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C. S. Jones and Ryan P. Abernathey

Abstract

Paleo-proxy observations suggest that deep ocean water-mass distributions were different at the Last Glacial Maximum than they are today. However, even modern deep ocean water-mass distributions are not completely explained by observations of the modern ocean circulation. This paper investigates two processes that influence deep ocean water-mass distributions: 1) interior downwelling caused by vertical mixing that increases in the downward direction and 2) isopycnal mixing. Passive tracers are used to assess how changes in the circulation and in the isopycnal-mixing coefficient impact deep ocean water-mass distributions in an idealized two-basin model. We compare two circulations, one in which the upper cell of the overturning reaches to 4000m depth and one in which it shoals to 2500m depth. Previous work suggests that in the latter case, the upper cell and the abyssal cell of the overturning are separate structures. Nonetheless, high concentrations of North Atlantic Water (NAW) are found in our model’s abyssal cell: these tracers are advected into the abyssal cell by interior downwelling caused by our vertical mixing profile, which increases in the downward direction. Further experiments suggest that the NAW concentration in the deep South Atlantic and in the deep Pacific is influenced by the isopycnal-mixing coefficient in the top 2000m of the Southern Ocean. Both the strength and the vertical profile of isopycnal mixing are important for setting deep-ocean tracer concentrations. A 1-D advection-diffusion model elucidates how NAW concentration depends on advective and diffusive processes.

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R. S. Ajayamohan, Boualem Khouider, V. Praveen, and Andrew J. Majda

Abstract

The barrier effect of the Maritime Continent (MC) in stalling or modifying the propagation characteristics of MJO is widely accepted. The strong diurnal cycle of convection over the MC is believed to play a dominant role in this regard. This hypothesis is studied here, with the help of a coarse-resolution Atmospheric General Circulation Model (AGCM). The dry dynamical core of the AGCM is coupled to the multicloud parameterization piggybacked with a dynamical bulk boundary layer model. A set of sensitivity experiments is carried out by systematically varying the strength of the MC diurnal flux to assess the impact of the diurnal convective variability on the MJO propagation. The effect of deterministic and stochastic diurnal forcings on MJO characteristics are compared. It is found that the precipitation and zonal wind variance, on the intraseasonal timescales, over the Western Pacific region decreases with the increase in diurnal forcing, indicating the blocking of MC precipitation. An increase in precipitation variance over the MC associated with the weakening of precipitation variance over the West Pacific is evident in all experiments. The striking difference between deterministic and stochastic diurnal forcing experiments is that the strength needed for the deterministic case to achieve the same degree of blocking is almost double that of stochastic case. The stochastic diurnal flux over the MC seems to be more detrimental in blocking the MJO propagation. This hints at the notion that the models with inadequate representation of organized convection tend to suffer from the MC-barrier effect.

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Sang-Ki Lee, Hosmay Lopez, Dongmin Kim, Andrew T. Wittenberg, and Arun Kumar

Abstract

This study presents an experimental model for Seasonal Probabilistic Outlook for Tornadoes (SPOTter) in the contiguous U.S. for March, April and May, and evaluates its forecast skill. This forecast model uses the leading empirical orthogonal function modes of regional variability in tornadic environmental parameters (i.e., low-level vertical wind shear and convective available potential energy), derived from the NCEP Coupled Forecast System version 2, as the primary predictors. A multiple linear regression is applied to the predicted modes of tornadic environmental parameters to estimate U.S. tornado activity, which is presented as the probability for above-, near-, and below-normal categories. The initial forecast is carried out in late February for March-April U.S. tornado activity, and then updated in late March for April-May activity. A series of re-forecast skill tests, including the jackknife cross-validation test, shows that the probabilistic re-forecast is overall skillful for predicting the above- and below-normal area-averaged activity in the contiguous U.S. for the target months of both March-April and April-May. The forecast model also successfully re-forecasts the 2011 super tornado outbreak season as well as the other three most active U.S. tornado seasons in 1982, 1991 and 2008, and thus may be suitable for an operational use for predicting future active and inactive U.S. tornado seasons. However, additional tests show that the regional re-forecast is skillful for March-April activity only in the Ohio Valley and South, and for April-May activity only in the Southeast and Upper Midwest. These and other limitations of the current model, and the future advances needed to improve the U.S. regional-scale tornado forecast are discussed.

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Tao Zhu and Jing Yang

Abstract

Two types of mid-high-latitude low-frequency intraseasonal oscillations (LF ISOs), featuring eastward and westward propagation, have been identified over the Eurasian continent in the past 37 summers (1982–2018). The eastward and westward propagating modes commonly have a dominant periodicity of 30–50 days near the Ural Mountains (UM) but have different origins and evolutions. The eastward propagating LF-ISO initiates over the eastern North America, migrates northeastward across the northeastern North America-Western North Atlantic, central North Atlantic, Western Europe and the UM, then propagates southeastward to northwestern and eastern China, which is the Atlantic-Eurasian continental mode. In contrast, the westward propagating mode is quasi-circumpolar, initiating over the East Siberian Sea and moving southwestward across the UM, northern Europe and eventually reaching Greenland and Canadian Arctic Archipelago. These two mid-high-latitude LF-ISOs are accompanied by significant tropical intraseasonal variations with evident tropical-extratropical interactions. Meanwhile, these two LF-ISOs have different decadal preferences before and after 2000, which are ascribed to the decadal change of both intraseasonal efficient kinetic energy obtained from the mean flow over their genesis region and their background flow associated with the North Atlantic Oscillation/Arctic Oscillation decadal change. This study deepens the understanding of subseasonal variations for mid-high-latitudes and subseasonal prediction sources for low-latitude regions.

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Anders A. Jensen, James O. Pinto, Sean C. C. Bailey, Ryan A. Sobash, Gijs de Boer, Adam L. Houston, Phillip B. Chilson, Tyler Bell, Glen Romine, Suzanne W. Smith, Dale A. Lawrence, Cory Dixon, Julie K. Lundquist, Jamey D. Jacob, Jack Elston, Sean Waugh, and Matthias Steiner

Abstract

Uncrewed aircraft system (UAS) observations collected during the 2018 LAPSE-RATE field campaign were assimilated into a high-resolution configuration of the Weather Research and Forecasting model using an ensemble Kalman filter. The benefit of UAS observations was assessed for a terrain-driven (drainage and upvalley) flow event that occurred within Colorado’s San Luis Valley (SLV) using independent observations. The analysis and prediction of the strength, depth and horizontal extent of drainage flow from the Saguache Canyon and the subsequent transition to upvalley and up-canyon flow was improved compared to that obtained both without DA (benchmark) and when only surface observations were assimilated. Assimilation of UAS observations greatly improved the analyses of vertical variations in temperature, relative humidity, and winds at multiple locations in the northern portion of the SLV with reductions in both bias and the root mean square error of roughly 40% for each variable compared to the benchmark run. Despite these noted improvements, some biases remain that were tied to measurement error and/or the impact of the boundary layer parameterization on vertically spreading the observations, both of which require further exploration. The results presented here highlight how observations obtained with a fleet of profiling UAS improve limited-area, high-resolution analyses and short-term forecasts in complex terrain.

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Johannes Mayer, Michael Mayer, and Leopold Haimberger

Abstract

This study uses advanced numerical and diagnostic methods are used to evaluate the atmospheric energy budget with the fifth generation European Re-Analysis (ERA5) in combination with observed and reconstructed top-of-the-atmosphere (TOA) energy fluxes for the period 1985–2018. We assess the meridional as well as ocean-to-land energy transport and perform internal consistency checks using mass-balanced data. Furthermore, the moisture and mass budgets in ERA5 are examined and compared with previous budget evaluations using ERA-Interim as well as observation-based estimates. Results show that peak annual mean meridional atmospheric energy transports in ERA5 (4.58±0.07 PW in the northern hemisphere) are weaker compared to ERA-Interim (4.74±0.09 PW), where the higher spatial and temporal resolution of ERA5 can be excluded as possible reason. The ocean-to-land energy transport in ERA5 is reliable at least from 2000 onwards (∼2.5 PW) such that the imbalance between net TOA fluxes and lateral energy fluxes over land are on the order of ∼1W m-2. Spin-up/-down effects as revealed from inconsistencies between analyses and forecasts are generally smaller and temporally less variable in ERA5 compared to ERA-Interim. Evaluation of the moisture budget shows that the ocean-to-land moisture transport and parameterized freshwater fluxes agree well in ERA5, while there are large inconsistencies in ERA-Interim. Overall, the quality of the budgets derived from ERA5 is demonstrably better than estimates from ERA-Interim. Still some particularly sensitive budget quantities (e.g., precipitation, evaporation, and ocean-land energy transport) show apparent inhomogeneities, especially in the late 1990s, which warrant further investigation and need to be considered in studies of interannual variability and trends.

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Sean R. Santellanes, George S. Young, David J. Stensrud, Matthew R. Kumjian, and Ying Pan

Abstract

Typical environmental conditions associated with horizontal convective rolls (HCRs) and cellular convection have been known for over 50 years. Yet our ability to predict whether HCRs, cellular convection, or no discernable organized (null) circulation will occur within a well-mixed convective boundary layer based upon easily observed environmental variables has been limited. Herein, a large data base of 50 cases each of HCR, cellular convection, and null events is created that includes observations of mean boundary layer wind and wind shear, boundary layer depth, and surface observations of wind, temperature, relative humidity, and estimates of surface sensible heat flux. Results from a multi-class linear discriminant analysis applied to these data indicate that environmental conditions can be useful in predicting whether HCRs, cellular convection, or no circulation occurs, with the analysis identifying the correct circulation type on 72% of the case days. This result is slightly better than using a mean CBL wind speed of 6 m s-1 to discriminate between HCRs and cells. However, the mean CBL wind speed has no ability to further separate out cases with no CBL circulation. The key environmental variables suggested by the discriminant analysis are mean sensible heat flux, friction velocity, and the Obukhov length.

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Stephanie Contardo, Ryan J. Lowe, Jeff E. Hansen, Dirk P. Rijnsdorp, François Dufois, and Graham Symonds

Abstract

Long waves are generated and transform when short-wave groups propagate into shallow water, but the generation and transformation processes are not fully understood. In this study we develop an analytical solution to the linearized shallow-water equations at the wave-group scale, which decomposes the long waves into a forced solution (a bound long wave) and free solutions (free long waves). The solution relies on the hypothesis that free long waves are continuously generated as short-wave groups propagate over a varying depth. We show that the superposition of free long waves and a bound long wave results in a shift of the phase between the short-wave group and the total long wave, as the depth decreases prior to short-wave breaking. While it is known that short-wave breaking leads to free long generation, through breakpoint forcing and bound wave release mechanisms, we highlight the importance of an additional free long wave generation mechanism due to depth variations, in the absence of breaking. This mechanism is important because as free long waves of different origins combine, the total free long wave amplitude is dependent on their phase relationship. Our free and forced solutions are verified against a linear numerical model, and we show how our solution is consistent with prior theory that does not explicitly decouple free and forced motions. We also validate the results with data from a nonlinear phase-resolving numerical wave model and experimental measurements, demonstrating that our analytical model can explain trends observed in more complete representations of the hydrodynamics.

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Paulo Rodrigo Zanin and Prakki Satyamurty

Abstract

The inter-seasonal and inter-basins hydrological couplings between the Amazon and the La Plata basins are obtained with the help of ERA-5 atmospheric reanalysis, MERGE/CPTEC precipitation, GLEAM evapotranspiration and the GLDAS/Noah soil moisture datasets. The hypotheses formulated by Zanin and Satyamurty (2020a) about the hydrological processes interconnecting the Amazon Basin and the La Plata Basin are tested. A new method for finding the source-sink relationships among the boxes (regions) is presented. The precipitation recycling, frequency of source-sink behaviors, the soil moisture memory and the continental moisture transport between remote regions are evaluated. The main result of this study is that the amount of water precipitated over the Southeastern region of the Amazon Basin at the end of the South American Monsoon during autumn season, influences the amount of precipitation during winter season over the Central-western region of the La Plata Basin.

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Christopher J. Cardinale, Brian E. J. Rose, Andrea L. Lang, and Aaron Donohoe

Abstract

The flux of moist static energy into the polar regions plays a key role in the energy budget and climate of the polar regions. While usually studied from a vertically integrated perspective (Fwall), this analysis examines its vertical structure, using the NASA-MERRA-2 reanalysis to compute climatological and anomalous fluxes of sensible, latent, and potential energy across 70°N and 65°S for the period 1980–2016. The vertical structure of the climatological flux is bimodal, with peaks in the mid- to lower-troposphere and mid- to upper-stratosphere. The near zero flux at the tropopause defines the boundary between stratospheric (Fstrat) and tropospheric (Ftrop) contributions to Fwall. Especially at 70°N, Fstrat is found to be important to the climatology and variability of Fwall, contributing 20.9 Wm−2 to Fwall (19% of Fwall) during the winter and explaining 23% of the variance of Fwall. During winter, an anomalous poleward increase in Fstrat preceding a sudden stratospheric warming is followed by an increase in outgoing longwave radiation anomalies, with little influence on the surface energy budget of the Arctic. Conversely, a majority of the energy input by an anomalous poleward increase in Ftrop goes toward warming the Arctic surface. Ftrop is found to be a better metric than Fwall for evaluating the influence of atmospheric circulations on the Arctic surface climate.

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