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D. G. Andrews and M. E. McIntyre

Abstract

Using a new generalization of the Eliassen-Palm relations, we discuss the zonal-mean-flow tendency ∂ū/∂t due to waves in a stratified, rotating atmosphere, with particular attention to equatorially trapped modes. Wave transience, forcing and dissipation are taken into account in a very general way. The theory makes it possible to discuss the latitudinal (y) and vertical (z) dependence of ∂ū/∂t qualitatively and calculate it directly from an approximate knowledge of the wave structure. For equatorial modes it reveals that the y profile of ∂ū/∂t is strongly dependent on the nature of the forcing or dissipation mechanism. A by-product of the theory is a far-reaching generalization of the theorems of Charney-Drazin, Dickinson and Holton on the forcing of ∂ū/∂t by conservative linear waves.

Implications for the quasi-biennial oscillation in the equatorial stratosphere are discussed. Graphs of y profiles of ∂ū/∂t are given for the equatorial waves considered in the recent analysis of observational data by Lindzen and Tsay (1975). The y profile of ∂ū/∂t for Rossby-gravity and inertio-gravity modes, in Lindzen and Tsay's parameter ranges, prove extremely sensitive to whether or not small amounts of mechanical dissipation are present alongside the radiative-photochemical dissipation of the waves.

The probable importance of low-frequency Rossby waves for the momentum budget of the descending easterlies is suggested.

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D. G. Andrews and M. E. McIntyre

Abstract

The theorems, which exhibit the role of wave dissipation, excitation and transience in the forcing of mean flow changes of second order in wave amplitude by arbitrary, small-amplitude disturbances, are obtained 1) for the primitive equations in pressure coordinates on a sphere, and 2) in a more general form (applicable for instance to nonhydrostatic disturbances in tornadoes or hurricanes) establishing that no approximations beyond axisymmetry of the mean flow are necessary. It is shown how the results reduce to those found by Boyd (1976) for the case of sinusoidal, hydrostatic waves with exponentially growing or decaying amplitude, and it is explained why the approximation used by Boyd in the thermodynamic equation is not needed. The reduction to Boyd's results entails the use of a virial theorem. This theorem amounts to a generalization of the “equipartition” law derived in an earlier paper (Andrews and Mclntyre, 1976). That derivation appeared to rely on an assumption about relative phases of disturbance Fourier components; the present derivation shows that no such assumption is in fact necessary.

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E. D. Andrews, Ronald C. Antweiler, Paul J. Neiman, and F. Martin Ralph

Abstract

The influence of the El Niño–Southern Oscillation (ENSO) phenomenon on flooding in California coastal streams is investigated by analyzing the annual peak floods recorded at 38 gauging stations. The state of ENSO prior to and during flooding is characterized by the multivariate ENSO index (MEI), where MEI < −0.5 is defined as the La Niña phase and MEI > 0.5 as the El Niño phase. Flood magnitude in all 20 streams located south of 35°N has a significant positive correlation (r = 0.3 to 0.6), whereas in 3 of the 4 streams located north of 41°N flood magnitude has a significant negative correlation (r = −0.3 to −0.4), with MEI from −2.2 to +3.2. Correlations with MEI are uniformly weak and insignificant, however, when the floods are subdivided into El Niño and non–El Niño phases. A comparison of the geometric mean El Niño flood to the geometric mean non–El Niño flood determined that the means were statistically different at gauging stations south of 35°N and north of 41°N. For 20 streams located south of 35°N, the geometric mean of annual peak floods recorded at a stream gauge during El Niño phases is 2–14 times the geometric mean of annual peak floods recorded during non–El Niño phases. Thus, south of 35°N along the California coast, floods are significantly larger during an El Niño phase than a non–El Niño phase. For the three streams located north of 41°N, the geometric mean of annual peak floods during an El Niño phase was less than 70% of the geometric mean of annual peak floods during a non–El Niño phase. The relative strength of the El Niño phase, however, has, at most, a weak influence on flood magnitude. Flood exceedance probabilities for the El Niño and non–El Niño periods were calculated for all gauging stations using a three-parameter log gamma distribution. For exceedance probabilities from 0.50 to 0.02, the ratio of the El Niño to non–El Niño floods varies from greater than 10 near 32°N to less than 0.7 near 42°N. Latitude explains 76%–90% of the observed variation in the relative magnitude of El Niño versus non–El Niño floods over the range of exceedance probabilities.

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I. Veselovskii, D. N. Whiteman, A. Kolgotin, E. Andrews, and M. Korenskii

Abstract

The feasibility of using a multiwavelength Mie–Raman lidar based on a tripled Nd:YAG laser for profiling aerosol physical parameters in the planetary boundary layer (PBL) under varying conditions of relative humidity (RH) is studied. The lidar quantifies three aerosol backscattering and two extinction coefficients and from these optical data the particle parameters such as concentration, size, and complex refractive index are retrieved through inversion with regularization. The column-integrated, lidar-derived parameters are compared with results from the AERONET sun photometer. The lidar and sun photometer agree well in the characterization of the fine-mode parameters, however the lidar shows less sensitivity to coarse mode. The lidar results reveal a strong dependence of particle properties on RH. The height regions with enhanced RH are characterized by an increase of backscattering and extinction coefficient and a decrease in the Ångström exponent coinciding with an increase in the particle size. The hygroscopic growth factor calculated for a select case is consistent with previous literature results despite the lack of collocated radiosonde data. These results demonstrate the potential of the multiwavelength Raman lidar technique for the study of aerosol humidification process.

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Mark D. Zelinka, Stephen A. Klein, Karl E. Taylor, Timothy Andrews, Mark J. Webb, Jonathan M. Gregory, and Piers M. Forster

Abstract

Using five climate model simulations of the response to an abrupt quadrupling of CO2, the authors perform the first simultaneous model intercomparison of cloud feedbacks and rapid radiative adjustments with cloud masking effects removed, partitioned among changes in cloud types and gross cloud properties. Upon CO2 quadrupling, clouds exhibit a rapid reduction in fractional coverage, cloud-top pressure, and optical depth, with each contributing equally to a 1.1 W m−2 net cloud radiative adjustment, primarily from shortwave radiation. Rapid reductions in midlevel clouds and optically thick clouds are important in reducing planetary albedo in every model. As the planet warms, clouds become fewer, higher, and thicker, and global mean net cloud feedback is positive in all but one model and results primarily from increased trapping of longwave radiation. As was true for earlier models, high cloud changes are the largest contributor to intermodel spread in longwave and shortwave cloud feedbacks, but low cloud changes are the largest contributor to the mean and spread in net cloud feedback. The importance of the negative optical depth feedback relative to the amount feedback at high latitudes is even more marked than in earlier models. The authors show that the negative longwave cloud adjustment inferred in previous studies is primarily caused by a 1.3 W m−2 cloud masking of CO2 forcing. Properly accounting for cloud masking increases net cloud feedback by 0.3 W m−2 K−1, whereas accounting for rapid adjustments reduces by 0.14 W m−2 K−1 the ensemble mean net cloud feedback through a combination of smaller positive cloud amount and altitude feedbacks and larger negative optical depth feedbacks.

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F. Martin Ralph, Paul J. Neiman, David E. Kingsmill, P. Ola G. Persson, Allen B. White, Eric T. Strem, Edmund D. Andrews, and Ronald C. Antweiler

Abstract

Data from the California Land-Falling Jets Experiment (CALJET) are used to explore the causes of variations in flood severity in adjacent coastal watersheds within the Santa Cruz Mountains on 2–3 February 1998. While Pescadero Creek (rural) experienced its flood of record, the adjacent San Lorenzo Creek (heavily populated), attained only its fourth-highest flow. This difference resulted from conditions present while the warm sector of the storm, with its associated low-level jet, high moisture content, and weak static stability, was overhead. Rainfall in the warm sector was dominated by orographic forcing. While the wind speed strongly modulated rain rates on windward slopes, the wind direction positioned the edge of a rain shadow cast by the Santa Lucia Mountains partially over the San Lorenzo basin, thus protecting the city of Santa Cruz from a more severe flood. Roughly 26% ± 9% of the streamflow at flood peak on Pescadero Creek resulted from the warm-sector rainfall. Without this rainfall, the peak flow on Pescadero Creek would likely not have attained record status.

These results are complemented by a climatological analysis based on ∼50-yr-duration streamflow records for these and two other nearby windward watersheds situated ∼20 to 40 km farther to the east, and a comparison of this climatological analysis with composites of NCEP–NCAR reanalysis fields. The westernmost watersheds were found to have their greatest floods during El Niño winters, while the easternmost watersheds peaked during non–El Niño episodes. These results are consistent with the case study, that showed that the composite 925-mb, meridionally oriented wind direction during El Niños favors a rain shadow over the eastern watersheds. During non–El Niño periods, the composite, zonally oriented wind direction indicates that the sheltering effect of the rain shadow on the eastern watersheds is reduced, while weaker winds, less water vapor, and stronger stratification reduce the peak runoff in the western watersheds relative to El Niño periods.

These case study and climatological results illustrate the importance of conditions in the moisture-rich warm sector of landfalling Pacific winter storms. Although many other variables can influence flooding, this study shows that variations of ±10° in wind direction can modulate the location of orographically enhanced floods. While terrain can increase predictability (e.g., rainfall typically increases with altitude), the predictability is reduced when conditions are near a threshold separating different regimes (e.g., in or out of a rain shadow).

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B. Schmid, J. M. Tomlinson, J. M. Hubbe, J. M. Comstock, F. Mei, D. Chand, M. S. Pekour, C. D. Kluzek, E. Andrews, S. C. Biraud, and G. M. McFarquhar

The Department of Energy Atmospheric Radiation Measurement (ARM) program is a climate research user facility operating stationary ground sites that provide long-term measurements of climate-relevant properties, mobile ground- and ship-based facilities to conduct shorter field campaigns (6–12 months), and the ARM Aerial Facility (AAF). The airborne observations acquired by the AAF enhance the surface-based ARM measurements by providing high-resolution in situ measurements for process understanding, retrieval-algorithm development, and model evaluation that are not possible using surface-or satellite-based techniques.

Several ARM aerial efforts were consolidated to form AAF in 2006. With the exception of a small aircraft used for routine measurements of aerosols and carbon cycle gases, AAF at the time had no dedicated aircraft and only a small number of instruments at its disposal. AAF successfully carried out several missions contracting with organizations and investigators who provided their research aircraft and instrumentation. In 2009, AAF started managing operations of the Battelle-owned Gulfstream I (G-1) large twin-turboprop research aircraft. Furthermore, the American Recovery and Reinvestment Act of 2009 provided funding for the procurement of over twenty new instruments to be used aboard the G-1 and AAF contracted aircraft. Depending on the requested scope, AAF now executes campaigns using the G-1 or contracted aircraft, producing freely available datasets for studying gas, aerosol, cloud, and radiative processes and their interactions in the atmosphere. AAF is also engaged in the maturation and testing of newly developed airborne sensors to help foster the next generation of airborne instruments.

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J. L. Kinter III, B. Cash, D. Achuthavarier, J. Adams, E. Altshuler, P. Dirmeyer, B. Doty, B. Huang, E. K. Jin, L. Marx, J. Manganello, C. Stan, T. Wakefield, T. Palmer, M. Hamrud, T. Jung, M. Miller, P. Towers, N. Wedi, M. Satoh, H. Tomita, C. Kodama, T. Nasuno, K. Oouchi, Y. Yamada, H. Taniguchi, P. Andrews, T. Baer, M. Ezell, C. Halloy, D. John, B. Loftis, R. Mohr, and K. Wong

The importance of using dedicated high-end computing resources to enable high spatial resolution in global climate models and advance knowledge of the climate system has been evaluated in an international collaboration called Project Athena. Inspired by the World Modeling Summit of 2008 and made possible by the availability of dedicated high-end computing resources provided by the National Science Foundation from October 2009 through March 2010, Project Athena demonstrated the sensitivity of climate simulations to spatial resolution and to the representation of subgrid-scale processes with horizontal resolutions up to 10 times higher than contemporary climate models. While many aspects of the mean climate were found to be reassuringly similar, beyond a suggested minimum resolution, the magnitudes and structure of regional effects can differ substantially. Project Athena served as a pilot project to demonstrate that an effective international collaboration can be formed to efficiently exploit dedicated supercomputing resources. The outcomes to date suggest that, in addition to substantial and dedicated computing resources, future climate modeling and prediction require a substantial research effort to efficiently explore the fidelity of climate models when explicitly resolving important atmospheric and oceanic processes.

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L. L. Pan, E. L. Atlas, R. J. Salawitch, S. B. Honomichl, J. F. Bresch, W. J. Randel, E. C. Apel, R. S. Hornbrook, A. J. Weinheimer, D. C. Anderson, S. J. Andrews, S. Baidar, S. P. Beaton, T. L. Campos, L. J. Carpenter, D. Chen, B. Dix, V. Donets, S. R. Hall, T. F. Hanisco, C. R. Homeyer, L. G. Huey, J. B. Jensen, L. Kaser, D. E. Kinnison, T. K. Koenig, J.-F. Lamarque, C. Liu, J. Luo, Z. J. Luo, D. D. Montzka, J. M. Nicely, R. B. Pierce, D. D. Riemer, T. Robinson, P. Romashkin, A. Saiz-Lopez, S. Schauffler, O. Shieh, M. H. Stell, K. Ullmann, G. Vaughan, R. Volkamer, and G. Wolfe

Abstract

The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was conducted from Guam (13.5°N, 144.8°E) during January–February 2014. Using the NSF/NCAR Gulfstream V research aircraft, the experiment investigated the photochemical environment over the tropical western Pacific (TWP) warm pool, a region of massive deep convection and the major pathway for air to enter the stratosphere during Northern Hemisphere (NH) winter. The new observations provide a wealth of information for quantifying the influence of convection on the vertical distributions of active species. The airborne in situ measurements up to 15-km altitude fill a significant gap by characterizing the abundance and altitude variation of a wide suite of trace gases. These measurements, together with observations of dynamical and microphysical parameters, provide significant new data for constraining and evaluating global chemistry–climate models. Measurements include precursor and product gas species of reactive halogen compounds that impact ozone in the upper troposphere/lower stratosphere. High-accuracy, in situ measurements of ozone obtained during CONTRAST quantify ozone concentration profiles in the upper troposphere, where previous observations from balloonborne ozonesondes were often near or below the limit of detection. CONTRAST was one of the three coordinated experiments to observe the TWP during January–February 2014. Together, CONTRAST, Airborne Tropical Tropopause Experiment (ATTREX), and Coordinated Airborne Studies in the Tropics (CAST), using complementary capabilities of the three aircraft platforms as well as ground-based instrumentation, provide a comprehensive quantification of the regional distribution and vertical structure of natural and pollutant trace gases in the TWP during NH winter, from the oceanic boundary to the lower stratosphere.

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Parthasarathi Mukhopadhyay, Peter Bechtold, Yuejian Zhu, R. Phani Murali Krishna, Siddharth Kumar, Malay Ganai, Snehlata Tirkey, Tanmoy Goswami, M. Mahakur, Medha Deshpande, V. S. Prasad, C. J. Johny, Ashim Mitra, Raghavendra Ashrit, Abhijit Sarkar, Sahadat Sarkar, Kumar Roy, Elphin Andrews, Radhika Kanase, Shilpa Malviya, S. Abhilash, Manoj Domkawale, S. D. Pawar, Ashu Mamgain, V. R. Durai, Ravi S. Nanjundiah, Ashis K. Mitra, E. N. Rajagopal, M. Mohapatra, and M. Rajeevan

Abstract

During August 2018 and 2019 the southern state of India, Kerala, received unprecedented heavy rainfall, which led to widespread flooding. We aim to characterize the convective nature of these events and the large-scale atmospheric forcing, while exploring their predictability by three state-of-the-art global prediction systems: the National Centers for Environmental Prediction (NCEP)-based India Meteorological Department (IMD) operational Global Forecast System (GFS), the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), and the Unified Model–based NCUM being run at the National Centre for Medium Range Weather Forecasting (NCMRWF). Satellite, radar, and lightning observations suggest that these rain events were dominated by cumulus congestus and shallow convection with strong zonal flow leading to orographically enhanced rainfall over the Ghats mountain range; sporadic deep convection was also present during the 2019 event. A moisture budget analyses using the fifth major global reanalysis produced by ECMWF (ERA5) and forecast output revealed significantly increased moisture convergence below 800 hPa during the main rain events compared to August climatology. The total column-integrated precipitable water tendency, however, is found to be small throughout the month of August, indicating a balance between moisture convergence and drying by precipitation. By applying a Rossby wave filter to the rainfall anomalies it is shown that the large-scale moisture convergence is associated with westward-propagating barotropic Rossby waves over Kerala, leading to increased predictability of these events, especially for 2019. Evaluation of the deterministic and ensemble rainfall predictions revealed systematic rainfall differences over the Ghats mountains and the coastline. The ensemble predictions were more skillful than the deterministic forecasts, as they were able to predict rainfall anomalies (greater than three standard deviations from climatology) beyond day 5 for August 2019 and up to day 3 for 2018.

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