Adele L. Igel

Make It a Double? Sobering Results from Simulations Using Single-Moment Microphysics Schemes

Single-moment microphysics schemes have long enjoyed popularity for their simplicity and efficiency. However, in this article it is argued through theoretical considerations, idealized thunderstorm simulations, and radiative‐convective equilibrium (RCE) simulations that the assumptions inherent in these parameterizations can induce large errors in the proper representation of clouds and their feedbacks to the atmosphere. For example, precipitation is shown to increase by 200% through changes to fixed parameters in a singlemoment scheme and low-cloud fraction in the RCE simulations drops from about 15% in double-moment simulations to about 2% in single-moment simulations. This study adds to the large body of work that has shown that double-moment schemes generally outperform single-moment schemes. Therefore, it is recommendedthatfuturestudies,regardlessoftheirfocusandespeciallythoseemployingcloud-resolvingmodelsto simulate a realistic atmosphere, strongly consider moving to the exclusive use of multimoment microphysics schemes.

Sensitivity of Warm-Frontal Processes to Cloud-Nucleating Aerosol Concentrations

An extratropical cyclone that crossed the United States on 9‐11 April 2009 was successfully simulated at high resolution (3-km horizontal grid spacing) using the Colorado State University Regional Atmospheric ModelingSystem.Thesensitivityoftheassociatedwarmfronttoincreasingpollutionlevelswasthenexplored by conducting the same experiment with three different background profiles of cloud-nucleating aerosol concentration. To the authors’ knowledge, no study has examined the indirect effects of aerosols on warm fronts. The budgets of ice, cloud water, and rain in the simulation with the lowest aerosol concentrations were examined. Theicemasswasfoundtobeproducedin equalamountsthroughvapordepositionandriming,and the melting of ice producedapproximately 75% of the total rain. Conversion of cloud water to rain accounted for the other 25%. When cloud-nucleating aerosol concentrations were increased, significant changes were seen in the budget terms, but total precipitation remained relatively constant. Vapor deposition onto ice increased, but riming of cloud water decreased such that there was only a small change in the total ice production and hence there was no significant change in melting. These responses can be understood in terms of a buffering effect in which smaller cloud droplets in the mixed-phase region lead to both an enhanced vapor deposition and decreased riming efficiency with increasing aerosol concentrations. Overall, while large changes were seen in the microphysical structure of the frontal cloud, cloud-nucleating aerosols had little impact on the precipitation production of the warm front.

The Free Troposphere as a Potential Source of Arctic Boundary Layer Aerosol Particles

This study investigates aerosol particle transport from the free troposphere to the boundary layer in the summertime high Arctic. Observations from the Arctic Summer Cloud Ocean Study field campaig ...

The Energetics and Magnitude of Hydrometeor Friction in Clouds

AbstractAs hydrometeors fall within or from a cloud, they reach a terminal velocity because of friction with the air through which they settle. This friction has previously been shown to result in ...

Meteorological and Land Surface Properties Impacting Sea Breeze Extent and Aerosol Distribution in a Dry Environment: Factors Impacting Sea Breezes

Author(s): Igel, AL; van den Heever, SC; Johnson, JS | Abstract: ©2017. American Geophysical Union. All Rights Reserved. The properties of sea breeze circulations are influenced by a variety of meteorological and geophysical factors that interact with one another. These circulations can redistribute aerosol particles and pollution and therefore can play an important role in local air quality, as well as impact remote sensing. In this study, we select 11 factors that have the potential to impact either the sea breeze circulation properties and/or the spatial distribution of aerosols. Simulations are run to identify which of the 11 factors have the largest influence on the sea breeze properties and aerosol concentrations and to subsequently understand the mean response of these variables to the selected factors. All simulations are designed to be representative of conditions in coastal sub tropical environments and are thus relatively dry, as such they do not support deep convection associated with the sea breeze front. For this dry sea breeze regime, we find that the background wind speed was the most influential factor for the sea breeze propagation, with the soil saturation fraction also being important. For the spatial aerosol distribution, the most important factors were the soil moisture, sea-air temperature difference, and the initial boundary layer height. The importance of these factors seems to be strongly tied to the development of the surface-based mixed layer both ahead of and behind the sea breeze front. This study highlights potential avenues for further research regarding sea breeze dynamics and the impact of sea breeze circulations on pollution dispersion and remote sensing algorithms.

Predicting Littoral Zone Aerosol Optical Properties under a Sea Breeze Regime: Part II

The underlying assumptions of aerosol composition and characteristics in littoral sea breeze regimes are evaluated, guided by recent in situ data, to determine the sensitivity of aerosol optical property retrievals to these assumptions.

Impacts of cloud condensation nuclei on deep stratus clouds

Simulations of a warm front associated with a deep, mixed-phase stratus cloud were performed with three different background cloud-nucleating aerosol profiles representative of clean, moderate, and polluted environments using the Regional Atmospheric Modeling System. While significant changes were seen in the cloud microphysical processes, the total precipitation reaching the surface only varied by 2% across the three simulations. This result arose from two buffering processes in the mixed-phase cloud. First, in the more polluted simulations, decreased riming of cloud water was compensated by increased vapor deposition onto ice leading to small changes in ice content and hence melting. Second, autoconversion of cloud droplets to rain was suppressed, but collection of cloud water by rain drops, which were primarily produced through melting, increased due to increased cloud water content in the more polluted simulations.