High-resolution Nonhydrostatic Model Research Articles

Modulation of internal solitary waves by the Kuroshio in the northern South China Sea

Internal solitary waves (ISWs) in the South China Sea (SCS) are considerably modulated by the background currents. In this study, a three-dimensional high-resolution non-hydrostatic model is configured to investigate how the Kuroshio influences the generation and evolution of ISWs in the northern SCS. Three runs are conducted, including one control experiment without the Kuroshio and two sensitivity experiments with the Kuroshio in different paths. In the Luzon Strait (LS), the Kuroshio reduces the westward baroclinic energy flux radiated into the SCS, resulting in weakened ISWs. In the SCS basin, the background currents further refract the ISWs. With the leaping Kuroshio, the A-waves have longer crest lines but lower amplitudes compared with those in the control run. In contrast, the B-waves are less affected by the leaping Kuroshio. In the presence of looping Kuroshio, the wave refraction caused by the intrusion currents in the SCS basin results in the weakest amplitudes and energy but the widest crest lines of ISWs. Moreover, the energy of the A-waves exhibits double-peak structure along the crest lines. The crest lines of the B-waves extend to 19.5° N, which are more south than those in summer. These results highlight the importance of the Kuroshio on the 3D features of ISWs in the SCS.

On the resonant triad interaction over mid-ocean ridges

Resonant triad interaction is an important mechanism via which the energy of internal tides (ITs) is dissipated. In this study, based on a two-dimensional high-resolution non-hydrostatic model, resonant triad interaction over mid-ocean ridges and corresponding energetics are investigated. Impacts of topographic criticality (the ratio of topographic slope to internal wave slope) on resonant triad interaction are examined by a series of simulations. Results indicate that the topographic criticality not only affects the occurrence of resonant triad interaction but also modulates the frequencies of generated subharmonic waves. Near-critical and supercritical topographies are beneficial to the occurrence of resonant triad interaction with enhanced energy production and dissipation. The impacts of topographic criticality are modulated by latitude. At the critical latitude where local Coriolis frequency is equal to half of the tidal frequency, subharmonic waves are intensified over critical and subcritical topographies, but their frequencies are fixed to local Coriolis frequency. In contrast, resonant triad interaction is suppressed poleward of the critical latitude despite the topographic criticality. Although the energy of subharmonic waves is less than a quarter of the total energy, they contribute to 50% of the total energy dissipation on average. Given that global ITs are dominated by the M2 and most of the M2 ITs are generated upon mid-ocean ridges equatorward of the critical latitude, a remarkable proportion of global IT energy may be dissipated through resonant triad interaction. Therefore, the contribution of resonant triad interaction should be taken into consideration when developing the parameterization of IT-driven mixing.

The Piedmont flood of November 1994: a testbed of forecasting capabilities of the CNR-ISAC meteorological model suite.

The celebration of the 25th anniversary of the Piedmont flood was organized by the University of Piemonte Orientale in Alessandria, a town that was severely affected in November 1994. It has been an opportunity to reexamine the meteorological event and assess the potential of CNR-ISAC models, after more than 20 years of development, to accurately simulate the heavy precipitation at different lead times. The predictability of this extreme event has been studied on a wide range of space and time scales, from subseasonal to convection resolving, using a variety of model setups and initial conditions. The subseasonal experiment produces a precipitation anomaly that, even if underestimated, indicates some predictability beyond the second forecast week. At shorter ranges, results indicate that there is a consistent improvement in the precipitation forecast going from low-resolution hydrostatic to high-resolution nonhydrostatic models. It is only at very high resolution (500 m) that the convective activity extending to the north of the divide line of the Ligurian Apennines, which was responsible for the flood of the Tanaro River, is predicted with an intensity comparable with rain gauge observations. The main mesoscale phenomena that played a role in the different phases of the event have been also identified.

Updraft Constraints on Entrainment: Insights from Amazonian Deep Convection

AbstractMixing of environmental air into clouds, or entrainment, has been identified as a major contributor to erroneous climate predictions made by modern comprehensive climate and numerical weather prediction models. Despite receiving extensive attention, the ad hoc treatment of this convective-scale process in global models remains poor. On the other hand, while limited-area high-resolution nonhydrostatic models can directly resolve entrainment, their sensitivity to model resolution, especially with the lack of benchmark mass flux observations, limits their applicability. Here, the dataset from the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign focusing on radar retrievals of convective updraft vertical velocities is used with the aid of cloud-resolving model simulations of four deep convective events over the Amazon to provide insights into entrainment. Entrainment and detrainment are diagnosed from the model simulations by applying the mass continuity equation over cloud volumes, in which grid cells are identified by some thresholds of updraft vertical velocity and cloud condensates, and accounting for the sources and sinks of the air mass. Entrainment is then defined as the environmental air intruding into convective cores causing cloud volume to shrink, while detrainment is defined as cloudy grid cells departing the convective core and causing cloud volume to expand. It is found that the diagnosed entrainment from the simulated convective events is strongly correlated to the inverse of the updraft vertical velocities in convective cores, which enables a more robust estimation of the mixing time scale. This highlights the need for improved observational capabilities for sampling updraft velocities across diverse geographic and cloud conditions. Evaluation of a number of assumptions used to represent entrainment in parameterization schemes is also presented, as contrasted against the diagnosed one.

The Governing Dynamics of the Secondary Eyewall Formation of Typhoon Sinlaku (2008)

Abstract Through successful convection-permitting simulations of Typhoon Sinlaku (2008) using a high-resolution nonhydrostatic model, this study examines the role of peripheral convection in the storm's secondary eyewall formation (SEF) and its eyewall replacement cycle (ERC). The study demonstrates that before SEF the simulated storm intensifies via an expansion of the tangential winds and an increase in the boundary layer inflow, which are accompanied by peripheral convective cells outside the primary eyewall. These convective cells, which initially formed in the outer rainbands under favorable environmental conditions and move in an inward spiral, play a crucial role in the formation of the secondary eyewall. It is hypothesized that SEF and ERC ultimately arise from the convective heating released from the inward-moving rainbands, the balanced response in the transverse circulation, and the unbalanced dynamics in the atmospheric boundary layer, along with the positive feedback between these processes.

Cold-Season Thunderstorms in Finland and Their Effect on Aviation Safety

A total of 13 commercial airplanes were struck by lightning in October (10 in 1 day) and December (3 on 3 separate days) 2011 in the main Finnish Helsinki–Vantaa airport. The number of lightning-struck airplanes is extremely large, considering the time of year and the small number of flashes by the storms. This paper indicates the characteristics of these cases regarding the synoptic situation as well as their forecasting. There were remarkable differences in the operational models; the high-resolution nonhydrostatic model was superior in predicting the convective nature of the event compared to the coarser-resolution hydrostatic model. The interview of the pilots of the struck airplanes shows that the pilots did not receive detailed information to avoid the situation; also, the lightning strike affected the pilots, even causing temporary loss of sight and hearing. Luckily, no fatalities or severe damage to the airplanes occurred. The most interesting case is 19 October 2011; during this single day, a total of 10 airplanes were struck. The analysis suggests that a major cause for the large number of struck airplanes is that the planes took off directly into the convective core of the storm and the planes initialized the flashes themselves. However, the time of the year, the near position of the storm area relative to the takeoff path, and the necessity to use only a certain takeoff path because of the direction of wind makes the convective scenario difficult to predict and avoid. The pilots have expressed interest in receiving training for these cold-season thunderstorms.

Reproducibility of Maximum Daily Precipitation Amount over Japan by a High-resolution Non-hydrostatic Model

In order to study changes in the regional climate in the vicinity of Japan during the summer rainy season due to global warming, preliminary experiments by a semi-cloud resolving non-hydrostatic model with a horizontal resolution of 5 km (NHM-5km) are conducted from June to October between 2002 and 2006 using 20-km horizontal grid operational regional analysis data of Japan Meteorological Agency (JMA) as the initial and boundary conditions.The total precipitation amount and appearance frequency for daily precipitation amount simulated by the NHM-5km show notable agreement with those of the surface observation data of Automated Meteorological Data Acquisition System (AMeDAS) of JMA. The temporal and spatial characteristics of maximum daily precipitation amounts (MDPs) from June to October also agree well with the observational results. The regional largest values among MDPs (R-MDPs) for 6 regions of the Japanese Islands are also estimated for the simulation results of the nearest grid points for each AMeDAS station and the AMeDAS observations. Those comparisons conclude the high performance of the NHM-5km for the reproducibility of MDPs and R-MDPs, which are highly related to extreme events.

The hypohydrostatic rescaling and its impacts on modeling of atmospheric convection

The atmospheric circulation spans a wide range of spatial scales, including the planetary scale (∼10,000 km), synoptic scale (∼2,000 km), mesoscale (∼200 km), and convective scales (<20 km). The wide scale separation between convective motions, responsible for the vertical energy transport, and the planetary circulation, responsible for the meridional energy transport, has prevented explicit representation of convective motions in global atmospheric models. Kuang et al. (Geophys. Res. Lett. 32: L02809, 2005) have suggested a way to circumvent this limitation through a rescaling that they refer to as Diabatic Acceleration and REscaling (DARE). We focus here on a modified version of the procedure that we refer to as hypohydrostatic rescaling. These two strategies are equivalent for inviscid and adiabatic flow in the traditional meteorological setting in which the vertical component of the Coriolis acceleration is ignored, but they differ when atmospheric physics is taken into account. It is argued here that, while the hypohydrostatic rescaling preserves the dynamics of the planetary scale circulation, it increases the horizontal scale of convective motions. This drastically reduces the computational cost for explicit simulation of hypohydrostatic convection in a global atmospheric model. A key question is whether explicit simulations of hypohydrostatic convection could offer a valid alternative to convective parameterization in global models. To do so, radiative-convective equilibrium is simulated with a high-resolution non-hydrostatic model using different model resolutions and values of the rescaling param- eter. When the behavior of hypohydrostatic convection is compared with coarse-resolution simulations of convection, the latter set of simulations reproduce more accurately the result from a reference high-resolution simulation. This is particularly true for the convective velocity and cloud ice distributions. Scaling arguments show that hypohydrostatic rescaling increases the convective overturning time. In particular, this convective slowdown associated with the hypohydrostatic rescaling is more significant than the slowdown resulting from

A Radar Simulator for High-Resolution Nonhydrostatic Models

Abstract A full radar simulator for high-resolution (1–5 km) nonhydrostatic models has been developed within the research nonhydrostatic mesoscale atmospheric (Meso-NH) model. This simulator is made up of building blocks, each of which describes a particular physical process (scattering, beam bending, etc.). For each of these blocks, several formulations have been implemented. For instance, the radar simulator offers the possibility to choose among Rayleigh, Rayleigh–Gans, Mie, or T-matrix scattering methods, and beam bending can be derived from an effective earth radius or can depend on the vertical gradient of the refractive index of air. Moreover, the radar simulator is fully consistent with the microphysical parameterizations used by the atmospheric numerical model. Sensitivity experiments were carried out using different configurations for the simulator. They permitted the specification of an observation operator for assimilation of radar reflectivities by high-resolution nonhydrostatic numerical weather prediction systems, as well as for their validation. A study of the flash flood of 8–9 September 2002 in southeastern France, which was well documented with volumetric data from an S-band radar, serves to illustrate the capabilities of the radar simulator as a validation tool for a mesoscale model.

Performance of Long-Term Integrations of the Japan Meteorological Agency Nonhydrostatic Model Using the Spectral Boundary Coupling Method

Abstract The spectral boundary coupling (SBC) method, which is an approach used to couple a limited-area model with a large-scale model, was introduced into a nonhydrostatic model. To investigate whether the SBC method works well in a long-term integration of a high-resolution nonhydrostatic model, two numerical experiments were conducted with a model having a horizontal grid interval of 5 km. In one experiment, the SBC method was employed, while it was not in the other experiment. The time integration in both experiments was over a 40-day period. The nonhydrostatic model was nested into objectively analyzed fields, instead of the forecasts from an extended-area model. Predicted patterns of sea level pressure and precipitation were compared with objective analyses, and data provided by the Global Precipitation Climatology Project (GPCP), respectively. The predicted rainfall amounts and surface temperature over the Japanese islands were statistically evaluated, making use of the analyzed rainfall and surface data observed by the Japan Meteorological Agency (JMA). All results examined in the present study exhibited better performances with use of the SBC method than those without the SBC method. It was found that the SBC method was highly useful in long-term simulations by a high-resolution nonhydrostatic model.

Inner-Core Structure of Typhoon Rusa (2002) Simulated by a High-Resolution Nonhydrostatic Model

Inner-Core Structure of Typhoon Rusa (2002) Simulated by a High-Resolution Nonhydrostatic Model

Comparison of methods for area-averaging surface energy fluxes over heterogeneous land surfaces using high-resolution non-hydrostatic simulations

The quantification of subgrid land surface heterogeneity effects on the scale of climate and numerical weather prediction models is of vital interest for the energy budget of the atmospheric boundary layer and for the atmospheric branch of the hydrological cycle. This paper focuses on heterogeneity effects for the exchange processes between land surfaces and the atmosphere. The results are based on high-resolution non-hydrostatic model simulations for the LITFASS area near Berlin. This area represents a highly heterogeneous landscape of 20 × 20 km 2 around the Meteorological Observatory Lindenberg of the German Weather Service (DWD). Model simulations were carried out using the non-hydrostatic model FOOT3DK of the University of K¨ oln with resolutions of 1 km and 250 m. The performance of different area-averaging methods for the turbulent surface fluxes was tested for the LITFASS area, namely the aggregation, mosaic and tile methods. For one tile method (station-tile), the experimental setup of the surface energy balance stations of the LITFASS98 experiment was investigated. Two different simulation types are considered: (1) realistic topography and idealized synoptic forcing; (2) realistic topography and realistic synoptic forcing for LITFASS98 cases. A double one-way nesting procedure is used for nesting FOOT3DK in ‘Lokalmodell’ of the DWD. The mosaic method shows good results, if the wind speed is sufficiently high. During weak-wind convective conditions, errors are particularly large for the latent heat flux on the 20 × 20 km 2 scale. The aggregation method yields generally higher errors than the mosaic method, which even increase for higher wind speeds. The main reason is the strong surface heterogeneity associated with the lakes and forests in the LITFASS area. The main uncertainty of the station-tile method is the knowledge of the area coverage in combination with the representativity of the stations for the land-use type and surface conditions. The results of this study lead to the recommendation to use a mosaic approach or at least a tile approach for downscaling fluxes over heterogeneous surfaces in mesoscale and regional climate models. Copyright  2005 Royal Meteorological Society.

An Evaluation of Atmospheric Models for GPS data Retrieval by Output from a Numerical Weather Model

Atmospheric delays in GPS processing are estimated by fitting parameters of atmospheric models to the observed delays. GPS-related error sources include multipath effects and phase center variations, as well as uncertainties in atmospheric models. An atmospheric model must accurately capture the delay distribution if errors are to be minimized. Large GPS position errors occurred on 7 March 1997 on the Izu Peninsula and at Hatsu-shima, about 100 km southwest of Tokyo, concomitant with a mountain lee wave. The coinciding of the large position errors and the mountain lee wave suggests that small-scale fluctuations in water vapor and air density associated with lee waves could cause large position errors. This study used a high-resolution non-hydrostatic model to simulate the mountain lee wave. Slant delays for the GPS satellites were calculated from the reproduced water vapor and air density fields using a ray-tracing method. The atmospheric delays are obtained by fitting the atmospheric models to the reproduced slant delays, instead of the actual delay data. The atmospheric models were evaluated by determining the difference in position error between the reproduced delays and the model-fitted delays. The mountain lee wave case was used to evaluate three different atmospheric models. The first, the “constant model”, has only zenith delay as an unknown parameter. Large position errors occurred when the constant model was used to fit the data in this case. The “linear gradient model”, adds two horizontal gradient parameters to the zenith parameter, and yielded significantly reduced horizontal position errors. Horizontal and vertical position errors were reduced further with a “second order model”, which adds second-order terms. Evaluation of the atmospheric models using the mountain lee wave case indicated that 1) the linear gradient model cannot express complicated atmospheric disturbances; and, 2) a second-order model reduces horizontal and vertical position errors.

Turbulent Mixing in the Red Sea Outflow Plume from a High-Resolution Nonhydrostatic Model

Abstract Given the motivation that overflow processes, which supply source waters for most of the deep and intermediate water masses in the ocean, pose significant numerical and dynamical challenges for ocean general circulation models, an intercomparison study is conducted between field data collected in the Red Sea overflow and a high-resolution, nonhydrostatic process model. The investigation is focused on the part of the outflow that flows along a long narrow channel, referred to as the “northern channel,” that naturally restricts motion in the lateral direction such that the use of a two-dimensional model provides a reasonable approximation to the dynamics. This channel carries about two-thirds of the total Red Sea overflow transport, after the overflow splits into two branches in the western Gulf of Aden. The evolution of the overflow in the numerical simulations can be characterized in two phases: the first phase is highly time dependent, during which the density front associated with the overflow propagates along the channel. The second phase corresponds to that of a statistically steady state. The primary accomplishment of this study is that the model adequately captures the general characteristics of the system: (i) the gradual thickening of the overflow with downstream distance, (ii) the advection of high salinity and temperature signals at the bottom along the channel with little dilution, and (iii) ambient water masses sandwiched between the overflow and surface mixed layer. To quantify mixing of the overflow with the ambient water masses, an entrainment parameter is determined from the transport increase along the slope and is expressed explicitly as a function of mean slope angle. Bulk Richardson numbers are estimated both from data and model and are related to the entrainment parameter. The range of entrainment parameter and its functional dependence on bulk Richardson number in this study are found to be in reasonable agreement with those reported from various laboratory experiments and that based on measurements of the Mediterranean overflow. The results reveal a complex dynamical interaction between shear-induced mixing and internal waves and illustrate the high computational and modeling requirements for numerical simulation of overflows to capture (at least in part) turbulent transports explicitly.

Turbulent Mixing in the Red Sea Outflow Plume from a High-Resolution Nonhydrostatic Model

Given the motivation that overflow processes, which supply source waters for most of the deep and intermediate water masses in the ocean, pose significant numerical and dynamical challenges for ocean general circulation models, an intercomparison study is conducted between field data collected in the Red Sea overflow and a high-resolution, nonhydrostatic process model. The investigation is focused on the part of the outflow that flows along a long narrow channel, referred to as the “northern channel,” that naturally restricts motion in the lateral direction such that the use of a two-dimensional model provides a reasonable approximation to the dynamics. This channel carries about two-thirds of the total Red Sea overflow transport, after the overflow splits into two branches in the western Gulf of Aden. The evolution of the overflow in the numerical simulations can be characterized in two phases: the first phase is highly time dependent, during which the density front associated with the overflow propagates along the channel. The second phase corresponds to that of a statistically steady state. The primary accomplishment of this study is that the model adequately captures the general characteristics of the system: (i) the gradual thickening of the overflow with downstream distance, (ii) the advection of high salinity and temperature signals at the bottom along the channel with little dilution, and (iii) ambient water masses sandwiched between the overflow and surface mixed layer. To quantify mixing of the overflow with the ambient water masses, an entrainment parameter is determined from the transport increase along the slope and is expressed explicitly as a function of mean slope angle. Bulk Richardson numbers are estimated both from data and model and are related to the entrainment parameter. The range of entrainment parameter and its functional dependence on bulk Richardson number in this study are found to be in reasonable agreement with those reported from various laboratory experiments and that based on measurements of the Mediterranean overflow. The results reveal a complex dynamical interaction between shear-induced mixing and internal waves and illustrate the high computational and modeling requirements for numerical simulation of overflows to capture (at least in part) turbulent transports explicitly.

Simulation of the katabatic flow near the Greenland ice margin using a high-resolution nonhydrostatic model

Simulations of the katabatic wind system in the stable boundary layer near the margin of the Greenland ice sheet are presented using the high-resolution (2.8 km) non-hydrostatic Lokal-Modell (LM) of the Deutscher Wetterdienst (DWD). The LM is nested into numerical forecasts provided by the hydrostatic Norwegian Limited Area Model (NORLAM). The area of Kangerlussuaq in West Greenland was selected as the LM domain, since comprehensive measurements of the katabatic wind structure are available for that area (aircraft and AWS). One focus of the LM simulations is to study processes in the transition region between the ice sheet and the tundra, which are not captured by previous model studies because of the micro-scale complexity of the topography. The first simulation is performed for an idealized case using a wintertime atmosphere at rest as initial conditions. For this situation lacking synoptic forcing, a well-developed katabatic wind system is simulated over the inland ice, while a system of fjord winds is simulated in the tundra area. In the second simulation study a realistic case of strong katabatic winds (exceeding 20 m s -1 ) is investigated, and AWS data and vertical profiles from aircraft measurements are used for the model validation. For this real case, the simulation results are in good agreement with instrumented aircraft and automatic weather station observations. While NORLAM fails to reproduce the wind field in the tundra region, the LM simulations show again a pattern of fjord winds and a complex wind field near the ice margin, which agrees with the observed atmospheric state. Because the intensity and the three-dimensional structure of the katabatic winds depend on the specific synoptic environment, the success of the LM simulations is also dependent on the quality of the initial and boundary conditions, i.e. the forecasts of synoptic and mesoscale processes by NORLAM.

Multicell Squall-Line Structure as a Manifestation of Vertically Trapped Gravity Waves

Abstract Two-dimensional and three-dimensional simulations of a midlatitude squall line with a high-resolution non-hydrostatic model suggest that the multicellular structure of the storm may be associated with gravity waves generated by convection. Time-lapse display of model output demonstrates that the commonly described “cut-off” process is actually a gravity wave phenomenon. The convective cells arise as gravity waves, which are forced by continuous strong low-level convergence at the storm's gust front. The waves propagate to both sides of the gust front. The stronger westward (front to rear) mode dominates at the mature stage of the squall line. Continuous low-level updraft is generated at the nose of the cold pool, which propagates at the speed of a density current. Updraft cells periodically break away from this persistent low-level gust-front updraft and move at phase speeds of their associated gravity waves, not at the surrounding airflow speeds as implied by the traditional multicell model. Lin...

Anelastic Modeling of Explosive Cyclogenesis

Abstract A three-dimensional anelastic model is used to perform simulations of explosive cyclones. The major goals are 1) to investigate the importance of horizontal resolution on the simulation of mesoscale features during explosive cyclogenesis using a very high resolution nonhydrostatic model and 2) to determine the impact of the destabilization of the lower troposphere by strong surface fluxes on the development of the storm. The results from six experiments are presented. The main conclusions are: 1) By using analytic initial conditions based on typical wintertime conditions prior to explosive cyclogenesis, it is possible to obtain very realistic simulations of rapid cyclogenesis. 2) The use of high horizontal resolution is important in simulating the mesoscale features of rapidly deepening cyclones. In particular, the structure of the intense warm front observed ahead of the cyclone is very sensitive to changes in horizontal resolution. 3) The destabilization of the lower troposphere prior to the pe...