falling through the updraft core. Strong outflows near the inversion led to the formation of sloped radar echo overhangs surrounding the cells. The bases of the overhangs descended to the surface with time, leading to an overall increase in the width of the rainbands. Short-lived downdrafts were present in the upper part of the clouds in mature and dissipating stages of cells’ life cycles but were not observed in the lower parts of the cloud, even in intense precipitation shafts.

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Marcin J. Szumowski
,
Robert M. Rauber
,
Harry T. Ochs III
, and
Kenneth V. Beard

Abstract

The microphysical structure of high reflectivity cores and surrounding weaker echo regions in Hawaiian rainbands is documented using aircraft data. These data show that high reflectivity cores are associated with giant raindrops (D > 4 mm) present in narrow (∼500 m wide) columns coincident with the core updraft. Updrafts were found to be strong enough to suspend 1–2-mm raindrops near cloud top. As these raindrops subsequently fall through the updraft core, they are exposed to high liquid water content, allowing them to grow to large sizes, provided that updrafts are not significantly sheared. The data indicate that size sorting due to differential terminal velocities of the larger and smaller raindrops occurs initially in the updraft. As a result, the larger raindrops fall through an environment in which there is a low concentration of smaller raindrops, decreasing the probability of breakup. Calculations of raindrop growth rates and breakup probabilities are used to demonstrate that high reflectivity cores in the rainbands can result from simple accretional growth of 1–2-mm raindrops falling from cloud top. In regions outside of the main updraft, drop size distributions were approximately exponential, with higher concentrations of small raindrops and no giant raindrops. Consequently radar reflectivities and rainfall rates were lower. In these regions, collisional breakup played a more significant role in eliminating the large size tail of the spectra.

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