The PIRE Cirrus research team seeks to shed light on a series of questions using a variety of approaches. Each question involves collaboration among different parts of the team, as is highlighted below:
How does deep convection affect the tropical tropopause layer (TTL), in terms of moisture and energy budgets?
Groups at the University of Washington, Princeton, Harvard and ETH Zurich are working with large-scale simulations and observations to learn more about the role of deep convection in the TTL. This involves both the analysis of existing simulations and making new ones, as well as bringing a new approach to analyzing observations.
The DYAMOND project has brought together a new generation of high-resolution, storm-resolving global weather models to benchmark their ability to simulate weather, precipitation and clouds. At the University of Washington, graduate students, Jacqueline Nugent and Sami Turbeville, along with postdoctoral researcher Suqin Duan at Princeton, are analyzing these exciting new simulations and comparing them with observations to understand how well they represent the TTL and how deep convection contributes to cloud in the TTL.
At Harvard, Zeyuan Hu, a graduate student, is making idealized simulations that ask fundamental questions about the role of convection in the energy budget of the TTL, while and Fayçal Lamraoui, a postdoctoral researcher, is simulating observed conditions during the StratoClim field campaign (a period of intense measurements over India, Nepal and Bangladesh in July and August 2017) to learn about convective influence on the TTL during the South Asian monsoon. Andries de Vries, a postdoctoral researcher at ETH Zurich, is also simulating the influence of deep convection on cirrus over the African continent.
The Princeton group has also been analyzing satellite and aircraft observations to estimate the flows of water (in the form of both vapor and ice) into and out of the TTL. These methods are described in Bolot and Fueglistaler (2020), a paper led by postdoctoral researcher, Max Bolot.
How do internal dynamics within cirrus affect their characteristics and persistence?
Blaž Gasparini, a postdoctoral researcher at the University of Washington who has participated in the PIRE project, has explored the interactions of radiation, microphysics and dynamics in the maintenance of cirrus clouds detrained from convection. This work has been published in Hartmann et al (2018) and Gasparini et al (2019).
The properties of the ice crystals in cirrus clouds — the distribution of ice crystal sizes, for example --- are important to their longevity, and the group of Martina Krämer in Jülich, Germany makes measurements of cirrus clouds from aircraft to help us better understand those properties.
What factors determine where, when, and how cirrus forms?
The groups mentioned above who use simulations to evaluate the influence of deep convection on the TTL will also be looking at how all cirrus is generated in those simulations, including both cirrus detrained from convection and that formed in situ. The group at ETH Zurich is already analyzing the origins of the air that contributes to cirrus formation using an analysis of air parcel trajectories in their simulations. The analysis of air parcel trajectories and their contribution to cirrus formation in the upper troposphere and the TTL has also been employed by PIRE participants, Bernard Legras (LMD, France), Stephan Fueglistaler (Princeton), Tra Dinh (Univ. of Auckland, NZ) and Eric Jensen (NCAR), among others.
Cirrus clouds that form in situ --- meaning that they form out of clear air high in the atmosphere — may have different properties depending on whether they form on ice-friendly particles in the atmosphere called ice nucleating particles, or on liquid-coated aerosols that subsequently freeze. These processes have been studied in detail in laboratory experiments in the AIDA chamber at KIT by the group of Ottmar Möhler, including his graduate student and PIRE participant, Julia Kaufmann.
And how do cirrus in turn affect large-scale circulations?
Cirrus clouds affect both the incoming radiation from the sun as well as the outgoing thermal radiation from the earth’s surface and its atmosphere. The radiative heating associated with clouds in the upper troposphere and TTL can influence circulations in the atmosphere both locally and on larger scales. Local, or mesoscale, circulations associated with TTL cirrus have been studies by PIRE participants, Tra Dinh (Univ. of Auckland, New Zealand), Tom Ackerman (Univ. of Washington) and Stephan Fueglistaler (Princeton). The groups of Tiffany Shaw (Univ. of Chicago) and Aiko Voigt (Karlsruhe Institute of Technology, KIT) are interested in the impact of cirrus on large-scale circulations in the atmosphere. Their earlier work in Voigt and Shaw (2015) highlighted the importance of tropical cirrus in the way atmospheric circulations change with climate. Sylvia Sullivan, a postdoctoral researcher at KIT, will be looking at these connections between cirrus and the large-scale circulation.