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Past MMM Events

Nedjelika ŽagarUniversity of LjubljanaLjubljana, Slovenia

Many studies of the forecast error growth focused on the extra-tropical quasi-geostrophic dynamics and often considered the error-free large-scale initial state.  In contrast, the operational global numerical weather prediction and ensemble prediction systems are characterized by uncertainties in the initial state at all scales, especially in the tropics.  In this seminar the evidence will be discussed about the dominant role of the large-scale error growth early in the forecasts in comparison with the errors cascades from the smaller scales.   A new parametric model for the representation of the error growth will be derived.  In contrast to the commonly used models, the new model does not involve computation of the time derivatives of the empirical data. The asymptotic error is not a fitting parameter, but it is computed from the model constants. 

Simulated forecast errors by the operational ensemble prediction system of the European Centre for Medium-Range Weather Forecasts are decomposed into scales and the new model is applied independently to every zonal wavenumber.  A combination of hyperbolic tangent functions in the parametrization of the error growth proves robust to reliably model complex growth dynamics across many scales.  The range of useful prediction skill, estimated as a scale where forecast errors exceeds 60% of their asymptotic values is around 7 days on large scales and 2-3 days at 1000 km scale.  The new model is easily transformed to the widely used model of Dalcher and Kalnay (1987) to discuss the scale-dependent growth as a sum of two terms, the so-called a and b terms.  Their comparison shows that at planetary scales their contributions to the growth in the first 2 days are similar whereas at small scales the b term describes most of a rapid exponential growth of errors towards saturation. 

Refreshments: 3:15pm

Building:
Room Number: 
1022
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, August 11, 2017 - 3:30am to 4:30am

Nedjelika Žagar
University of Ljubljana
Ljubljana, Slovenia

Many studies of the forecast error growth focused on the extra-tropical quasi-geostrophic dynamics and often considered the error-free large-scale initial state.  In contrast, the operational global numerical weather prediction and ensemble prediction systems are characterized by uncertainties in the initial state at all scales, especially in the tropics.  In this seminar the evidence will be discussed about the dominant role of the large-scale error growth early in the forecasts in comparison with the errors cascades from the smaller scales.   A new parametric model for the representation of the error growth will be derived.  In contrast to the commonly used models, the new model does not involve computation of the time derivatives of the empirical data. The asymptotic error is not a fitting parameter, but it is computed from the model constants. 

Simulated forecast errors by the operational ensemble prediction system of the European Centre for Medium-Range Weather Forecasts are decomposed into scales and the new model is applied independently to every zonal wavenumber.  A combination of hyperbolic tangent functions in the parametrization of the error growth proves robust to reliably model complex growth dynamics across many scales.  The range of useful prediction skill, estimated as a scale where forecast errors exceeds 60% of their asymptotic values is around 7 days on large scales and 2-3 days at 1000 km scale.  The new model is easily transformed to the widely used model of Dalcher and Kalnay (1987) to discuss the scale-dependent growth as a sum of two terms, the so-called a and b terms.  Their comparison shows that at planetary scales their contributions to the growth in the first 2 days are similar whereas at small scales the b term describes most of a rapid exponential growth of errors towards saturation. 

Refreshments: 3:15pm


First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1022
Host lab/program/group:
Calendar Timing: 
Thursday, August 10, 2017 - 3:30pm to 4:30pm

James Done NCAR/MMM

As populations increase in hazard-prone regions, the human, cultural and economic costs rise, and will continue to rise in the future. The likely scenario of the weather and climate hazards themselves changing in the future will compound the problem. A transformation of how weather and climate risk is assessed and integrated with risk management practice is needed for society to confront this new era of weather and climate risk. Bringing physics to bear on risk assessment has the potential to transform our understanding of weather and climate risk. Furthermore, physically based risk assessments that are informed by risk management practice are a potentially powerful component of climate resilience. Three recent examples will be presented to illustrate the flow between physically based weather and climate risk assessments and community action.

The first example is the development of a terrain-aware tropical cyclone wind probability assessment at the global scale. In collaboration with a reinsurance broker, an approach to modeling tropical cyclone wind footprints is developed by fitting a parametric wind field model to historical and synthetic cyclone track data, and bringing the winds down to the surface using a 3-dimensional numerical boundary model, accounting for terrain and surface roughness effects. The new wind probability assessments are being used to understand inland wind risk in regions of complex topography, and assess public and private risk management strategies in regions of sparse historical data. The second example explores how the relationship between residential losses and hurricane winds is modified through building codes. Adherence to the Florida building code drives down losses by up to 70%, and the code is cost-effective with a return on investment after 12 years under current climate. The final example explores the role of decadal climate predictions in water resource and flood risk management. The multi-disciplinary UDECIDE (Understanding Decision-Climate Interactions on Decadal Scales) project combines statistical and physical assessments of climate prediction skill with data from interviews with managers to identify intersections at the decadal scale in support of effective management.

Refreshments:  3:15pm

Building:
Room Number: 
1001 (Note Location)
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, August 4, 2017 - 3:30am to 4:30am

James Done 
NCAR/MMM

As populations increase in hazard-prone regions, the human, cultural and economic costs rise, and will continue to rise in the future. The likely scenario of the weather and climate hazards themselves changing in the future will compound the problem. A transformation of how weather and climate risk is assessed and integrated with risk management practice is needed for society to confront this new era of weather and climate risk. Bringing physics to bear on risk assessment has the potential to transform our understanding of weather and climate risk. Furthermore, physically based risk assessments that are informed by risk management practice are a potentially powerful component of climate resilience. Three recent examples will be presented to illustrate the flow between physically based weather and climate risk assessments and community action.

The first example is the development of a terrain-aware tropical cyclone wind probability assessment at the global scale. In collaboration with a reinsurance broker, an approach to modeling tropical cyclone wind footprints is developed by fitting a parametric wind field model to historical and synthetic cyclone track data, and bringing the winds down to the surface using a 3-dimensional numerical boundary model, accounting for terrain and surface roughness effects. The new wind probability assessments are being used to understand inland wind risk in regions of complex topography, and assess public and private risk management strategies in regions of sparse historical data. The second example explores how the relationship between residential losses and hurricane winds is modified through building codes. Adherence to the Florida building code drives down losses by up to 70%, and the code is cost-effective with a return on investment after 12 years under current climate. The final example explores the role of decadal climate predictions in water resource and flood risk management. The multi-disciplinary UDECIDE (Understanding Decision-Climate Interactions on Decadal Scales) project combines statistical and physical assessments of climate prediction skill with data from interviews with managers to identify intersections at the decadal scale in support of effective management.

Refreshments:  3:15pm

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1001 (Note Location)
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, August 3, 2017 - 3:30pm to 4:30pm

Lotte BierdelLudwig Maximilians University Munich, Germany 

The current literature discussing predictability of atmospheric flow and the nature of the underlying scale interactions considers the problem from two main perspectives. One approach is based on statistical closure models in a homogeneous and isotropic turbulent flow, where the predictability time is determined solely by the background kinetic energy spectrum and not by the underlying dynamical model. An alternative approach is based on results from numerical weather prediction models that suggest that latent heat release associated with deep moist convection is a primary mechanism for small-scale error growth. From this point of view error growth in the atmosphere is an initially localized, highly intermittent phenomenon that expands upscale, leading to a complete loss of predictability on scales below 100 km within a few hours. The error growth process then depends on the underlying dynamics of the respective scale range and the errors in particular have to transition from geostrophically unbalanced to balanced motion while propagating through the mesoscale. In this talk a study will be presented that examines the geostrophic adjustment process as possibly underlying this transition. To that end, an analytical framework for the geostrophic adjustment of an initial pointlike pulse of heat (modeling a convective cloud or an error within the prediction of a cloud) is developed. Spatial and temporal scales of the geostrophic adjustment mechanism are deduced and three characteristics of the solution are shown to be potentially useful for identifying the geostrophic adjustment process in numerical simulations. These three predictions are then tested in the framework of error growth experiments in idealized numerical simulations of a convective cloud field. Three different rotation rates are employed in order to identify the geostrophic adjustment mechanism and allow a quantitative comparison with the predictions of the analytical model. As will be shown, the numerical simulations agree well with the predictions developed from the analytical model. Based on these findings it is suggested that the geostrophic adjustment process governs upscale error growth through the atmospheric mesoscales.

Refreshments: 3:15 PM

Building:
Room Number: 
1001 (Note Location)
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, July 21, 2017 - 3:30am to 4:30am

Lotte Bierdel
Ludwig Maximilians University
Munich, Germany 

The current literature discussing predictability of atmospheric flow and the nature of the underlying scale interactions considers the problem from two main perspectives. One approach is based on statistical closure models in a homogeneous and isotropic turbulent flow, where the predictability time is determined solely by the background kinetic energy spectrum and not by the underlying dynamical model. An alternative approach is based on results from numerical weather prediction models that suggest that latent heat release associated with deep moist convection is a primary mechanism for small-scale error growth. From this point of view error growth in the atmosphere is an initially localized, highly intermittent phenomenon that expands upscale, leading to a complete loss of predictability on scales below 100 km within a few hours. The error growth process then depends on the underlying dynamics of the respective scale range and the errors in particular have to transition from geostrophically unbalanced to balanced motion while propagating through the mesoscale. In this talk a study will be presented that examines the geostrophic adjustment process as possibly underlying this transition. To that end, an analytical framework for the geostrophic adjustment of an initial pointlike pulse of heat (modeling a convective cloud or an error within the prediction of a cloud) is developed. Spatial and temporal scales of the geostrophic adjustment mechanism are deduced and three characteristics of the solution are shown to be potentially useful for identifying the geostrophic adjustment process in numerical simulations. These three predictions are then tested in the framework of error growth experiments in idealized numerical simulations of a convective cloud field. Three different rotation rates are employed in order to identify the geostrophic adjustment mechanism and allow a quantitative comparison with the predictions of the analytical model. As will be shown, the numerical simulations agree well with the predictions developed from the analytical model. Based on these findings it is suggested that the geostrophic adjustment process governs upscale error growth through the atmospheric mesoscales.

Refreshments: 3:15 PM

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1001 (Note Location)
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, July 20, 2017 - 3:30pm to 4:30pm

Stipo SenticNew Mexico Tech

Tropical convective organization is the process in which disorganized convection organizes into regions of intense convective activity surrounded by dry, convectively inactive regions. Well known examples are tropical cyclones and the Madden-Julian Oscillation (MJO)—they affect atmospheric energetics, and the MJO affects virtually all weather on our planet. Recent advances in idealized modelling of tropical convection, namely the weak temperature gradient (WTG) approximation, enable us to study convective organization in idealized settings. The WTG approximation parameterizes the effects of the large-scale on local convection, and can be used in idealized sensitivity studies of convection to changes in large-scale convective environment. To model organized convection in the context of the MJO, we used observations from the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign to force WTG simulations in a cloud resolving model, and test how well the WTG approximation reproduces variations in convective diagnostics: precipitation rate, stability, moisture content, and large-scale transport (gross moist stability). We find that the WTG approximation reproduces variations in these diagnostics, and relationships between them.  The ability of WTG approximation to reproduce important observed diagnostics provides confidence that this is a good strategy for exploring tropical phenomena.  An example that I'll talk about is the behavior of convective organization at different SSTs.

(Special Date) Tuesday, 30 May 2017, 3:30 PMRefreshments: 3:15 PM NCAR-Foothills Laboratory 3450 Mitchell LaneBldg. 2, Small Seminar Room 1001 (Note Location)

Building:
Room Number: 
1001 (Please note location)
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Wednesday, May 31, 2017 - 3:30am to 4:30am

Stipo Sentic
New Mexico Tech

Tropical convective organization is the process in which disorganized convection organizes into regions of intense convective activity surrounded by dry, convectively inactive regions. Well known examples are tropical cyclones and the Madden-Julian Oscillation (MJO)—they affect atmospheric energetics, and the MJO affects virtually all weather on our planet. Recent advances in idealized modelling of tropical convection, namely the weak temperature gradient (WTG) approximation, enable us to study convective organization in idealized settings. The WTG approximation parameterizes the effects of the large-scale on local convection, and can be used in idealized sensitivity studies of convection to changes in large-scale convective environment. To model organized convection in the context of the MJO, we used observations from the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign to force WTG simulations in a cloud resolving model, and test how well the WTG approximation reproduces variations in convective diagnostics: precipitation rate, stability, moisture content, and large-scale transport (gross moist stability). We find that the WTG approximation reproduces variations in these diagnostics, and relationships between them.  The ability of WTG approximation to reproduce important observed diagnostics provides confidence that this is a good strategy for exploring tropical phenomena.  An example that I'll talk about is the behavior of convective organization at different SSTs.

(Special Date) Tuesday, 30 May 2017, 3:30 PM
Refreshments: 3:15 PM 
NCAR-Foothills Laboratory 
3450 Mitchell Lane
Bldg. 2, Small Seminar Room 1001 (Note Location)


First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1001 (Please note location)
Host lab/program/group:
Type of event:
Calendar Timing: 
Tuesday, May 30, 2017 - 3:30pm to 4:30pm

Michael TjernströmDepartment of Meteorology & Bolin Centre for Climate ResearchStockholm University, Sweden

Arctic climate is ultimately determined by a balance between meridional heat transport into the area, and radiation heat loss at the top of the atmosphere over the same area. Since the net radiation loss is due to small-scale processes parameterized in models, and the meridional heat flux is due to larger scale atmospheric dynamics resolved by the models, the two has usually been studied separately. In this seminar this concept will be called into question.

In an episode during the Arctic Clouds in Summer Experiment (ACSE) in the summer of 2014, warm air from the Siberian mainland flowed in over melting sea-ice in the East-Siberian Sea for over a week. As the ~25 °C warm air flowed over the melting surface, maintained at the melting point, a strong surface inversion formed in which dense fog also formed. This resulted in a positive net longwave radiation while the sensible heat flux was downward. Although solar radiation was attenuated by the fog, this led to an additional 10-20 Wm-2 energy to the surface. This led us to hypothesize a zone from the ice edge where the surface will receive enhanced energy when the atmospheric flow is northward onto the ice. 

To test this hypothesis, we analyzed the observation from the entire ACSE expedition. All temperature profiles taken over sea ice were categorized into cases with or without a surface inversion; the inversion cases where further divided into two categories using the humidity profiles. When projecting other observations onto these three classes, many are systematically different. Surface inversion with increasing moisture with height systematically added 10-20 Wm-2 energy to the surface energy budget, indicating that meridional heat flux must be considered together with the small-scale processes caused by the air mass transformation.

Please note the location change.

Thursday, 1 June 2017, 3:30 PMRefreshments:  3:15 PM NCAR-Foothills Laboratory3450 Mitchell Lane Bldg. 2, Small Seminar Room 1001 

Building:
Room Number: 
1001 (Please note location change)
Type of event:
Will this event be webcast to the public by NCAR|UCAR?: 
Calendar Timing: 
Friday, June 2, 2017 - 3:30am to 4:30am

Michael Tjernström
Department of Meteorology & Bolin Centre for Climate Research
Stockholm University, Sweden

Arctic climate is ultimately determined by a balance between meridional heat transport into the area, and radiation heat loss at the top of the atmosphere over the same area. Since the net radiation loss is due to small-scale processes parameterized in models, and the meridional heat flux is due to larger scale atmospheric dynamics resolved by the models, the two has usually been studied separately. In this seminar this concept will be called into question.

In an episode during the Arctic Clouds in Summer Experiment (ACSE) in the summer of 2014, warm air from the Siberian mainland flowed in over melting sea-ice in the East-Siberian Sea for over a week. As the ~25 °C warm air flowed over the melting surface, maintained at the melting point, a strong surface inversion formed in which dense fog also formed. This resulted in a positive net longwave radiation while the sensible heat flux was downward. Although solar radiation was attenuated by the fog, this led to an additional 10-20 Wm-2 energy to the surface. This led us to hypothesize a zone from the ice edge where the surface will receive enhanced energy when the atmospheric flow is northward onto the ice. 

To test this hypothesis, we analyzed the observation from the entire ACSE expedition. All temperature profiles taken over sea ice were categorized into cases with or without a surface inversion; the inversion cases where further divided into two categories using the humidity profiles. When projecting other observations onto these three classes, many are systematically different. Surface inversion with increasing moisture with height systematically added 10-20 Wm-2 energy to the surface energy budget, indicating that meridional heat flux must be considered together with the small-scale processes caused by the air mass transformation.

Please note the location change.

Thursday, 1 June 2017, 3:30 PM
Refreshments:  3:15 PM
NCAR-Foothills Laboratory
3450 Mitchell Lane
Bldg. 2, Small Seminar Room 1001 

First Name: 
Bobbie
Last Name: 
Weaver
Phone Extension (4 digits): 
8946
Email: 
weaver@ucar.edu
Building:
Room Number: 
1001 (Please note location change)
Host lab/program/group:
Type of event:
Calendar Timing: 
Thursday, June 1, 2017 - 3:30pm to 4:30pm

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