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Insights from ARM observations into aerosol processing and transport by extratropical cyclones and aerosol effects on cyclone clouds

Active Dates 8/15/2021-8/14/2024
Program Area Atmospheric System Research
Project Description
We propose to investigate how aerosols affect, and are affected by, clouds and precipitation in the fronts of extratropical cyclones in the North Atlantic and Southern Oceans. Extratropical cyclones occur frequently in these regions and play a critical role in setting the transport of momentum, heat, moisture and aerosols. We will test the following two hypotheses using observations from the Atmospheric Radiation Measurements (ARM) Graciosa Island observatory, Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) and the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field projects. These observations will be complemented with satellite datasets, reanalysis and simulations with a weather prediction model.

First, we hypothesize that aerosols suppress precipitation in clouds associated with fronts. These clouds cover substantial fractions of the mid-latitudes, so significant aerosol effects on cloud albedo, lifetime or precipitation along the fronts may have substantial consequences for global albedo and aerosol budgets. 

Second, we hypothesize that clouds in cold fronts and warm conveyor belts are critical to aerosol transport and scavenging in the mid-latitudes. It is well known that warm conveyor belts in extratropical cyclones act to transport aerosol from continental outflow regions across the Pacific and Atlantic. However, the effects of frontal clouds both on aerosols associated with these conveyor belts and on aerosols in cyclone cold sectors, in both the Northern Hemisphere and Southern Ocean, remain uncertain. These uncertainties have hindered critical efforts to understand aerosol budgets, cloud droplet concentrations and precipitation in these regions. In the Southern Ocean, substantial biases in climate models result.

To understand aerosol interactions with mid-latitude cyclones and quantify the extent to which our hypotheses are true, we will determine the relationships between cloud and aerosol properties observed by ARM surface and aircraft instrumentation as well as complementary satellite datasets. The atmospheric processes we will focus on are aerosol activation, warm rain formation, and aerosol removal by precipitation.   

To gain process understanding as we test the hypotheses we outlined above, we will examine whether the relationships we will identify also exist in simulations and whether they can be associated with clear causal relationships, building on prior studies of causality in non-cyclonic conditions. The simulations will be performed with the UK Met Office Unified Model at a horizontal resolution of a few kilometers, avoiding parameterized convection. Deficiencies in this model highlighted by our analysis may well be common to Department of Energy-supported models such as the Energy Exascale Earth System Model (E3SM).

We are proposing to combine ARM data with simulations to deliver an improved understanding of aerosol-cloud interaction processes in cyclones. We intend that this work will facilitate improvements to climate model simulations of aerosol indirect effects, global aerosol budgets and anthropogenic radiative forcing.
Award Recipient(s)
  • Carnegie Mellon University (PI: Gordon, Hamish)