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J. Mailhot and C. Chouinard

Abstract

Mesoscale numerical forecasts of cases of explosive cyclogenesis during the Canadian Atlantic Storms Program are presented in order to examine the evolution and structure of the simulated storms, and to assess the model's skill in forecasting significant weather elements. The investigation focuses on a series of sensitivity experiments with various horizontal resolutions, sea surface temperatures (SST), surface energy fluxes and condensation schemes in order to understand the physical mechanisms responsible for explosive deepening.

The results confirm the conclusions of several other investigators that realistic simulations of explosive storms and many of their subsynoptic and mesoscale features can be obtained using high resolution models with a complete set of physical processes, even when starting with synoptic-scale analyses only. In particular, the formation and maintenance of an intense southerly low-level jet (LLJ) ahead of the surface cold front appears to be instrumental throughout the rapid deepening phase.

Variations in physical parameterization schemes or SST analyses have an impact mostly in the lower levels. However, a critical factor for simulating marine explosive cyclogenesis is identified as evaporation from the ocean. The results indicate that a large fraction of the moisture available for condensation processes and further deepening originates from the air–sea interactions. The coupling provided by the LLJ then becomes a very efficient mechanism by which heat and moisture are carried from the source region into the developing storm.

The overall skill of the model in forecasting quantitative precipitation is found to be quite good. Horizontal distributions of cloud and precipitation compare favorably with satellite imagery and the simulated splitting of precipitation into different types agrees quite well with surface reports.

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J. Mailhot and R. Benoit

Abstract

We give a detailed description of an atmospheric boundary layer model capable of simulating the diurnal cycles of wind, temperature and humidity. The model includes a formulation of various physical processes (radiative effects, variation of soil surface temperature and humidity, etc.) and uses a first-order closure for turbulent fluxes that relies upon a time-dependent equation for turbulent kinetic energy and on a mixing length governed by a relaxation process. The exchange processes taking place in the surface layer are dealt with in a separate micrometeorological module.

The one-dimensional model uses a Galerkin technique based on linear finite elements, variable resolution in the vertical, and a time discretization of the Crank-Nicholson type. A simulation test based on day 33 of the Wangara Australian experiment indicates that the model, despite its relative simplicity, gives realistic results that compare favorably with those from higher order models while taking much less space and time on the computer. This could make feasible its use by operational numerical weather prediction models.

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J. Pudykiewicz, R. Benoit, and J. Mailhot

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The implementation of a predictive cloud-water scheme in a regional finite-element weather prediction model is presented. The model employed in this study includes efficient and accurate numerical techniques and is equipped with a relatively extensive parameterization of the planetary boundary layer, surface process, and radiation. The modifications made to the meteorological model in this study include the addition of the advection equation for cloud water to the set of primitive meteorological equations. In our implementation of the predictive cloud-water scheme, the cloud-water equation represents the grid-resolved cloud-water field, whereas the effects of subgrid convective clouds are parameterized with the convective scheme of Kuo. The numerical solution of the advection equation for cloud water is analogous to the solution of the moisture equation using the semi-Lagrangian advection algorithm applied previously in regional weather forecast. The advection of cloud water is performed using the horizontal wind as well as vertical motions. The choice of a fully three-dimensional advection scheme instead of advection by the horizontal wind only is motivated by the importance of vertical motion in frontal zones. The performance of the cloud-water scheme is demonstrated by a numerical simulation of the great storm that severely affected the British Isles and France in October 1987. This case was selected because of the well-known importance of condensation processes in rapidly developing storm systems. Our study shows that the model equipped with the predictive cloud-water scheme is remarkably successful in predicting the explosive development of the low pressure system. One of the most interesting features of the simulation is the very realistic depiction of the cold-frontal structure including the conveyor bell the low-level jet, and the distribution of liquid water. An objective evaluation of the cloud-water forecast in this study is performed using the microwave radiances observed by the Special Sensor Microwave/Imager (SSM/I) of the Defense Meteorological Satellite Program (DMSP). The application of the microwave sounding technique for evaluation of the cloud scheme in a numerical weather prediction model is one of the lust attempts of its kind. The verification of the predicted cloud-water distribution using the microwave sounding technique indicates that the model is able to correctly represent the distribution of hydrometeors in a rapidly developing low pressure system. The horizontal structure of the system reflected by the cloud-water field and the precipitation field is reproduced with relatively good accuracy.

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R. Benoit, J. Côté, and J. Mailhot

Abstract

The formulation of the regional model recently implemented by the Atmospheric Environment Service of Canada for its operational 48 h NWP forecasts is presented. The emphasis is put on the parameterization of the physical processes, especially those affecting the atmospheric boundary layer. The originality of this model, in addition to the use of 3-D finite elements, of variable meshes in both the horizontal and vertical, and of being non-nested (as previously described by Staniforth and Daley), consists in the treatment of the time-dependent turbulent Kinetic energy (TKE) and the inclusion of the full diurnal cycle. The overall organization of the model calculations it also presented in order to convey a more accurate description of this integrated system. Sample results from the well-known case of the Presidents' Day Cyclone of 1979 and general performance are covered in the last section.

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S. Desjardins, J. Mailhot, and R. Lalbeharry

Abstract

To take into account the change of surface roughness length induced by ocean waves, a coupled atmospheric and ocean wave model system is developed. A two-way coupling is done between a mesoscale atmospheric model, MC2, and an oceanic wave model, a regional version of WAM Cycle-4. Two different approaches, based on the wave age of Smith et al. and the wave-induced stress of Janssen, are used to compute a coupling parameter, called the Charnock parameter, expressed as the nondimensional surface roughness length. The coupling between the two models is accomplished by the use of this parameter, which is a function of sea state, instead of the constant value obtained from empirical studies using the well-known Charnock relation.

The coupled atmospheric and ocean wave system was applied to four intense storms in the western Atlantic, to examine the impact of the two-way interaction using real midlatitude storm cases. In Part I, the atmospheric forecasts resulting from this two-way coupling are discussed for these different synoptic cases. The two approaches are evaluated by comparing atmospheric outputs obtained from the coupled and uncoupled systems against buoy observations. The authors conclude that, at least for short-term forecasts, the effects of sea-state-dependent surface roughness on the evolution and synoptic-scale aspects of the storm are rather weak, while small beneficial effects are noted on surface variables.

The most significant changes are a reduction of about 10% of the surface winds associated with enhanced surface roughness lengths in areas of younger and rougher seas. The coupling also modifies the sea surface fluxes by as much as 30%, but these are not located in the active areas of the storm. Consequently, precipitation only increases slightly and does not affect the storm development. The impact on the ocean wave forecasts is discussed in Part II.

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J. A. Milbrandt, M. K. Yau, J. Mailhot, and S. Bélair

Abstract

This paper reports the first evaluation of the Milbrandt–Yau multimoment bulk microphysics scheme against in situ microphysical measurements. The full triple-moment version of the scheme was used to simulate a case of orographically enhanced precipitation with a 3D mesoscale model at high resolution (4- and 1-km grid spacings). The simulations described in this paper also serve as the control runs for the sensitivity experiments that will be examined in Part II of this series. The 13–14 December 2001 case of heavy orographically enhanced precipitation, which occurred over the Oregon Cascades, was selected since it was well observed during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) observational campaign. The simulated fields were compared with observed radar reflectivity, vertical velocity, precipitation quantities from rain gauges, and microphysical quantities measured in situ by two instrumented aircraft. The simulated reflectivity structure and values compared favorably to radar observations during the various precipitation stages of the event. The vertical motion field in the simulations corresponded reasonably well to the mountain-wave pattern obtained from in situ and dual-Doppler radar inferred measurements, indicating that biases in the simulations can be attributed in part to the microphysics scheme. The patterns of 18-h accumulated precipitation showed that the model correctly simulated the bulk of the precipitation to accumulate along the coastal mountains and along the windward slope of the Cascades, with reduced precipitation on the lee side of the crest. However, both the 4- and 1-km simulations exhibited a general overprediction of precipitation quantities. The model also exhibited a distinct bias toward overprediction of the snow mass concentration aloft and underprediction of the mass and vertical extent of the pockets of cloud liquid water on the windward side of the Cascades. Nevertheless, the overall spatial distribution of the hydrometeor fields was simulated realistically, including the mean-mass particle diameters for each category and the observed trend of larger snow sizes to be located at lower altitudes.

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Stéphane Bélair, Jocelyn Mailhot, J. Walter Strapp, and J. Ian MacPherson

Abstract

In this study, the ability of a turbulent kinetic energy (TKE)–based boundary layer scheme to reproduce the rapid evolution of the planetary boundary layer (PBL) observed during two clear convective days is examined together with the impact of including nonlocal features in the boundary layer scheme. The two cases are chosen from the Montreal-96 Experiment on Regional Mixing and Ozone (MERMOZ): one is characterized by strong buoyancy, a strong capping inversion, and weak vertical wind shear; the other displays moderate buoyancy, a weaker subsidence inversion, and significant wind shear near the PBL top. With the original local version of the turbulence scheme, the model reproduces the vertical structures and turbulent quantities observed in the well-developed boundary layer for the first case. For the second case, the model fails to reproduce the rapid evolution of the boundary layer even though the TKE and sensible heat fluxes are greatly overpredicted.

Some nonlocal aspects of the turbulence scheme are tested for these two cases. Inclusion of nonlocal (countergradient) terms in the vertical diffusivity equation has little impact on the simulated PBL. In contrast, alternative formulations of the turbulent length scales that follow the strategy proposed by Bougeault and Lacarrère have a greater influence. With the new turbulent lengths, entrainment at the top of the boundary layer is enhanced so that the depth of the well-mixed layer is much larger compared to that of the local simulations even though the turbulent sensible heat fluxes are smaller. Comparison with observations reveals, however, that the inclusion of these modifications does not improve all aspects of the simulation. To improve the performance and reduce somewhat the arbitrariness in the Bougeault–Lacarrère technique, a relationship between the two turbulent length scales (mixing and dissipation) used in the turbulence scheme is proposed. It is shown that, in addition to reducing the sensitivity of the results to the particular formulations, the simulated boundary layer agrees better with observations.

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R. Lalbeharry, J. Mailhot, S. Desjardins, and L. Wilson

Abstract

A coupled atmospheric and ocean wave system has been developed to study the impact of changes of surface roughness length induced by ocean waves. A two-way coupling between a mesoscale atmospheric model, MC2, and an oceanic wave model, a regional version of WAM Cycle-4, was designed to ensure consistency in the treatment of the atmospheric boundary layer parameterizations between the two models. Two different approaches, based on the wave age of Smith et al. and the wave-induced stress of Janssen, are used to compute a coupling parameter, called the Charnock parameter, expressed as the nondimensional surface roughness length. The coupling between the two models is accomplished by the use of this parameter, which is a function of sea state, instead of the constant value obtained from empirical studies using the well-known Charnock relation.

The impacts on the atmospheric forecasts are discussed in Part I. In Part II, the ocean wave forecasts resulting from this two-way coupling are discussed for four different real cases. The two approaches are evaluated by comparing ocean wave model outputs obtained from the coupled and uncoupled systems against buoy observations. The coupling has some beneficial impact, especially in areas of extreme sea states. The significant wave heights are reduced in the coupled runs and generally show better agreement with the buoy observations. The impact of the coupling also exhibits some dependence on the intensity of the cyclone development, with larger changes occurring in the case of rapidly deepening storms.

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A. Lemonsu, S. Bélair, J. Mailhot, and S. Leroyer

Abstract

Using the Montreal Urban Snow Experiment (MUSE) 2005 database, surface radiation and energy exchanges are simulated in offline mode with the Town Energy Balance (TEB) and the Interactions between Soil, Biosphere, and Atmosphere (ISBA) parameterizations over a heavily populated residential area of Montreal, Quebec, Canada, during the winter–spring transition period (from March to April 2005). The comparison of simulations with flux measurements indicates that the system performs well when roads and alleys are snow covered. In contrast, the storage heat flux is largely underestimated in favor of the sensible heat flux at the end of the period when snow is melted. An evaluation and an improvement of TEB’s snow parameterization have also been conducted by using snow property measurements taken during intensive observational periods. Snow density, depth, and albedo are correctly simulated by TEB for alleys where snow cover is relatively homogeneous. Results are not as good for the evolution of snow on roads, which is more challenging because of spatial and temporal variability related to human activity. An analysis of the residual term of the energy budget—including contributions of snowmelt, heat storage, and anthropogenic heat—is performed by using modeling results and observations. It is found that snowmelt and anthropogenic heat fluxes are reasonably well represented by TEB–ISBA, whereas storage heat flux is underestimated.

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J. A. Milbrandt, M. K. Yau, J. Mailhot, S. Bélair, and R. McTaggart-Cowan

Abstract

This is the second in a series of papers examining the behavior of the Milbrandt–Yau multimoment bulk microphysics scheme for the simulation of the 13–14 December 2001 case of orographically enhanced precipitation observed during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) experiment. The sensitivity to the number of predicted moments of the hydrometeor size spectra in the bulk scheme was investigated. The triple-moment control simulations presented in were rerun using double- and single-moment configurations of the multimoment scheme as well the single-moment Kong–Yau scheme. Comparisons of total precipitation and in-cloud hydrometeor mass contents were made between the simulations and observations, with the focus on a 2-h quasi-steady period of heavy stratiform precipitation. The double- and triple-moment simulations were similar; both had realistic precipitation fields, though generally overpredicted in quantity, and had overprediction of snow mass and an underprediction of cloud water aloft. Switching from the triple- to single-moment configuration resulted in a simulation with a precipitation pattern shifted upwind and with a larger positive bias, but with hydrometeor mass fields that corresponded more closely to the observations. Changing the particular single-moment scheme used had a greater impact than changing the number of moments predicted in the same scheme, with the Kong–Yau simulations greatly overpredicting the total precipitation in the lee side of the mountain crest and producing too much snow aloft. Further sensitivity tests indicated that the leeside overprediction in the Kong–Yau runs was most likely due to the combination of the absence of the latent heat effect term in the diffusional growth rate for snow combined with the assumption of instantaneous snow melting in the scheme.

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