Bibliography - Bruce Wyman

1. Held, Isaac, Ming Zhao, and Bruce Wyman, January 2007: Dynamic radiative-convective equilibria using GCM column physics. Journal of the Atmospheric Sciences, 64(1), 228-238.
   [ Abstract PDF ]

   The behavior of a GCM column physics package in a nonrotating, doubly periodic, homogeneous setting with prescribed SSTs is examined. This radiative–convective framework is proposed as a useful tool for studying some of the interactions between convection and larger-scale dynamics and the effects of differing modeling assumptions on convective organization and cloud feedbacks. For the column physics utilized here, from the Geophysical Fluid Dynamics Laboratory (GFDL) AM2 model, many of the properties of the homogeneous, nonrotating model are closely tied to the fraction of precipitation that is large-scale, rather than convective. Significant large-scale precipitation appears above a critical temperature and then increases with further increases in temperature. The amount of large-scale precipitation is a function of horizontal resolution and can also be controlled by modifying the convection scheme, as is illustrated here by modifying assumptions concerning entrainment into convective plumes. Significant similarities are found between the behavior of the homogeneous model and that of the Tropics of the parent GCM when ocean temperatures are increased and when the convection scheme is modified.

2. Delworth, Thomas L., Anthony J Broccoli, Anthony Rosati, Ronald J Stouffer, Ventakramani Balaji, J A Beesley, W F Cooke, Keith W Dixon, John Dunne, Krista A Dunne, J W Durachta, Kirsten L Findell, Paul Ginoux, Anand Gnanadesikan, C Tony Gordon, Stephen Griffies, Rich Gudgel, Matthew J Harrison, Isaac Held, Richard S Hemler, Larry Horowitz, Stephen A Klein, Thomas R Knutson, P J Kushner, A R Langenhorst, H C Lee, Shian-Jiann Lin, Jian Lu, S Malyshev, P C D Milly, V Ramaswamy, J L Russell, M Daniel Schwarzkopf, Elena Shevliakova, Joseph J Sirutis, Michael J Spelman, William F Stern, Michael Winton, Andrew T Wittenberg, Bruce Wyman, Fanrong Zeng, and Rong Zhang, 2006: GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics. Journal of Climate, 19(5), doi:10.1175/JCLI3629.1.
   [ Abstract ]

   The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved. Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments. The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic. Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/). Manuscript received 8 December 2004, in final form 18 March 2005

3. Anderson, Jeffrey L., Bruce Wyman, Shoaqing Zhang, and T Hoar, 2005: Assimilation of surface pressure observations using an ensemble filter in an idealized global atmospheric prediction system. Journal of the Atmospheric Sciences, 62(8), doi:10.1175/JAS3510.1.
   [ Abstract ]

   An ensemble filter data assimilation system is tested in a perfect model setting using a low resolution Held-Suarez configuration of an atmospheric GCM. The assimilation system is able to reconstruct details of the model's state at all levels when only observations of surface pressure (PS) are available. The impacts of varying the spatial density and temporal frequency of PS observations are examined. The error of the ensemble mean assimilation prior estimate appears to saturate at some point as the number of PS observations available once every 24 h is increased. However, increasing the frequency with which PS observations are available from a fixed network of 1800 randomly located stations results in an apparently unbounded decrease in the assimilation's prior error for both PS and all other model state variables. The error reduces smoothly as a function of observation frequency except for a band with observation periods around 4 h. Assimilated states are found to display enhanced amplitude high-frequency gravity wave oscillations when observations are taken once every few hours, and this adversely impacts the assimilation quality. Assimilations of only surface temperature and only surface wind components are also examined. The results indicate that, in a perfect model context, ensemble filters are able to extract surprising amounts of information from observations of only a small portion of a model's spatial domain. This suggests that most of the remaining challenges for ensemble filter assimilation are confined to problems such as model error, observation representativeness error, and unknown instrument error characteristics that are outside the scope of perfect model experiments. While it is dangerous to extrapolate from these simple experiments to operational atmospheric assimilation, the resulrts also suggest that exploring the frequency with which observations are used for assimilation may lead to significant enhancements to assimilated state estimates.

4. Wyman, Bruce, 1996: A comparison of the sigma and eta vertical coordinate In Research Activities in Atmospheric and Oceanic Modelling, CAS/JSC Working Group on Numerical Experimentation, Report No. 23 WMO/TD No. 734, World Meteorological Organization, 3.42-3.43.
5. Wyman, Bruce, 1996: A step-mountain coordinate general circulation model: Description and validation of medium-range forecasts. Monthly Weather Review, 124(1), 102-121.
   [ Abstract PDF ]

   The step-mountain or eta vertical coordinate has been a proposed solution for eliminating the numerical errors encountered when calculating the pressure gradient force along sloping surfaces. The main objectives of this paper are to describe the development of a global general circulation model using the eta coordinate and to verify the capabilities of the model for medium-range forecasts. First, the treatment of the polar boundary and the polar filtering are presented. To verify the polar treatment, numerical results using the shallow-water equations are presented. Second, various physical parameterizations are incorporated into the multilevel eta coordinate model. Model integrations for several January cases are presented to validate the model. The similarity of the eta coordinate formulation to the terrain-following sigma coordinate allows the model to be run using either vertical coordinate. Thus, model comparisons are performed with the eta and sigma coordinate versions of the general circulation model, keeping the same physical parameterizations. Additional comparisons are made with a sigma coordinate spectral model. As a validation of the model, 10-day integrations are made from four observed initial conditions at several horizontal resolutions. At relatively low resolution, forecast results slightly favor the spectral and sigma coordinate models. However, at higher resolution, forecast skill scores for the eta coordinate model are indistinguishable from those of the sigma models. Additional results are presented to demonstrate the advantages the advantages of the eta coordinate near steep topography and the potential deficiency of the eta coordinate in connection with the surface boundary layer treatment.

6. Pierrehumbert, R T., and Bruce Wyman, 1985: Upstream effects of mesoscale mountains. Journal of the Atmospheric Sciences, 42(10), 977-1003.
   [ Abstract PDF ]

   The Alpine Experiment (ALPEX) has revealed that low-level air is typically diverted around the Alps without reaching the mountaintop. In pursuit of an understanding of the physical basis of this phenomenon and of its generality, we have explored the characteristics of orographic blocking of a rotating continuously stratified fluid, as revealed in a simple model problem retaining full nonlinear and transient effects. Hydrostatic dynamics is assumed, and the obstacleis taken to be an infinitely long ridge with height h(x). The key questions treated are the strength of the upstream deceleration of cross-mountain flow and the length scale over which the decelerated region extends. By means of scale analysis, the controlling parameters are found to be the Rossby number Ro = U/fL and the Froude number Fr = Nhm/U, where U is the speed of the oncoming flow, f is the Coriolis parameter, L the mountain half-width, N the Brunt Väisälä frequency, and hm is the maximum mountain height. The scale analysis also determines the qualitative dependence of the strength of the blocking on Ro and Fr; these predictions were confirmed and made quantitative via extensive numerical simulation. In the nonrotating limit, Fr is the sole parameter. In this case, it is found that for sufficiently large Fr a decelerated layer of fluid forms near the obstacle and propagates arbitrarily far upstream with time, in a manner similar to that familiar in one-layer hydraulic theory. The upstream influence requires neither downstream lee wave trains nor vertical confinement by a rigid lid; rather, the upstream modes appear to be generated by wave breaking above the lee slope of the mountain. For a Gaussian mountain profile, wave breaking and upstream influence set in near Fr = 0.75; low-level flow upstream of the mountain is decelerated to rest for Fr > 1.5. In the rotating case, the decelerated zone does not propagate infinitely far. Instead, it attains a maximum extent on the order of the radius of deformation Nhm/f before retreating toward the mountain. The upstream scales remaining after a long time has passed are also discussed. The theory accounts for a number of aspects of the ALPEX data, as well as for features seen in earlier observations of barrier winds elsewhere. It appears though that the sharp transition between flow over and flow around found in certain ALPEX vertical soundings obtained from aircraft cannot be explained in terms of inviscid theory. It is conjectured that the sharp division is due to low-level convective mixing.

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