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409 results on '"Kiladis, George N."'

1. Revisiting the Quasi Biennial Oscillation as Seen in ERA5.Part II: Evaluation of Waves and Wave Forcing

Author

Pahlavan, Hamid A., Wallace, John M., Fu, Qiang, and Kiladis, George N.

Subjects
Physics - Atmospheric and Oceanic Physics
Abstract

This paper describes stratospheric waves in ERA5 reanalysis and evaluates the contributions of different types of waves to the driving of the quasi-biennial oscillation (QBO). Because of its higher spatial resolution compared to its predecessors, ERA5 is capable of resolving a broader spectrum of waves. It is shown that the resolved waves contribute to both eastward and westward accelerations near the equator, mainly by the way of the vertical flux of zonal momentum. The eastward accelerations by the resolved waves are mainly due to Kelvin waves and small-scale gravity (SSG) waves with zonal wavelengths smaller than 2000 km, whereas the westward accelerations are forced mainly by SSG waves, with smaller contributions from inertio-gravity and mixed-Rossby-gravity waves. Extratropical Rossby waves propagate into the tropical region and impart a westward acceleration to the zonal flow. They appear to be responsible for at least some of the irregularities in the QBO cycle.

Published
2020
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2. Revisiting the Quasi Biennial Oscillation as Seen in ERA5. Part I: Description and Momentum Budget

Author

Pahlavan, Hamid A., Fu, Qiang, Wallace, John M., and Kiladis, George N.

Subjects
Physics - Atmospheric and Oceanic Physics
Abstract

The dynamics and momentum budget of the quasi-biennial oscillation (QBO) are examined in the ERA5 reanalysis and compared with those in ERA-I. Because of ERA5's higher spatial resolution it is capable of resolving a broader spectrum of atmospheric waves and allows for a better representation of the wave-mean flow interactions, both of which are of crucial importance for QBO studies. It is shown that the QBO-induced mean meridional circulation, which is mainly confined to the winter hemisphere, is strong enough to interrupt the tropical upwelling during the descent of the westerly shear zones. Since the momentum advection tends to damp the QBO, the wave forcing is responsible for both the downward propagation and for the maintenance of the QBO. It is shown that half the required wave forcing is provided by resolved waves during the descent of both westerly and easterly regimes. Planetary-scale waves account for most of the resolved wave forcing of the descent of westerly shear zones and small-scale gravity (SSG) waves with wavelengths shorter than 2000 km account for the remainder. SSG waves account for most of the resolved forcing of the descent of the easterly shear zones. The representation of the mean fields in the QBO is very similar in ERA5 and ERA-I but the resolved wave forcing is shown to be substantially stronger in ERA5. The contributions of the various equatorially-trapped wave modes to the QBO forcing are documented in Part II.

Published
2020
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4. The influence of tropical forcing on extreme winter precipitation in the western Himalaya

Author

Cannon, Forest, Carvalho, Leila MV, Jones, Charles, Hoell, Andrew, Norris, Jesse, Kiladis, George N, and Tahir, Adnan A

Subjects
Climate Action, Extreme precipitation events, Tropical forcing, Karakoram, Madden-Julian oscillation, El Nino southern oscillation, Atmospheric Sciences, Oceanography, Physical Geography and Environmental Geoscience, Meteorology & Atmospheric Sciences
Published
2017

6. ADVANCING SCIENCE AND SERVICES DURING THE 2015/16 EL NIÑO : The NOAA El Niño Rapid Response Field Campaign

Author

Dole, Randall M., Spackman, J. Ryan, Newman, Matthew, Compo, Gilbert P., Smith, Catherine A., Hartten, Leslie M., Barsugli, Joseph J., Webb, Robert S., Hoerling, Martin P., Cifelli, Robert, Wolter, Klaus, Barnet, Christopher D., Gehne, Maria, Gelaro, Ronald, Kiladis, George N., Abbott, Scott, Akish, Elena, Albers, John, Brown, John M., Cox, Christopher J., Darby, Lisa, de Boer, Gijs, DeLuisi, Barbara, Dias, Juliana, Dunion, Jason, Eischeid, Jon, Fairall, Christopher, Gambacorta, Antonia, Gorton, Brian K., Hoell, Andrew, Intrieri, Janet, Jackson, Darren, Johnston, Paul E., Lataitis, Richard, Mahoney, Kelly M., McCaffrey, Katherine, McColl, H. Alex, Mueller, Michael J., Murray, Donald, Neiman, Paul J., Otto, William, Persson, Ola, Quan, Xiao-Wei, Rangwala, Imtiaz, Ray, Andrea J., Reynolds, David, Dellaripa, Emily Riley, Rosenlof, Karen, Sakaeda, Naoko, Sardeshmukh, Prashant D., Slivinski, Laura C., Smith, Lesley, Solomon, Amy, Swales, Dustin, Tulich, Stefan, White, Allen, Wick, Gary, Winterkorn, Matthew G., Wolfe, Daniel E., and Zamora, Robert

Published
2018

11. The Role of Convectively Coupled Equatorial Waves in Sub‐Seasonal Predictions.

Author

Dias, Juliana, Gehne, Maria, Kiladis, George N., and Magnusson, Linus

Subjects
LONG-range weather forecasting, PRECIPITATION forecasting, NUMERICAL weather forecasting, WEATHER forecasting, RAINFALL, THEORY of wave motion, FORECASTING
Abstract

Convectively coupled equatorial waves (CCEWs) are modes of tropical variability that are often seen as potentially beneficial for sub‐seasonal predictions. Quantifying this potential has been proven difficult in operational forecast systems because tropical variability tends to be poorly represented in such models. The European Center for Medium‐Range Weather Forecasts (ECMWF) is a potential exception because it has been shown to have much improved representation of the tropics. Here, ECMWF reforecasts are used to investigate the predicted evolution of CCEWs and how they might impact sub‐seasonal predictions. It is shown that the ECMWF system is able to generate variability consistent with various modes of CCEWs, but CCEWs tend to be weaker in amplitude and their propagation characteristics often deviate from observations. This study suggests that model advancements aimed at improving the deterministic representation of CCEWs are still needed in order to better understand and utilize tropical sources of sub‐seasonal predictive skill. Plain Language Summary: Accurate weather forecasts leverage on proper characterization of the current state of the atmosphere and on predictable atmospheric phenomena that constrain the evolution of that state. Tropical weather forecasting is challenging in both aspects: observations are more sparse than in the extratropics, and the evolution of the atmosphere is constrained by large‐scale forcings such as tropical rainfall. This study focuses on the contribution of equatorial waves to low latitude weather forecast skill because these types of disturbances are known to modulate tropical rainfall, which feed back strongly onto the disturbances themselves. The main finding is that the initial state tends to be fairly well represented in forecast systems; however, within a day or two the amplitude of these waves becomes weaker than in observations, which is a decay time much faster than their typical one‐to‐three week lifetimes. Along with errors in the waves propagation characteristics, this loss of amplitude suggests that equatorial waves do not impact precipitation forecast within the models as much as theory and observations imply they should. Key Points: Convectively coupled equatorial waves (CCEWs) are a potential source of global sub‐seasonal precipitation skillOperational forecast model's representation of CCEWs has greatly advanced over the last two decadesDeviations from observations often attenuate the role of CCEWs as sources of sub‐seasonal predictive skill [ABSTRACT FROM AUTHOR]

Published
2023
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16. The intricacies of identifying equatorial waves

Author

Knippertz, Peter, primary, Gehne, Maria, additional, Kiladis, George N., additional, Kikuchi, Kazuyoshi, additional, Rasheeda Satheesh, Athul, additional, Roundy, Paul E., additional, Yang, Gui‐Ying, additional, Žagar, Nedjeljka, additional, Dias, Juliana, additional, Fink, Andreas H., additional, Methven, John, additional, Schlueter, Andreas, additional, Sielmann, Frank, additional, and Wheeler, Matthew C., additional

Published
2022
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45. A Python Package to Calculate the OLR-Based Index of the Madden- Julian-Oscillation (OMI) in Climate Science and Weather Forecasting

Author

Hoffmann, Christoph G., Kiladis, George N., Gehne, Maria, von Savigny, Christian, German Research Foundation, 393148499, and Open Access Publication Fund of the University of Greifswald

Subjects
python, QA76.75-76.765, intraseasonal variation, Madden-Julian Oscillation, OLR-based MJO Index, OMI, atmosphere, weather forecasting, climate, Python, Climate, Weather, Atmosphere, omi, madden-julian oscillation, olr-based mjo index, Computer software
Abstract

The Madden-Julian Oscillation (MJO) is a prominent feature of the intraseasonal variability of the atmosphere. The MJO strongly modulates tropical precipitation and has implications around the globe for weather, climate and basic atmospheric research. The time-dependent state of the MJO is described by MJO indices, which are calculated through sometimes complicated statistical approaches from meteorological variables. One of these indices is the OLR-based MJO Index (OMI; OLR stands for outgoing longwave radiation). The Python package mjoindices, which is described in this paper, provides the first open source implementation of the OMI algorithm, to our knowledge. The package meets state-of-the-art criteria for sustainable research software, like automated tests and a persistent archiving to aid the reproducibility of scientific results. The agreement of the OMI values calculated with this package and the original OMI values is also summarized here. There are several reuse scenarios; the most probable one is MJO-related research based on atmospheric models, since the index values have to be recalculated for each model run.

Published
2021

50. Three-dimensional structure and dynamics of African easterly waves. Part III: genesis

Author

Thorncroft, Chris D., Hall, Nicholas M.J., and Kiladis, George N.

Subjects
Jets -- Research, Earth sciences, Science and technology
Abstract

This paper promotes the view that African easterly waves (AEWs) are triggered by localized forcing, most likely associated with latent heating upstream of the region of observed AEW growth. A primitive equation model is used to show that AEWs can be triggered by finite-amplitude transient and localized latent heating on a zonally varying basic state that is linearly stable. Heating close to the entrance region of the African easterly jet (AEJ) is shown to initiate AEWs downstream. The heating leads to an initial trough that reaches the West African coast about 5-7 days later, depending on the nature of the heating profile. After this, a structure that projects strongly onto the leading linear normal mode of the basic state becomes established, characterized by a number of westward-propagating disturbances that strongly resemble AEWs. The sensitivity of the forced AEWs to the nature and location of the heating profile is examined. AEWs are most efficiently triggered by heating profiles that establish lower tropospheric circulations close to the entrance region of the AEJ. In the present study, this was best achieved by lower tropospheric heating from shallow convection or upper-level heating and lower-level cooling from a strati-form precipitation profile. Both profiles have significant heating gradients in the vertical in the mid-to-lower troposphere. This triggering paradigm for the genesis of AEWs has consequences for the variability and predictability of AEWs at weather and climate time scales. In addition to the nature of the AEJ, often emphasized, it is crucial to consider the nature and variability of upstream heating triggers.

Published
2008

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