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Turbulence and Droplet Clustering in Shallow Cumulus: The Effects of Aerosols and Cloud Height.

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Title:Turbulence and Droplet Clustering in Shallow Cumulus: The Effects of Aerosols and Cloud Height.
Authors:Dodson, Dillon S.
Contributors:Atmospheric Sciences (department)
Date Issued:May 2017
Publisher:University of Hawaiʻi at Mānoa
Abstract:Aerosol{cloud interactions are complex including albedo and lifetime e ects that cause
modi cations to cloud characteristics, as well as the resulting e ects on climate. With most
cloud{aerosol interactions focused on the previously stated phenomena, few studies have
focused explicitly on how aerosols can e ect turbulence within a cloud, especially cloud edge
entrainment which impacts cloud lifetime and size. Along with turbulence, little research has
been conducted outside the lab on droplet clustering within clouds and its relationships with
turbulence. Cloud droplets are impacted by turbulence through droplets being preferentially
concentrated into regions of lower vorticity, leaving regions of higher vorticity relatively free
of droplets. This research aims to gain a better understanding on how turbulence can be
used to map droplet clustering within a cumulus cloud and how aerosol number concentration
a ects droplet clustering. Not only is turbulence a tool for analyzing droplet clustering, but
it can also have implications related to precipitation formation through collision{coalescence.
In this work, aerosol{cloud relationships are derived from warm continental cumuli subjected
to various levels of anthropogenic in
uence sampled during the 2006 Gulf of Mexico Atmospheric
Composition and Climate Study (GoMACCS) by the Center for interdisciplinary
Remotely{Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. Drop size distributions,
cloud liquid water content (LWC), and a time stamp (allowing for droplet calculations down
to 10􀀀4 m scale) for when each cloud droplet was encountered were measured using the Artium
Flight Phase{Doppler interferometry (PDI). These data along with other meteorological
observations are used to investigate turbulence and droplet spacing within the cumuli. The
pair{correlation function (PCF) is used to identify the scale of preferential concentration,
with more clustering signifying a more turbulent environment and vice versa.
Results using four complete days of data with 81 non{precipitating cloud penetrations
(minimum 300 m in length) organized into two
ights of low (L1, L2) and high (H1, H2)
pollution data show a more turbulent environment near cloud edge as compared to the
cloud center for all four cases, with low polluted clouds showing more droplet clustering
for both cloud edge and center. Although low polluted clouds have more overall clustering,
high polluted clouds have more statistically signi cant clustering than low polluted clouds.
Further analysis shows that the higher clustering experienced in low polluted clouds is due
to L2, which has a signi cantly higher amount of clustering than the other three
ights. It
is believed that most of the clouds within the L2
ight are decaying, making cloud lifetime
more signi cant to clustering than the aerosol number concentration, although more research
must be done. Using a single cloud in which multiple
ight passes were made at di erent
altitudes (ranging from 806 to 3381 m) shows that clustering increases particularly at cloud
top where cloud top entrainment is occurring. Negligible change in the PCF as a function of
altitude is seen before the cloud top entrainment zone. This work helps lay the foundation
for being able to model the a ects that droplet clustering has on collision{coalescence and
rain formation time.
Description:M.S. Thesis. University of Hawaiʻi at Mānoa 2017.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: M.S. - Atmospheric Sciences

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