Ayrton Zadra

Staggered Vertical Discretization of the Canadian Environmental Multiscale (GEM) Model Using a Coordinate of the Log-Hydrostatic-Pressure Type

AbstractThe Global Environmental Multiscale (GEM) model is the Canadian atmospheric model used for meteorological forecasting at all scales. A limited-area version now also exists. It is a gridpoint model with an implicit semi-Lagrangian iterative space–time integration scheme. In the “horizontal,” the equations are written in spherical coordinates with the traditional shallow atmosphere approximations and are discretized on an Arakawa C grid. In the “vertical,” the equations were originally defined using a hydrostatic-pressure coordinate and discretized on a regular (unstaggered) grid, a configuration found to be particularly susceptible to noise. Among the possible alternatives, the Charney–Phillips grid, with its unique characteristics, and, as the vertical coordinate, log-hydrostatic pressure are adopted. In this paper, an attempt is made to justify these two choices on theoretical grounds. The resulting equations and their vertical discretization are described and the solution method of what is formi...

Impacts of parameterized orographic drag on the Northern Hemisphere winter circulation

Abstract A recent intercomparison exercise proposed by the Working Group for Numerical Experimentation (WGNE) revealed that the parameterized, or unresolved, surface stress in weather forecast models is highly model‐dependent, especially over orography. Models of comparable resolution differ over land by as much as 20% in zonal mean total subgrid surface stress (τtot). The way τtot is partitioned between the different parameterizations is also model‐dependent. In this study, we simulated in a particular model an increase in τtot comparable with the spread found in the WGNE intercomparison. This increase was simulated in two ways, namely by increasing independently the contributions to τtot of the turbulent orographic form drag scheme (TOFD) and of the orographic low‐level blocking scheme (BLOCK). Increasing the parameterized orographic drag leads to significant changes in surface pressure, zonal wind and temperature in the Northern Hemisphere during winter both in 10 day weather forecasts and in seasonal integrations. However, the magnitude of these changes in circulation strongly depends on which scheme is modified. In 10 day forecasts, stronger changes are found when the TOFD stress is increased, while on seasonal time scales the effects are of comparable magnitude, although different in detail. At these time scales, the BLOCK scheme affects the lower stratosphere winds through changes in the resolved planetary waves which are associated with surface impacts, while the TOFD effects are mostly limited to the lower troposphere. The partitioning of τtot between the two schemes appears to play an important role at all time scales.

Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice : the Larcform 1 single column model intercomparison

Weather and climate models struggle to represent lower tropospheric temperature and moisture profiles and surface fluxes in Arctic winter, partly because they lack or misrepresent physical processes that are specific to high latitudes. Observations have revealed two preferred states of the Arctic winter boundary layer. In the cloudy state, cloud liquid water limits surface radiative cooling, and temperature inversions are weak and elevated. In the radiatively clear state, strong surface radiative cooling leads to the build-up of surface-based temperature inversions. Many large-scale models lack the cloudy state, and some substantially underestimate inversion strength in the clear state. Here, the transformation from a moist to a cold dry air mass is modelled using an idealized Lagrangian perspective. The trajectory includes both boundary layer states, and the single-column experiment is the first Lagrangian Arctic air formation experiment (Larcform 1) organized within GEWEX GASS (Global atmospheric system studies). The intercomparison reproduces the typical biases of large-scale models: Some models lack the cloudy state of the boundary layer due to the representation of mixed-phase micro-physics or to the interaction between micro-and macrophysics. In some models, high emissivities of ice clouds or the lack of an insulating snow layer prevent the build-up of surface-based inversions in the radiatively clear state. Models substantially disagree on the amount of cloud liquid water in the cloudy state and on turbulent heat fluxes under clear skies. Observations of air mass transformations including both boundary layer states would allow for a tighter constraint of model behaviour.

Comparison of Estimated Atmospheric Boundary Layer Mixing Height in the Arctic and Southern Great Plains under Statically Stable Conditions: Experimental and Numerical Aspects

Abstract The atmospheric boundary layer mixing height (MH) is an important bulk parameter in air quality (AQ) modelling. Formulating this parameter under statically stable conditions, such as in the Arctic, has historically been difficult. In an effort to improve AQ modelling capacity in North America, MH is studied in two geographically distinct areas: the Arctic (Barrow, Alaska) and the southern Great Plains (Lamont, Oklahoma). Observational data from the Atmospheric Radiation Measurement program, Climate Research Facility and numerical weather forecasting data from Environment Canadas Regional Global Environmental Multiscale (GEM15) model have been used in order to examine the suitability of available parameterizations for MH under statically stable conditions and also to compare the level of agreement between observed and modelled MH. The analysis period is 1 October 2011 to 1 October 2012. The observations alone suggest that profile methods are preferred over surface methods in defining MH under statically stable conditions. Surface methods exhibit poorer comparison statistics with observations than profile methods. In addition, the fitted constants for surface methods are site-dependent, precluding their applicability for modelling under general conditions. The comparison of observations and GEM15 MH suggests that although the agreement is acceptable in Lamont, the default model surface method contributes to a consistent overprediction of MH in Barrow in all seasons. An alternative profile method for MH is suggested based on the bulk Richardson number. This method is shown to reduce the model bias in Barrow by a factor of two without affecting model performance in Lamont.

Representing Richardson Number Hysteresis in the NWP Boundary Layer

AbstractTurbulence in the planetary boundary layer (PBL) transports heat, momentum, and moisture in eddies that are not resolvable by current NWP systems. Numerical models typically parameterize this process using vertical diffusion operators whose coefficients depend on the intensity of the expected turbulence. The PBL scheme employed in this study uses a one-and-a-half-order closure based on a predictive equation for the turbulent kinetic energy (TKE). For a stably stratified fluid, the growth and decay of TKE is largely controlled by the dynamic stability of the flow as represented by the Richardson number. Although the existence of a critical Richardson number that uniquely separates turbulent and laminar regimes is predicted by linear theory and perturbation analysis, observational evidence and total energy arguments suggest that its value is highly uncertain. This can be explained in part by the apparent presence of turbulence regime-dependent critical values, a property known as Richardson number h...

Evaluation of Tropical Cyclones in the Canadian Global Modeling System: Sensitivity to Moist Process Parameterization

AbstractDeep convection is one of various complex processes driving the evolution of tropical cyclones (TCs). The scales associated with deep convection are too small to be resolved by global NWP models. In the deep convection parameterization used by the Canadian Global Deterministic Prediction System (GDPS), the trigger function depends on various criteria, one of which is the adjustable “trigger velocity” parameter, a vertical velocity threshold used in the parcel stability test of the scheme. In this study, the sensitivity of the GDPS TC activity and precipitation distribution to convective triggering parameters is investigated by varying this threshold. Multiple basins are considered for three TC seasons, and the impacts of trigger velocity variations on TC statistics (forecast hits, bias, false alarms, and track and intensity errors) and on the model’s genesis potential index (GPI) are measured. It is shown that a reduction of the trigger velocity, from 0.05 to 0.01 m s−1, over the tropical oceans l...

Simulating wind channelling over Frobisher Bay and its interaction with downslope winds during the 7–8 November 2006 wind event

Abstract Previous observational studies have identified wind channelling over Frobisher Bay induced by pressure fields associated with cyclones as the main cause for the occurrence of strong and sustained surface winds at Iqaluit. The wind event of 7–8 November 2006, when a surface cyclone moved over the Labrador Sea, is representative of many such occurrences. Our simulations of the event with the Global Environmental Multiscale – Limited Area Model (GEM‐LAM 2.5 km) show an almost simultaneous development of wind channelling over Frobisher Bay and of downslope winds over the lee slopes of Hall Peninsula, as well as their interaction. The winds are caused by the passage of the same cyclone, but they advect different air masses. The channelled wind is initiated by a barrier jet generated as a result of the low‐level blocking of northeasterly winds by steep orography near the head of the bay. Later, it intensifies, while being driven primarily by large‐scale pressure gradients. The cross‐bay development of the channelled wind against the downslope wind over the lee slopes of Hall Peninsula explains the shift in surface wind direction and the high surface wind speeds recorded at Iqaluit. Some of the findings are also supported by data from radiosondes launched at Iqaluit. A short sensitivity study shows the beneficial effect of a distributed orographic drag parametrization on the near‐surface winds. The impact of using a modified Lenderink‐Holtslag mixing length and of increasing the vertical resolution near the surface are also addressed.

Adapting Two-Moment Microphysics Schemes across Model Resolutions: Subgrid Cloud and Precipitation Fraction and Microphysical Sub–Time Step

AbstractTwo-moment multiclass microphysics schemes are very promising tools to be used in high-resolution NWP models. However, they must be adapted for coarser resolutions. Here, a twofold solution is proposed—namely, a simple representation of subgrid cloud and precipitation fraction—as well as a microphysical sub-time-stepping method. The scheme is easy to implement, allows supersaturation in ice cloud, and exhibits flexibility for adoption across model grid spacing. It is implemented in the Milbrandt and Yau two-moment microphysics scheme with prognostic precipitation in the context of a simple 1D kinematic model as well as a mesoscale NWP model [the Canadian regional Global Environmental Multiscale model (GEM)]. Sensitivity tests were performed and the results highlighting the advantages and disadvantages of the two-moment multiclass cloud scheme relative to the classical Sundqvist scheme. The respective roles of subgrid cloud fraction, precipitation fraction, and time splitting were also studied. Whe...

A stable and accurate scheme for nonlinear diffusion equations: Application to atmospheric boundary layer

Stability concerns are always a factor in the numerical solution of nonlinear diffusion equations, which are a class of equations widely applicable in different fields of science and engineering. In this study, a modified extended backward differentiation formulae (ME BDF) scheme is adapted for the solution of nonlinear diffusion equations, with a special focus on the atmospheric boundary layer diffusion process. The scheme is first implemented and examined for a widely used nonlinear ordinary differential equation, and then extended to a system of two nonlinear diffusion equations. A new temporal filter which leads to significant improvement of numerical results is proposed, and the impact of the filter on the stability and accuracy of the results is investigated. Noteworthy improvements are obtained as compared to other commonly used numerical schemes. Linear stability analysis of the proposed scheme is performed for both systems, and analytical stability limits are presented.

Optimal high-order diagonally-implicit Runge–Kutta schemes for nonlinear diffusive systems on atmospheric boundary layer

Abstract Nonlinear diffusion equations are extensively applicable in diverse fields of science and engineering. Numerical stability is a common concern in this class of equations. In the present study, a three-stage third-order diagonally-implicit Runge–Kutta (DIRK) scheme is introduced by optimizing the error and linear stability analysis for a commonly used nonlinear diffusive system in atmospheric boundary layer. The proposed scheme is stable for a wide range of time steps and able to resolve different diffusive systems with diagnostic turbulence closures, or prognostic ones with a diagnostic length scale, with enhanced accuracy and stability compared to current schemes. It maintains A-stability, which makes it appropriate for the solution of stiff problems. The procedure implemented in this study is quite general and can be used in other diffusive systems as well.