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Jimy Dudhia and James F. Bresch

Abstract

A global version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (PSU–NCAR MM5) is described. The new model employs two polar stereographic projection domains centered on each pole. These domains interact at their equators, thereby eliminating the need for a lateral boundary condition file.

This paper describes the method, and contrasts this fully compressible nonhydrostatic Eulerian global model with other global models. There are potential advantages over spherical polar grids in the resolution distribution and the treatment of curvature forces near the poles. The model also selectively damps acoustic modes, which appears to have some benefits in real-data initialization. The split-explicit time steps are different from the semi-implicit schemes used in several global nonhydrostatic models, and this localized scheme avoids the need for global elliptic solvers, making it particularly adept for distributed-memory platforms and the use of composite meshes.

Tests of the model show that acoustic and gravity waves as well as advective features propagate across the equator without distortion. A trial 100-day perpetual January simulation shows realistic rain patterns as compared to climatology with no evidence of equatorial effects. Nesting is also available to focus on areas of interest, and this is demonstrated with a 72-h nested forecast over North America.

While the time step is shorter than that typically used in semi-Lagrangian global models with a comparable resolution, the model is efficient enough to have allowed the running of daily 120-km grid forecasts on nondedicated computers as small as four-processor workstations since October 1999. Results from this real-time application of the model to 5-day forecasts are shown, and demonstrate that the model performs well at this scale.

The model is consistent with the regular regional MM5 and shares dynamics and physics packages without modification. It can also make use of pre- and postprocessing packages developed for the MM5 system. This tight linkage between a regional and global model will have a clear benefit as future global models move toward higher resolutions. It allows current mesoscale numerical weather prediction research to directly feed into the next generation of global models.

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Richard H. Johnson and James F. Bresch

Abstract

Characteristics of Mei-Yu precipitating cloud systems over Taiwan during the May–June 1987 Taiwan Area Mesoscale Experiment (TAMEX) have been studied using sounding, surface precipitation, and radar data. Vertical motion has been computed over the island at 6-h intervals from 13 May to 15 June using a modification of the kinematic method that takes into account the mountainous lower boundary within a four-station sounding polygon.

Two primary characteristics of the precipitation have been found. First, the major rainfall event were linked to the passage of midlatitude disturbances and typically consisted of both deep convective and stratiform components. Deep convection was primarily prefrontal or frontal, while the stratiform precipitation was postfrontal, presumably in association with overrunning and orographic lifting. Second, there was a pronounced diurnal variability in the rainfall.

Vertical motion, heating (Q 1), and moistening (Q 2) profiles have been used to define the character of the precipitating systems. During periods of deep convection (as indicated by radar and surface rainfall measurements), a separation of the Q 1 and Q 2 peaks is observed, whereas at times of stratiform precipitation, the Q 1 and Q 1 peaks are nearly coincident. The findings for Taiwan generally support those of Luo and Yanai, indicating a predominance of stratiform rainfall over the entire southern China and Yangtze regions (including Taiwan) during the Mei-Yu; however, they also suggest that in at least a portion of this region (Taiwan), precipitation may consist of a mixture of deep convective and stratiform components. The occurrence of coincident Q 1 and Q 1 peaks in the mid- to lower troposphere (600–800 mb) during moderate-to-heavy stratiform rain events indicates the importance of shallow cold-frontal and/or stable orographic lifting. Thus, it appears that in the Taiwan area, heavy rain in stable situations may depend critically on low-level forcing mechanisms.

The evolution of the sea breeze, the development of the afternoon mixed layer, and the diurnal cycle of Q 1 and Q 1 have been examined on a synoptically undisturbed day (24 May) when afternoon thunderstorms occurred over Taiwan. Moistening of the boundary layer by the daytime sea breeze was evident. A high-level heating peak and a midtropospheric drying peak were observed in the afternoon in association with the sea breeze and deep convection. In the evening, heating and drying aloft and cooling and moistening at low levels occurred, suggestive of stratiform precipitation during the decaying stage of the convection.

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William A. Gallus Jr. and James F. Bresch

Abstract

An intense small-scale low pressure system that moved across portions of the midwest United States is examined. The system produced a continuous band of significant snowfall, typically only 50 km wide but extending over 1500 km in length. The system traveled across the Iowa Department of Transportation surface mesonetwork, allowing high-resolution surface analyses that show a closed circulation and intense pressure gradients around the mesolow, comparable to those occurring in warm season MCS events. Radar and satellite images also revealed the small-scale low-level circulation, which apparently was confined below about 800 mb. Although the strong vorticity advection aloft and baroclinicity at lower levels present in this system are typical of baroclinic cyclones, the unusually small scale and short lifetime of the surface system are more reminiscent of polar lows.

Mesoscale simulations of the system using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 with 20-km horizontal grid spacing and initialized with standard synoptic-scale data were unable to capture the closed circulation and significantly underestimated the strength of the mesolow. The inclusion of mesonet surface data in an initialization significantly improved the initial pressure field but did not significantly change the simulation. The simulation was also not strongly sensitive to variations in horizontal and vertical resolution, surface characteristics, convective parameterizations, and the use of nudging toward observations. However, an adjustment of upper-level fields to support the surface mesoscale low did result in a significantly improved simulation of the event, apparently due to better simulation of forcing from warm advection in low levels.

A simulation neglecting latent heating produced a surface low that was at least 1 mb weaker than the full-physics run and had much weaker and disorganized upward vertical motion. The mesoscale low was apparently the result of upper-tropospheric forcing, which eliminated a capping inversion in a small region, permitting precipitating convection and latent heat release.

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William A. Gallus Jr. and James F. Bresch

Abstract

A series of simulations for 15 events occurring during August 2002 were performed using the Weather Research and Forecasting (WRF) model over a domain encompassing most of the central United States to compare the sensitivity of warm season rainfall forecasts with changes in model physics, dynamics, and initial conditions. Most simulations were run with 8-km grid spacing. The Advanced Research WRF (ARW) and the nonhydrostatic mesoscale model (NMM) dynamic cores were used. One physics package (denoted NCEP) used the Betts–Miller–Janjic convective scheme with the Mellor–Yamada–Janjic planetary boundary layer (PBL) scheme and GFDL radiation package; the other package (denoted NCAR) used the Kain–Fritsch convective scheme with the Yonsei University PBL scheme and the Dudhia rapid radiative transfer model radiation. Other physical schemes were the same (e.g., the Noah land surface model, Ferrier et al. microphysics) in all runs. Simulations suggest that the sensitivity of the model to changes in physics is a function of which the dynamic core is used, and the sensitivity to the dynamic core is a function of the physics used. The greatest sensitivity in general is associated with a change in physics packages when the NMM core is used. Sensitivity to a change in physics when the ARW core is used is noticeably less. For light rainfall, the spread in the rainfall forecasts when physics are changed under the ARW core is actually less at most times than when the dynamic core is changed while NCAR physics are used. For light rainfall, the WRF model using NCAR physics is much more sensitive to a change in dynamic core than the WRF model using NCEP physics. For heavier rainfall, the opposite is true with a greater sensitivity occurring when NCEP physics is used. Sensitivity to initial conditions (Eta versus the Rapid Update Cycle with an accompanying small change in grid spacing) is generally less substantial than the sensitivity to changes in dynamic core or physics, except in the first 6–12 h of the forecast when it is comparable. As might be expected for warm season rainfall, the finescale structure of rainfall forecasts is more affected by the physics used than the dynamic core used. Surprisingly, however, the overall areal coverage and rain volume within the domain may be more influenced by the dynamic core choice than the physics used.

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James F. Bresch and Elmar R. Reiter

Abstract

Observations of extremely high fine particulate sulfur concentrations during early April 1983 in the western United States are linked to a strong cyclone over the midwestern United States. The strong winds around this cyclone circulated polluted midwestern air as far west as the Pacific Coast. A retrograding upper wave pattern was conducive for this polluted air to move southwestward. Both a long-range trajectory analysis and a subjective evaluation of synoptic conditions confirm this hypothesis.

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Rujin Shen, Elmar R. Reiter, and James F. Bresch

Abstract

The initialization of numerical prediction models usually requires the transformation of variables observed in a p-coordinate system into some other coordinate frame of reference (e.g., α-coordinates or Θ-coordinates). Such transformations require the application of interpolation or curve-fitting techniques. The present study demonstrates that the choice of an appropriate interpolation scheme can become a critical issue for the skill of a low-resolution prediction model. First we show that the interpolation scheme, when applied to more than one meteorological variable, should satisfy the balance requirements that exist between these variables. Not all of the currently used schemes meet this condition. Next we provide evidence indicating that interpolation schemes used to convert p-into α-coordinates, and then back into p-coordinates, do not necessarily replicate the original, observed field distributions of these meteorological variables. Such double transformations usually are required, because the numerical output in model coordinates has to be translated back to p-coordinates for verification of model results. Because of the limitations of certain interpolation procedures, even a correct model prediction may exhibit low predictive skill because of errors introduced in this final coordinate transformation process.

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Rujin Shen, Elmar R. Reiter, and James F. Bresch

Abstract

The influence of sensible heating from the earth's surface on the development of summertime vortices over the Tibetan Plateau was investigated using a numerical model. It was found that sensible heating could cause local intensification of vortices over high elevations and sometimes act in combination with topography to block intrusions of cold air. Sensible heating can play an important role, not indicated by its magnitude, when it is combined with topography and the proper synoptic situation. Sensible heating had a greater impact over higher elevations, areas with strong cold advection, and areas under the upper-tropospheric jet stream. Sensible heating tends to destabilize an air column, permitting downward transfer of westerly momentum in the vicinity of the jet stream and causing an increase in cyclonic vorticity in the lower troposphere north of the upper-level jet. During the premonsoon period, when the upper-level jet was located over the southern plateau, sensible heating acted to intensify plateau vortices. After the transition into the summer monsoon period, the jet was north of the plateau and sensible heating had only localized and gradual effects on plateau vortices.

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James F. Bresch, Richard J. Reed, and Mark D. Albright

Abstract

A polar low that developed over the western Bering Sea on 7 March 1977 and tracked across St. Paul Island is investigated using observations and the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). A series of fine-mesh (20 km) simulations are performed in order to examine the structure of the cyclone and the airflow within it and to determine the physical processes important for its development. Observations show that the low formed near the ice edge in a region of moderate low-level baroclinicity and cold-air advection when an upper-level trough, or lobe of anomalously large potential vorticity (PV), broke off from a migratory, upper-level cold low over Siberia and advanced into the region.

A full physics model experiment, initialized 24 h prior to the appearance of the polar low, produced a small, intense cyclone having characteristics similar to the observed low. The simulated low more closely resembled an extratropical cyclone than a typical circularly symmetric hurricane, possessing a thermal structure with frontlike features and an asymmetric precipitation shield. Although the simulated low developed southeast of, and earlier than, the observed low, the basic similarity of the observed and modeled systems was revealed by a comparison of the sequence of weather elements at a point in the path of the simulated low with the sequence of observations from nearby St. Paul Island, Alaska.

A series of experiments was performed to test the sensitivity of the simulated polar low development to various physical processes. Four experiments of 48-h duration each were initialized 24 h before the low appeared. In the first experiment, in which surface fluxes were turned off, the low failed to develop. In the second experiment, in which the fluxes were switched on after a 24-h delay, only a weak low formed. In the third experiment, in which the ice edge was shifted a degree of latitude to the north, thus increasing the overwater fetch of the cold air, the low’s evolution was slightly altered but the final outcome was little changed. A fourth high-horizontal resolution experiment (6.67-km spacing) displayed more plentiful and sharper mesoscale features but on the storm scale yielded results that were similar to those of the full-physics run. A full-physics experiment initialized 24 h later, at the time the low first appeared, and run for 24 h, produced a system of similar intensity to that in the 48-h full-physics run but somewhat better positioned. Corresponding sensitivity experiments showed that with both surface fluxes and latent heating omitted, the low weakened and nearly died away. Experiments retaining only surface fluxes in one case and only latent heating in the other, produced similar cyclones of moderate depth.

The results suggest that the development of some, if not most, polar lows can be regarded as fundamentally similar to that of midlatitude ocean cyclones. In both cases a mobile upper-level PV anomaly interacts with a low-level thermal or PV anomaly produced by thermal advection and/or diabatic heating. The polar low lies at the end of the spectrum of extratropical cyclogenesis in which concurrent surface fluxes of sensible and latent heat and the immediately ensuing condensation heating in organized convection dominate the development of the low-level anomaly.

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Laura L. Pan, Kenneth P. Bowman, Elliot L. Atlas, Steve C. Wofsy, Fuqing Zhang, James F. Bresch, Brian A. Ridley, Jasna V. Pittman, Cameron R. Homeyer, Pavel Romashkin, and William A. Cooper

The Stratosphere–Troposphere Analyses of Regional Transport 2008 (START08) experiment investigated a number of important processes in the extratropical upper troposphere and lower stratosphere (UTLS) using the National Science Foundation (NSF)–NCAR Gulfstream V (GV) research aircraft. The main objective was to examine the chemical structure of the extratropical UTLS in relation to dynamical processes spanning a range of scales. The campaign was conducted during April–June 2008 from Broomfield, Colorado. A total of 18 research flights sampled an extensive geographical region of North America (25°–65°N, 80°–120°W) and a wide range of meteorological conditions. The airborne in situ instruments measured a comprehensive suite of chemical constituents and microphysical variables from the boundary layer to the lower stratosphere, with flights specifically designed to target key transport processes in the extratropical UTLS. The flights successfully investigated stratosphere–troposphere exchange (STE) processes, including the intrusion of tropospheric air into the stratosphere in association with the secondary tropopause and the intrusion of stratospheric air deep into the troposphere. The flights also sampled the influence of convective transport and lightning on the upper troposphere as well as the distribution of gravity waves associated with multiple sources, including fronts and topography. The aircraft observations are complemented by satellite observations and modeling. The measurements will be used to improve the representation of UTLS chemical gradients and transport in Chemistry–Climate models (CCMs). This article provides an overview of the experiment design and selected observational highlights.

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Britton B. Stephens, Matthew C. Long, Ralph F. Keeling, Eric A. Kort, Colm Sweeney, Eric C. Apel, Elliot L. Atlas, Stuart Beaton, Jonathan D. Bent, Nicola J. Blake, James F. Bresch, Joanna Casey, Bruce C. Daube, Minghui Diao, Ernesto Diaz, Heidi Dierssen, Valeria Donets, Bo-Cai Gao, Michelle Gierach, Robert Green, Justin Haag, Matthew Hayman, Alan J. Hills, Martín S. Hoecker-Martínez, Shawn B. Honomichl, Rebecca S. Hornbrook, Jorgen B. Jensen, Rong-Rong Li, Ian McCubbin, Kathryn McKain, Eric J. Morgan, Scott Nolte, Jordan G. Powers, Bryan Rainwater, Kaylan Randolph, Mike Reeves, Sue M. Schauffler, Katherine Smith, Mackenzie Smith, Jeff Stith, Gregory Stossmeister, Darin W. Toohey, and Andrew S. Watt

Abstract

The Southern Ocean plays a critical role in the global climate system by mediating atmosphere–ocean partitioning of heat and carbon dioxide. However, Earth system models are demonstrably deficient in the Southern Ocean, leading to large uncertainties in future air–sea CO2 flux projections under climate warming and incomplete interpretations of natural variability on interannual to geologic time scales. Here, we describe a recent aircraft observational campaign, the O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) study, which collected measurements over the Southern Ocean during January and February 2016. The primary research objective of the ORCAS campaign was to improve observational constraints on the seasonal exchange of atmospheric carbon dioxide and oxygen with the Southern Ocean. The campaign also included measurements of anthropogenic and marine biogenic reactive gases; high-resolution, hyperspectral ocean color imaging of the ocean surface; and microphysical data relevant for understanding and modeling cloud processes. In each of these components of the ORCAS project, the campaign has significantly expanded the amount of observational data available for this remote region. Ongoing research based on these observations will contribute to advancing our understanding of this climatically important system across a range of topics including carbon cycling, atmospheric chemistry and transport, and cloud physics. This article presents an overview of the scientific and methodological aspects of the ORCAS project and highlights early findings.

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