Events (Upcoming & Past)

Past MMM Events

Charles Knight
NCAR/MMM  

“The box” represents classical nucleation theory, CNT, a conceptually simple and at first appealing mechanism in which the interfacial energy between an initial, unstable phase (liquid water, here) and a stable one (ice) constitutes an energy barrier against the stable one’s first appearance.  Ice nucleation obviously involves crystal growth, but the theory of CNT in general has been based upon thermodynamics and chemical reaction theory, independent of crystal structure.  Outside the box here is treating crystal growth and nucleation in terms of growth of the known hydrogen-bond network of ice: the ice crystal structure and its tetrahedral bonding.  The initial context here was trying to explain the observed correlations between ice crystal growth in liquid water and the ice crystal structure, part of which appeared to involve two-dimensional nucleation of new molecular layers at an interface between ice and liquid water.  This explanation turned out not to work well whereas a simple model of growth of the bonding network does seem to provide conceptual understanding.  A bonding-network approach to homogeneous nucleation is unwieldy but interesting, and from that point of view, CNT (for nucleating ice) seems dubious.  The actual mechanism may be dominated by structural effects, not interfacial energy.

Refreshments: 3:15 PM

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1022
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, March 22, 2018 - 3:30pm to 4:30pm

Xiang-Yu LiDepartment of MeteorologyStockholm University, Sweden

We investigate the effect of turbulence on the collisional growth of μm-sized droplets by high-resolution numerical simulations with well resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. We show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collision depends strongly on the turbulent energy-dissipation rate, but only weakly on the Reynolds number. The size distribution exhibits power law behavior with a slope of −3.7 in the size range of about 10 ∼40 μm, which is close to the power law size distribution found for interstellar dust grains. When gravity is invoked, the strong dependency becomes weakened. Turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. With combined turbulence and gravity, the time scale to reach drizzle sized droplets is about 900 s, which is close to the time scale of rapid warm rain formation. The collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when the turbulence-generated collision is invoked.

Refreshments: 3:15 PM

Building:
Room Number: 
1022
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, March 9, 2018 - 5:30am to 6:30am

Xiang-Yu Li
Department of Meteorology
Stockholm University, Sweden

We investigate the effect of turbulence on the collisional growth of μm-sized droplets by high-resolution numerical simulations with well resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. We show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collision depends strongly on the turbulent energy-dissipation rate, but only weakly on the Reynolds number. The size distribution exhibits power law behavior with a slope of −3.7 in the size range of about 10 ∼40 μm, which is close to the power law size distribution found for interstellar dust grains. When gravity is invoked, the strong dependency becomes weakened. Turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. With combined turbulence and gravity, the time scale to reach drizzle sized droplets is about 900 s, which is close to the time scale of rapid warm rain formation. The collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when the turbulence-generated collision is invoked.

Refreshments: 3:15 PM

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1022
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, March 8, 2018 - 3:30pm to 4:30pm

Andy Wood Martyn Clark NCAR/RAL/HAP 

Ensemble hydrologic (streamflow) prediction provides critical inputs for water, energy and hazard management, particularly in the face of extremes such as floods and droughts.  Following steady advances in operational ensemble numerical weather prediction since the 1990s, US and international operational prediction groups have invested heavily in developing datasets, methods, and models to enable a seamless suite of probabilistic hydrologic predictions spanning timescales from hours to seasons.  Ensemble hydrologic forecasting systems are now operational in a number of countries (including the US), and are enhanced by an increasingly crowded field of operational continental and global ensemble hydrologic prediction services.  In this presentation, we provide background describing the evolution of ensemble hydrologic prediction systems, and highlight the role of the HEPEX (Hydrologic Ensemble Prediction Experiment; www.hepex.org) initiative since 2004 in defining and promoting an integrative, scientific view of the elements of a hydrologic ensemble prediction approach.  These include methods for the probabilistic downscaling and calibration of meteorological forecast ensembles, hydrologic model parameter estimation and uncertainty quantification, hydrologic model data assimilation, model output post-processing, and ensemble forecast verification and communication for use in risk-based decision-making.  We summarize the current state of practice in applying these methods to achieve reliable ensemble streamflow forecasts (locally and globally), and discuss long-standing and new challenges identified by the ensemble hydrologic prediction community.

Refreshments:  3:15 PM  

Building:
Room Number: 
1022
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, March 2, 2018 - 5:30am to 6:30am

Andy Wood
Martyn Clark
NCAR/RAL/HAP 

Ensemble hydrologic (streamflow) prediction provides critical inputs for water, energy and hazard management, particularly in the face of extremes such as floods and droughts.  Following steady advances in operational ensemble numerical weather prediction since the 1990s, US and international operational prediction groups have invested heavily in developing datasets, methods, and models to enable a seamless suite of probabilistic hydrologic predictions spanning timescales from hours to seasons.  Ensemble hydrologic forecasting systems are now operational in a number of countries (including the US), and are enhanced by an increasingly crowded field of operational continental and global ensemble hydrologic prediction services.  In this presentation, we provide background describing the evolution of ensemble hydrologic prediction systems, and highlight the role of the HEPEX (Hydrologic Ensemble Prediction Experiment; www.hepex.org) initiative since 2004 in defining and promoting an integrative, scientific view of the elements of a hydrologic ensemble prediction approach.  These include methods for the probabilistic downscaling and calibration of meteorological forecast ensembles, hydrologic model parameter estimation and uncertainty quantification, hydrologic model data assimilation, model output post-processing, and ensemble forecast verification and communication for use in risk-based decision-making.  We summarize the current state of practice in applying these methods to achieve reliable ensemble streamflow forecasts (locally and globally), and discuss long-standing and new challenges identified by the ensemble hydrologic prediction community.

Refreshments:  3:15 PM  

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1022
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, March 1, 2018 - 3:30pm to 4:30pm

Hugh Morrison MMM/NCAR

The representation of cloud and precipitation microphysics is a critical element in atmospheric models of all scales. It affects the thermodynamics and dynamics from latent heating/cooling and condensate drag, strongly influences cloudy radiative transfer, and is a key component of the hydrological cycle through the generation and fallout of precipitation. An overview of the historical development of microphysics schemes in cloud and mesoscale models will be presented first. Advances over the last decade will be covered in more detail, particularly the recent development of a scheme called Predicted Particle Properties (P3) that predicts and smoothly evolves ice particle properties such as density and fall speed. This approach is a significant departure from traditional microphysics schemes that separate ice into categories with fixed properties corresponding to particular ice types (small ice, snow, graupel, hail, etc.). Simulations using P3 implemented in the Weather Research and Forecasting (WRF) model will be presented and contrasted with those using traditional schemes. Additional developments related to P3, including an improved numerical treatment of cloud and precipitation transport, will also be presented. Finally, more “outside of the box” ideas for parameterizing microphysics will be highlighted, including a Bayesian statistical-physical parameterization framework that facilitates observational constraint of process rates and a rigorous characterization of uncertainty. The talk with conclude with a broader outlook and commentary on future microphysics scheme developments over the next 5-10 years and beyond.

Refreshments: 3:15 PM

Building:
Room Number: 
1022
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, February 16, 2018 - 5:30am to 6:30am

Hugh Morrison
MMM/NCAR

The representation of cloud and precipitation microphysics is a critical element in atmospheric models of all scales. It affects the thermodynamics and dynamics from latent heating/cooling and condensate drag, strongly influences cloudy radiative transfer, and is a key component of the hydrological cycle through the generation and fallout of precipitation. An overview of the historical development of microphysics schemes in cloud and mesoscale models will be presented first. Advances over the last decade will be covered in more detail, particularly the recent development of a scheme called Predicted Particle Properties (P3) that predicts and smoothly evolves ice particle properties such as density and fall speed. This approach is a significant departure from traditional microphysics schemes that separate ice into categories with fixed properties corresponding to particular ice types (small ice, snow, graupel, hail, etc.). Simulations using P3 implemented in the Weather Research and Forecasting (WRF) model will be presented and contrasted with those using traditional schemes. Additional developments related to P3, including an improved numerical treatment of cloud and precipitation transport, will also be presented. Finally, more “outside of the box” ideas for parameterizing microphysics will be highlighted, including a Bayesian statistical-physical parameterization framework that facilitates observational constraint of process rates and a rigorous characterization of uncertainty. The talk with conclude with a broader outlook and commentary on future microphysics scheme developments over the next 5-10 years and beyond.

Refreshments: 3:15 PM

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1022
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, February 15, 2018 - 3:30pm to 4:30pm

Shane KeatingUniversity of New South Wales Australia  

The era of earth-observing satellites has revolutionised our understanding of our planet and the dynamical processes that shape it. In many real-world geophysical systems, however, estimates of turbulent mixing and transport are limited by the resolution of available observations. In this talk, I will describe a suite of stochastic filtering strategies for estimating mixing in turbulent geophysical flows from “superresolved” satellite imagery obtained by combining coarse observations with an efficient stochastic parameterization for the unresolved scales. The method enhances the effective resolution of satellite observations by exploiting the effect of spatial aliasing and generates an optimal estimate of small scales using standard Bayesian inference. The technique is tested in quasigeostrophic simulations driven by realistic climatological shear and stratification profiles. Two applications are considered: calculating poleward ocean eddy heat flux from satellite altimetry, and estimating the three-dimensional upper ocean velocity field from superresolved sea-surface temperature imagery. In each case, the superresolved satellite observations result in a considerable improvement in estimates of turbulent fluxes compared with the raw observations.

Refreshments: 3:15 PM

Building:
Room Number: 
1022
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, February 9, 2018 - 5:30am to 6:30am

Shane Keating
University of New South Wales
Australia  

The era of earth-observing satellites has revolutionised our understanding of our planet and the dynamical processes that shape it. In many real-world geophysical systems, however, estimates of turbulent mixing and transport are limited by the resolution of available observations. In this talk, I will describe a suite of stochastic filtering strategies for estimating mixing in turbulent
geophysical flows from “superresolved” satellite imagery obtained by combining coarse observations with an efficient stochastic parameterization for the unresolved scales.

The method enhances the effective resolution of satellite observations by exploiting the effect of spatial aliasing and generates an optimal estimate of small scales using standard Bayesian inference. The technique is tested in quasigeostrophic simulations driven by realistic climatological shear and stratification profiles. Two applications are considered: calculating poleward ocean eddy heat flux from satellite altimetry, and estimating the three-dimensional upper ocean velocity field from superresolved sea-surface temperature imagery. In each case, the superresolved satellite observations result in a considerable improvement in estimates of turbulent fluxes compared with the raw observations.

Refreshments: 3:15 PM

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1022
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, February 8, 2018 - 3:30pm to 4:30pm

2018 NORTH AMERICAN WORKSHOP ON HAIL & HAILSTORMSAUGUST 14 - 16, 2018, BOULDER, COLORADONCAR CENTER GREEN CAMPUS

Across North America, hailstorms are responsible for over $10 billion dollars in annual property damage. The increase in the impact of hailstorms has outpaced advances in detection, forecasting, and mitigation. The National Science Foundation, the National Center for Atmospheric Research and the Insurance Institute for Business & Home Safety are organizing the first North American Workshop on hail, and hailstorms. The workshop will bring together public and private stakeholders to discuss the current state of the science regarding all facets of this peril and provide a look to the future. The workshop will be held at the NCAR Center Green 1 (CG1) campus, 3080 Center Green Drive, Boulder, Colorado.

Call For Abstracts: The deadline for abstract submissions is May 1, 2018.

For information: https://www.mmm.ucar.edu/north-american-hail-workshop 

Building:
Room Number: 
Auditorium
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
No
Calendar Timing: 
Repeats every day every Monday and every Tuesday and every Wednesday and every Thursday and every Friday until Thu Aug 16 2018.
Thursday, August 16, 2018 - 9:00pm to Sunday, August 19, 2018 - 5:00am

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