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D. E. Harrison and A. P. Craig

Abstract

A hindcast of the 1982–1983 ENSO event using a primitive equation ocean circulation model forced by monthly mean wind stresses based on the SADLER pseudostress fields shows very good agreement with observations at 0°, 159°W between June 1982 and March 1983. The hindcast experiment is analyzed to explore the processes that caused the large accelerations, decelerations, and thermal changes observed during this time. Several hindcast experiments incorporating variations of the SADLER wind field and several idealized experiments incorporating a western Pacific westerly wind event are analyzed and compared with the 1982–1983 SADLER hindcast to explore the importance of local and remote forcing, the relative importance of zonal and meridional wind stress changes, and the dynamical signatures of the processes at work. Meridional wind stress changes have little effect on either the zonal velocity or temperature fields. Local zonal wind stress variations can account for the qualitative changes in the upper-ocean zonal flow, but cannot reproduce the observed thermal changes or the timing and quantitative evolution of the zonal flow. Remote forcing is needed to account for these latter aspects of the observations. Eastward-propagating Kelvin response appears to be quite important, but westward-propagating Rossby variance forced during 1982 from east of 160°W does not appear to play any significant role. The idealized remote-forcing experiments indicate that westerly events can account for the variability not explained by local forcing; the essential aspect is how the forcing projects onto the vertical modes defined by the stratification under the forcing at the time of the wind event. Modes higher than the first and second can be strongly forced and the sum over modes produces vertical structures in the near field of the forcing similar to those observed. Simple linear Kelvin mode ideas thus are useful for understanding the response to remotely forced variability. However, nonlinear processes affect the quantitative response, both by changing the stratification under the forcing region as the forcing event proceeds (and thereby altering the modal projection of the forcing) and through zonal advection and interaction between the response and the background mean flow. The dynamical balance of terms for zonal momentum in the SADLER hindcast is quite complex and the difficulty of identifying remote forcing from the balance of terms, even during periods when remote forcing is the primary agent of change, is discussed. This detailed study of a particularly interesting period of equatorial flow and thermal variability illustrates the many processes at work on the equator in the central Pacific during periods of substantial local and remote wind stress variability. It also illustrates some of the challenges that might be encountered in interpreting the results of an oceanic local dynamics experiment under conditions like these.

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P. A. Sandery, G. B. Brassington, A. Craig, and T. Pugh

Abstract

This study investigates the impact of atmosphere–ocean coupling on predicted tropical cyclone (TC) intensity change and the ocean response in the Australian region. The coupled model comprises the Australian Bureau of Meteorology’s Tropical Cyclone Limited-Area Prediction System (TC-LAPS) and a regional version of the BLUElink ocean forecasting system. A series of case study forecasts are presented and the differences between coupled and uncoupled forecasts, operational forecasts, and posterior objective analyses are compared. A coupled model ensemble is also developed that uses different first-order approximations of the effects of surface waves on surface stress in an inertial coupling method. In each of the cases, the use of reanalyzed sea surface temperatures significantly improves the prediction of TC intensity change in the intensification phase. The results show that dynamic air–sea coupling has a modest impact on intensity in cases where SST cooling is significant and is likely to be important for predicting the rate of TC intensification, peak intensity, and deintensification. Results also show that there is a definite coupled signal and suggest inherent biases in the atmospheric model that could potentially be removed. With different parameterizations of surface wave effects, results show modest sensitivity in TC intensity of up to 10 hPa in minimum surface pressure; however, in some cases there was significant sensitivity in the predicted ocean response. The results also highlight the relative increased complexity of tropical cyclone prediction in the Australian region compared to other regions. In cases where the forecast TC track was reasonably skillful, there were improvements in the predicted ocean response with respect to observations compared to an ocean reanalysis.

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Peter A. Bogenschutz, Andrew Gettelman, Hugh Morrison, Vincent E. Larson, Cheryl Craig, and David P. Schanen

Abstract

This paper describes climate simulations of the Community Atmosphere Model, version 5 (CAM5), coupled with a higher-order turbulence closure known as Cloud Layers Unified by Binormals (CLUBB). CLUBB is a unified parameterization of the planetary boundary layer (PBL) and shallow convection that is centered around a trivariate probability density function (PDF) and replaces the conventional PBL, shallow convection, and cloud macrophysics schemes in CAM5. CAM–CLUBB improves many aspects of the base state climate compared to CAM5. Chief among them is the transition of stratocumulus to trade wind cumulus regions in the subtropical oceans. In these regions, CAM–CLUBB provides a much more gradual transition that is in better agreement with observational analysis compared to CAM5, which is too abrupt. The improvement seen in CAM–CLUBB can be largely attributed to the gradual evolution of the simulated turbulence, which is in part a result of the unified nature of the parameterization, and to the general improved representation of shallow cumulus clouds compared to CAM5. In addition, there are large differences in the representation and structure of marine boundary layer clouds between CAM–CLUBB and CAM5. CAM–CLUBB is also shown to be more robust, in terms of boundary layer clouds, to changes in vertical resolution for global simulations in a preliminary test.

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Raymond W. Arritt, Thomas D. Rink, Moti Segal, Dennis P. Todey, Craig A. Clark, Mark J. Mitchell, and Kenneth M. Labas

Abstract

Hourly wind profiler observations from the NOAA Profiler Network were used to develop a climatology of the low-level jet (LLJ) over the Great Plains of the central United States from April to September of 1993. The peak precipitation episode of the 1993 flood was associated with a sustained period of high incidence of strong low-level jets (over 20 m s−1). Consistent with previous studies, strong low-level jets were found to be promoted in the warm sector of an extratropical cyclone. Comparison of datasets formulated using velocity variance thresholds with unthresholded data similar to the operational hourly data suggests that the profiler observations often were contaminated by radar returns from migrating birds, especially during the months of April and May.

The strong low-level jets during the peak precipitation episode of the 1993 flood over the upper Mississippi River basin were associated with a high-amplitude upper-level wave pattern over and upstream of the continental United States. Separating the composite 850-mb wind for strong low-level jets into geostrophic and ageostrophic components showed that the magnitudes of the ageostrophic component and the anomalous geostrophic component were comparable.

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Jessie C. Carman, Daniel P. Eleuterio, Timothy C. Gallaudet, Gerald L. Geernaert, Patrick A. Harr, Jack A. Kaye, David H. McCarren, Craig N. McLean, Scott A. Sandgathe, Frederick Toepfer, and Louis W. Uccellini

Abstract

The United States has had three operational numerical weather prediction centers since the Joint Numerical Weather Prediction Unit was closed in 1958. This led to separate paths for U.S. numerical weather prediction, research, technology, and operations, resulting in multiple community calls for better coordination. Since 2006, the three operational organizations—the U.S. Air Force, the U.S. Navy, and the National Weather Service—and, more recently, the Department of Energy, the National Aeronautics and Space Administration, the National Science Foundation, and the National Oceanic and Atmospheric Administration/Office of Oceanic and Atmospheric Research, have been working to increase coordination. This increasingly successful effort has resulted in the establishment of a National Earth System Prediction Capability (National ESPC) office with responsibility to further interagency coordination and collaboration. It has also resulted in sharing of data through an operational global ensemble, common software standards, and model components among the agencies. This article discusses the drivers, the progress, and the future of interagency collaboration.

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Joseph A. Santanello Jr., Paul A. Dirmeyer, Craig R. Ferguson, Kirsten L. Findell, Ahmed B. Tawfik, Alexis Berg, Michael Ek, Pierre Gentine, Benoit P. Guillod, Chiel van Heerwaarden, Joshua Roundy, and Volker Wulfmeyer

Abstract

Land–atmosphere (L-A) interactions are a main driver of Earth’s surface water and energy budgets; as such, they modulate near-surface climate, including clouds and precipitation, and can influence the persistence of extremes such as drought. Despite their importance, the representation of L-A interactions in weather and climate models remains poorly constrained, as they involve a complex set of processes that are difficult to observe in nature. In addition, a complete understanding of L-A processes requires interdisciplinary expertise and approaches that transcend traditional research paradigms and communities. To address these issues, the international Global Energy and Water Exchanges project (GEWEX) Global Land–Atmosphere System Study (GLASS) panel has supported “L-A coupling” as one of its core themes for well over a decade. Under this initiative, several successful land surface and global climate modeling projects have identified hot spots of L-A coupling and helped quantify the role of land surface states in weather and climate predictability. GLASS formed the Local Land–Atmosphere Coupling (LoCo) project and working group to examine L-A interactions at the process level, focusing on understanding and quantifying these processes in nature and evaluating them in models. LoCo has produced an array of L-A coupling metrics for different applications and scales and has motivated a growing number of young scientists from around the world. This article provides an overview of the LoCo effort, including metric and model applications, along with scientific and programmatic developments and challenges.

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Yubao Liu, Thomas T. Warner, James F. Bowers, Laurie P. Carson, Fei Chen, Charles A. Clough, Christopher A. Davis, Craig H. Egeland, Scott F. Halvorson, Terrence W. Huck Jr., Leo Lachapelle, Robert E. Malone, Daran L. Rife, Rong-Shyang Sheu, Scott P. Swerdlin, and Dean S. Weingarten

Abstract

Given the rapid increase in the use of operational mesoscale models to satisfy different specialized needs, it is important for the community to share ideas and solutions for meeting the many associated challenges that encompass science, technology, education, and training. As a contribution toward this objective, this paper begins a series that reports on the characteristics and performance of an operational mesogamma-scale weather analysis and forecasting system that has been developed for use by the U.S. Army Test and Evaluation Command. During the more than five years that this four-dimensional weather system has been in use at seven U.S. Army test ranges, valuable experience has been gained about the production and effective use of high-resolution model products for satisfying a variety of needs. This paper serves as a foundation for the rest of the papers in the series by describing the operational requirements for the system, the data assimilation and forecasting system characteristics, and the forecaster training that is required for the finescale products to be used effectively.

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Michael A. Rawlins, Michael Steele, Marika M. Holland, Jennifer C. Adam, Jessica E. Cherry, Jennifer A. Francis, Pavel Ya Groisman, Larry D. Hinzman, Thomas G. Huntington, Douglas L. Kane, John S. Kimball, Ron Kwok, Richard B. Lammers, Craig M. Lee, Dennis P. Lettenmaier, Kyle C. McDonald, Erika Podest, Jonathan W. Pundsack, Bert Rudels, Mark C. Serreze, Alexander Shiklomanov, Øystein Skagseth, Tara J. Troy, Charles J. Vörösmarty, Mark Wensnahan, Eric F. Wood, Rebecca Woodgate, Daqing Yang, Ke Zhang, and Tingjun Zhang

Abstract

Hydrologic cycle intensification is an expected manifestation of a warming climate. Although positive trends in several global average quantities have been reported, no previous studies have documented broad intensification across elements of the Arctic freshwater cycle (FWC). In this study, the authors examine the character and quantitative significance of changes in annual precipitation, evapotranspiration, and river discharge across the terrestrial pan-Arctic over the past several decades from observations and a suite of coupled general circulation models (GCMs). Trends in freshwater flux and storage derived from observations across the Arctic Ocean and surrounding seas are also described.

With few exceptions, precipitation, evapotranspiration, and river discharge fluxes from observations and the GCMs exhibit positive trends. Significant positive trends above the 90% confidence level, however, are not present for all of the observations. Greater confidence in the GCM trends arises through lower interannual variability relative to trend magnitude. Put another way, intrinsic variability in the observations tends to limit confidence in trend robustness. Ocean fluxes are less certain, primarily because of the lack of long-term observations. Where available, salinity and volume flux data suggest some decrease in saltwater inflow to the Barents Sea (i.e., a decrease in freshwater outflow) in recent decades. A decline in freshwater storage across the central Arctic Ocean and suggestions that large-scale circulation plays a dominant role in freshwater trends raise questions as to whether Arctic Ocean freshwater flows are intensifying. Although oceanic fluxes of freshwater are highly variable and consistent trends are difficult to verify, the other components of the Arctic FWC do show consistent positive trends over recent decades. The broad-scale increases provide evidence that the Arctic FWC is experiencing intensification. Efforts that aim to develop an adequate observation system are needed to reduce uncertainties and to detect and document ongoing changes in all system components for further evidence of Arctic FWC intensification.

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Emily Shroyer, Amit Tandon, Debasis Sengupta, Harindra J.S. Fernando, Andrew J. Lucas, J. Thomas Farrar, Rajib Chattopadhyay, Simon de Szoeke, Maria Flatau, Adam Rydbeck, Hemantha Wijesekera, Michael McPhaden, Hyodae Seo, Aneesh Subramanian, R Venkatesan, Jossia Joseph, S. Ramsundaram, Arnold L. Gordon, Shannon M. Bohman, Jaynise Pérez, Iury T. Simoes-Sousa, Steven R. Jayne, Robert E. Todd, G.S. Bhat, Matthias Lankhorst, Tamara Schlosser, Katherine Adams, S.U.P Jinadasa, Manikandan Mathur, M. Mohapatra, E. Pattabhi Rama Rao, A. K. Sahai, Rashmi Sharma, Craig Lee, Luc Rainville, Deepak Cherian, Kerstin Cullen, Luca R. Centurioni, Verena Hormann, Jennifer MacKinnon, Uwe Send, Arachaporn Anutaliya, Amy Waterhouse, Garrett S. Black, Jeremy A. Dehart, Kaitlyn M. Woods, Edward Creegan, Gad Levy, Lakshmi H Kantha, and Bulusu Subrahmanyam

Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the US, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ~20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10−12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ~20 to 50 m), cooling SST (by ~ 1°C), and warming/drying of the lower to mid-troposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

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M. Ades, R. Adler, Rob Allan, R. P. Allan, J. Anderson, Anthony Argüez, C. Arosio, J. A. Augustine, C. Azorin-Molina, J. Barichivich, J. Barnes, H. E. Beck, Andreas Becker, Nicolas Bellouin, Angela Benedetti, David I. Berry, Stephen Blenkinsop, Olivier. Bock, Michael G. Bosilovich, Olivier. Boucher, S. A. Buehler, Laura. Carrea, Hanne H. Christiansen, F. Chouza, John R. Christy, E.-S. Chung, Melanie Coldewey-Egbers, Gil P. Compo, Owen R. Cooper, Curt Covey, A. Crotwell, Sean M. Davis, Elvira de Eyto, Richard A. M de Jeu, B.V. VanderSat, Curtis L. DeGasperi, Doug Degenstein, Larry Di Girolamo, Martin T. Dokulil, Markus G. Donat, Wouter A. Dorigo, Imke Durre, Geoff S. Dutton, G. Duveiller, James W. Elkins, Vitali E. Fioletov, Johannes Flemming, Michael J. Foster, Richard A. Frey, Stacey M. Frith, Lucien Froidevaux, J. Garforth, S. K. Gupta, Leopold Haimberger, Brad D. Hall, Ian Harris, Andrew K Heidinger, D. L. Hemming, Shu-peng (Ben) Ho, Daan Hubert, Dale F. Hurst, I. Hüser, Antje Inness, K. Isaksen, Viju John, Philip D. Jones, J. W. Kaiser, S. Kelly, S. Khaykin, R. Kidd, Hyungiun Kim, Z. Kipling, B. M. Kraemer, D. P. Kratz, R. S. La Fuente, Xin Lan, Kathleen O. Lantz, T. Leblanc, Bailing Li, Norman G Loeb, Craig S. Long, Diego Loyola, Wlodzimierz Marszelewski, B. Martens, Linda May, Michael Mayer, M. F. McCabe, Tim R. McVicar, Carl A. Mears, W. Paul Menzel, Christopher J. Merchant, Ben R. Miller, Diego G. Miralles, Stephen A. Montzka, Colin Morice, Jens Mühle, R. Myneni, Julien P. Nicolas, Jeannette Noetzli, Tim J. Osborn, T. Park, A. Pasik, Andrew M. Paterson, Mauri S. Pelto, S. Perkins-Kirkpatrick, G. Pétron, C. Phillips, Bernard Pinty, S. Po-Chedley, L. Polvani, W. Preimesberger, M. Pulkkanen, W. J. Randel, Samuel Rémy, L. Ricciardulli, A. D. Richardson, L. Rieger, David A. Robinson, Matthew Rodell, Karen H. Rosenlof, Chris Roth, A. Rozanov, James A. Rusak, O. Rusanovskaya, T. Rutishäuser, Ahira Sánchez-Lugo, P. Sawaengphokhai, T. Scanlon, Verena Schenzinger, S. Geoffey Schladow, R. W Schlegel, Eawag Schmid, Martin, H. B. Selkirk, S. Sharma, Lei Shi, S. V. Shimaraeva, E. A. Silow, Adrian J. Simmons, C. A. Smith, Sharon L Smith, B. J. Soden, Viktoria Sofieva, T. H. Sparks, Paul W. Stackhouse Jr., Wolfgang Steinbrecht, Dimitri A. Streletskiy, G. Taha, Hagen Telg, S. J. Thackeray, M. A. Timofeyev, Kleareti Tourpali, Mari R. Tye, Ronald J. van der A, Robin, VanderSat B.V. van der Schalie, Gerard van der SchrierW. Paul, Guido R. van der Werf, Piet Verburg, Jean-Paul Vernier, Holger Vömel, Russell S. Vose, Ray Wang, Shohei G. Watanabe, Mark Weber, Gesa A. Weyhenmeyer, David Wiese, Anne C. Wilber, Jeanette D. Wild, Takmeng Wong, R. Iestyn Woolway, Xungang Yin, Lin Zhao, Guanguo Zhao, Xinjia Zhou, Jerry R. Ziemke, and Markus Ziese
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