SEMINAR: MPO Special Seminar: Dr. Robert Seigel, TODAY, at 3:00 p.m., MSC 343

[Sandrine Apelbaum <sapelbaum@rsmas.miami.edu>]

MPO Special Seminar

Dr. Robert Seigel

Colorado State University

Title:"The Impacts of Mineral Dust on Organized Mesoscale Deep Convection"

Friday, November 9, at 3 p.m.

MSC 343

Abstract:

THE IMPACTS OF MINERAL DUST ON ORGANIZED MESOSCALE DEEP

CONVECTION

The overarching goal of this dissertation research is to investigate how mineral

dust impacts various aspects of organized deep moist convection (DMC) using numerical

modeling. From a bulk perspective, organized mesoscale DMC can be characterized by

heating (i.e. latent heating from precipitation production) above cooling (i.e. latent

cooling from melting and evaporation) that is long-lived. The balance between dynamics

and thermodynamics for mesoscale systems dictates their level of organization and

longevity, which can be modulated by mineral dust through direct and indirect effects.

When a layer of dust exists in the atmosphere, it can absorb radiation such that a

layer of increased stability results. Using experiments of idealized cold pools, it is shown

that the strength and location of a vertically thin stable layer modulates the characteristics

and propagation of cold pools. This direct aerosol forcing subsequently impacts the

organization of mesoscale DMC. Furthermore, observations have shown that dust can

effectively serve as both cloud condensation nuclei (CCN) and ice nuclei (IN), which can

indirectly impact the level of organization for mesoscale DMC. As increased dust aerosol

within anvils can alter the ice crystal size distribution by serving as an additional source

of IN, the impact of varying the mean hail diameter for squall lines is investigated. It is

demonstrated that as hail size decreases, the intensity of the simulated squall line

increases. The increase in squall line intensity is attributed to a hydrometeor recirculation

mechanism that is centered at the freezing level and more efficiently produces latent

heating, driving enhanced buoyancy within the updrafts. The mechanism stems from the

advection of hail and rain hydrometeors into the upshear flank of the mid-level updraft by

the rear-inflow jet of the squall line. This advection process, along with squall line

intensity, increases as hail size is reduced

Finally, analysis will be presented of a study that utilizes a nocturnal squall line to

assess and isolate the individual responses in a squall line that arise (1) from radiation, (2)

from dust altering the microphysics, as well as (3) from the synergistic effects between

(1) and (2). To accomplish these tasks, we use RAMS set up as a cloud-resolving model

(CRM) and represent mineral dust as a background aerosol, which is shown to be the

most effective source of dust ingestion. The CRM contains aerosol and microphysical

schemes that allow mineral dust particles to nucleate as cloud drops and ice crystals,

replenish upon evaporation and sublimation, be tracked throughout hydrometeor

transition, and be scavenged by precipitation and dry sedimentation. Factor separation is

used on four simulations of the squall line in order to isolate the individual roles of

radiation (RADIATION), microphysically active dust (DUST MICRO), and the

nonlinear interactions between these two factors (SYNERGY). Results indicate that

RADIATION acts to increase precipitation, intensify the cold pool, and enhance the

mesoscale organization of the squall line due to radiation-induced changes in the

microphysics that appear to initiate from cloud top cooling. Conversely, DUST MICRO

decreases precipitation, weakens the cold pool, and weakens the mesoscale organization

of the squall line due to an enhancement of the warm rain process. Surprisingly,

SYNERGY shows little impact on the squall line, except near the freezing level, where

an increase in mesoscale organization takes place.

Sandrine Apelbaum

Meteorology and Physical Oceanography 
Rosenstiel School of Marine and Atmospheric Science
University of Miami
4600 Rickenbacker Causeway
Miami, FL 33149-1098
Tel     (305) 421-4057
Fax     (305) 421-4696

sapelbaum@rsmas.miami.edu