Weather

The term "weather systems" broadly encompasses the processes that regulate atmospheric conditions on time scales of seconds to weeks and spatial scales ranging from the microscale (~ 0.1 to 1 km) to the planetary scale (~10,000 km). Research activity is particularly focused on mesoscale (~1 to 100s of km) and synoptic scale (~1000 km) processes. Phenomena that fall into this theme include extratropical cyclones, fronts, tropical cyclones and hurricanes, mountain waves, mesoscale convective systems, cloud initiation and evolution, and boundary-layer circulations. Enhancing the understanding and prediction of such processes requires strong expertise in atmospheric dynamics, thermodynamics, cloud physics, and radiation. Our faculty and research trainees use this background as a foundation for their studies. Research methodologies encompass observational, theoretical, and numerical analyses of varying weather phenomena, with a goal of improving the conceptual understanding and numerical prediction of weather systems.

The Weather group carries out a wide range of research centered around four major themes/ areas.  The areas and the associated faculty members are:

Radar and remote sensing

Faculty member: Fabry

Since we do not have sensors everywhere on the surface of the earth and in the air, we must rely on remote sensing (or sensing from a distance) to gather additional information on the atmosphere around us. Radars and satellite imagers are traditional remote sensing instruments routinely used for monitoring and studying weather. While satellite imagers are generally used to study clouds, radars are best at observing precipitation. Together with additional specialized sensors such as lidars (laser-based “radars”) and radiometers, they provide us a much more detailed picture of weather systems that would otherwise be available. At McGill, we run a variety of ground-based remote sensors, and we also use data from other systems for our work.

Research in remote sensing and weather has a few distinct fields of interest: 1) getting more or better information from remote sensing, 2) using remote-sensed information in combination with other sensors to better understand weather systems, 3) improving forecasts by better using the information obtained by remote sensing through data assimilation (a process of optimally combining information from a variety of sources). Although we want to improve the study and forecasting of all types of weather systems, we tend to focus on the varied winter-to-summer events that affect the North-East of the North-American continent.

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Cumulus convection

Faculty members: Fabry, Kirshbaum, Yau

Cumulus convection involves the buoyancy-driven vertical motion of saturated air. This process is widespread over the globe, producing large vertical transports of heat, moisture, and momentum. These transports feed back onto the larger-scale atmospheric circulations that regulate our weather and climate. The precipitation produced by deep cumulus clouds (cumulonimbi) is also vital for sustaining life on earth. However, the potential of such clouds to produce severe weather (thunderstorms, hail, flooding, etc.) render them potentially hazardous to life and property.

Due to the multiscale, multiphase, nonlinear, and turbulent nature of cumulus convection, its understanding and prediction pose formidable challenges that have not been completely overcome. Our research addresses these challenges by observing the internal details of cumulus clouds (and their surrounding environments) and simulating these clouds in numerical models. At McGill, the observational research focuses on remote sensing of clouds by radars and satellites, and the numerical research consists of both idealized cloud-resolving simulations and Numerical Weather Prediction models at coarser resolutions. The goal is to synthesize these different data sources to build an improved physical understanding that, in turn, leads to improved representation of cumulus convection in weather and climate models.

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Tropical Cyclones

Faculty members: Gyakum, Yau

Hurricanes represent one of the most violent natural phenomena in the atmosphere. They form over tropical regions and can be regarded as a heat engine by gaining heat from the warm tropical seas and releasing heat in the cold upper troposphere. They are warm core systems with their maximum winds in the lower levels. Hurricanes are highly destructive by means of their violent winds, copious precipitation, and the accompanying flash floods and storm surges. For hurricane forecasting, it is necessary to predict the track and the intensity of the storm.

Image showing the double Eyeball of hurricane Maria (2017).

In recent years, considerable progress has been made in track forecasting but major problems remain in intensity change prediction. Research is urgently needed to understand the different processes that can regulate hurricane intensity. They include environmental conditions like sea surface sensible/latent heat fluxes, wind shear, air humidity, and upper level temperature distribution; as well as internal dynamical processes like different wave motions and instabilities operating in the vicinity of the hurricane eyewall. Specific examples of the latter include the formation and dissipation of multiple eyewalls and their effects on the distribution of strong winds and precipitation. Observations, theoretical analysis, and numerical modeling are tools useful for such investigation.

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Extratropical cyclones

Faculty members: Gyakum, Kirshbaum

Extratropical cyclones (ECs) are characterized by cyclonic rotation of surface winds around a sea-level pressure minimum. Unlike tropical cyclones, ECs are associated with horizontal temperature contrasts. ECs are typically associated with precipitation and strong winds. The EC formation, its winds, and associated precipitation continue to be important scientific research topics. EC formation may occur along pre-existing fronts, in response to mid-tropospheric mobile troughs, or as a transformation from a tropical cyclone (an “Extratropical Transition”) traveling poleward into the extratropical latitudes. The EC’s meteorological impacts are also topics of ongoing research. ECs are responsible for producing precipitation on a wide range of spatial scales and severity.

Satellite image of the March 1993 superstorm

For example, ECs may produce environments that are conducive to slantwise or upright convection, squall lines and severe weather. ECs are primarily responsible for producing both cold and warm fronts, which are favored regions for significant precipitation. ECs transport both heat and water vapor poleward, occasionally in narrow bands of moisture, often referred to as “atmospheric rivers.” ECs can impact the dynamical environments for future cyclogenesis, either by increasing or diminishing the strength of the jet stream. They may impact the planetary-scale general circulation through wave-breaking processes that can alter the atmosphere’s available potential energy. Societal impacts of ECs extend beyond the effects of wind and precipitation. For example, ECs’ strong winds may produce storm surges along coastal regions, and blowing snow over snow-covered continental regions.

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