Changes In Horizontal Resolution Research Articles

Effect of the Grell‐Freitas Deep Convection Scheme in Quasi‐Uniform and Variable‐Resolution Aquaplanet CAM Simulations

AbstractThe Grell‐Freitas deep convection scheme (GF) has been implemented in the Community Atmosphere Model (CAM) with the nonhydrostatic Model for Prediction Across Scales dynamical core. Aquaplanet simulations have been performed with a global quasi‐uniform resolution mesh with ∼120 km grid spacing and a variable resolution mesh employing a circular refined region with ∼30 km grid spacing centering at the equator and ∼120 km elsewhere. GF produces overall patterns of the general circulations comparable to those of other aquaplanet experiments, including those produced using the default Zhang‐McFarlane convection scheme (ZM) in CAM Version 5.4. GF alleviates the overestimated occurrence of weak intensity precipitation characteristic of ZM due to GF's different closure methods and deep‐convection trigger conditions. Compared to ZM, the GF characteristics of the equatorial Kelvin waves are closer to observations. GF also better simulates precipitation at variable resolution, with no unrealistic wave distortion by the refinement. Overall, GF shows less sensitivity of convective precipitation and heating/moistening tendencies to changes in horizontal resolution compared to ZM. However, vertically overlapped total cloud fraction simulated with GF in the tropics is more sensitive to the resolution than ZM due to the larger resolution‐sensitivity of high‐level cloud fraction in the tropics.

Ensemble size: How suboptimal is less than infinity?

Ensemble forecasts are the method of choice in numerical weather prediction (NWP) to generate probabilistic forecasts. The number of members in an ensemble is an important factor in determining how well a probability distribution of a weather‐related variable can be estimated. Having only a finite number of members reduces the average skill such a probabilistic forecast can have. Increasing ensemble size is therefore desirable; however, ensemble size is also proportional to the computational cost. Having a small ensemble size limits the cost and makes other improvements, such as increases in spatial resolution, feasible.This article examines how average skill measures with metrics such as the continuous ranked probability score, the quantile score, and the Dawid–Sebastiani score converge with ensemble size. A numerical experiment with a 200 member ensemble using the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) model at a resolution of 29 km and a forecast range of 15 days provides data to compare the convergence of probabilistic skill in a current NWP system with theoretical expectations derived for perfectly reliable ensembles with exchangeable members.Results in the first part of the article can help users of operational NWP ensemble forecasts formulate their minimum requirement in terms of ensemble size. In the second part, requirements for scientists who test changes to NWP systems are examined. Using proper scores and fair scores, it is explored whether testing changes in the ensemble forecasts can be meaningful with fewer members than in the operational configuration. Results are based on medium‐range numerical experiments with 50 members. Two experiments test the activation of a representation of model uncertainty and three other experiments test changes in horizontal resolution from 29 to 18 km and from 29 to 45 km.

Does convection-permitting simulate better rainfall distribution and extreme over Guinean coast and surroundings?

This research work focuses on the problem of climate simulation of rainfall over West Africa and particularly over coastal countries of the Gulf of Guinea by Regional Climate Models. The sensitivities of Weather Research and Forecasting (WRF) Model were tested for changes in horizontal resolution (convection permitting versus parameterized) on the replication of West African monsoon for the year 2014. Sensitivity test was also performed for response of rainfall to changes in microphysics (MP) and planetary boundary layer (PBL) schemes. Generally, the result shows that WRF is able to replicates rainfall distribution with an adequate representation of the dynamical features of West African monsoon system. The high-resolution (wrf-4km) shows dry bias along the coast of the Gulf of Guinea but generally outperforms wrf-24km run especially in replication of the extreme rainfall distribution. The dry bias along the coastal area is suggested to not to be only related to convection but mostly to microphysics and PBL parameterisation schemes. Differences were noticed between the dynamics of WRF and ERA-interim outputs despite the use of spectral nudging in the experiment which then suggest strong interactions between scales. These differences were observed to be restricted mainly to the low-layer monsoon flow in JJA. Both runs at 24 and 4 km hardly simulate the typical diurnal distribution of rainfall. The sensitivity of WRF to MP (only sophisticated MP were tested) and PBL reveals a stronger impact of PBL than MP on rainfall distribution and the most significant added value over the Guinean coast and surroundings area was provided by the configurations using non-local PBL scheme (as ACM2). The changes in MP and PBL schemes in general seem to have less effect on the explicit runs (wrf-4km) in the replication of the rainfall over the Gulf of Guinea and the surroundings seaboard.

Regional Climate Simulations With the Community Earth System Model

AbstractThe spectral element (SE) variable‐resolution (VR) mesh dynamical core is tested in developmental versions of the Community Earth System Model version 2 (CESM2). The SE dynamical core is tested in baroclinic wave, aquaplanet and full physics configurations to evaluate variable‐resolution simulations against uniform high and uniform low‐resolution simulations. Different physical parameterization suites are also evaluated to gauge their sensitivity to resolution. Dry dynamical core variable‐resolution cases compare well to high‐resolution tests. More recent versions of the atmospheric physics, including cloud schemes for CESM2, are less sensitive to changes in horizontal resolution. Most of the sensitivity is due to sensitivity to time step and interactions between deep convection and large‐scale condensation, which is expected from the closure methods. The resulting full physics SE‐VR model produces a similar climate to the global low‐resolution mesh and similar high‐frequency statistics in the high‐resolution region. The SE‐VR simulations are able to reproduce uniform high‐resolution results, making them an effective tool for regional climate simulations at lower computational cost. Some biases are reduced (orographic precipitation in Western United States), but biases do not necessarily go away at high resolution (e.g., summertime surface temperatures). Variable‐resolution grids are a viable alternative to traditional nesting for regional climate studies and are available in CESM2.

Sensitivity of resolved and parametrized surface drag to changes in resolution and parametrization

The relative contributions of resolved and parametrized surface drag towards balancing the atmospheric angular momentum flux convergence (AMFC) and their sensitivity to horizontal resolution and parametrization are investigated in an atmospheric model. This sensitivity can be difficult to elucidate in free‐running climate models, in which the AMFC varies with changing climatologies and, as a result, the relative contributions of surface terms balancing the AMFC also vary. While the sensitivity question has previously been addressed using short‐range forecasts, we demonstrate that a nudging framework is an effective method for constraining the AMFC. The Met Office Unified Model is integrated at three horizontal resolutions ranging from 130 (N96) to 25 km (N512), while relaxing the model's wind and temperature fields towards the ERA‐Interim reanalysis within the altitude regions of maximum AMFC. This method is validated against short‐range forecasts and good agreement is found. These experiments are then used to assess the fidelity of the exchange between parametrized and resolved orographic torques with changes in horizontal resolution. Although the parametrized orographic torque reduces substantially with increasing horizontal resolution, there is little change in resolved orographic torque over 20–50°N. The tendencies produced by the nudging routine indicate that the additional drag at lower horizontal resolution is excessive. When parametrized orographic blocking is removed at the coarsest of these resolutions, there is a lack of compensation, and even compensation of the opposite sense, by the boundary layer and resolved torques, which is particularly pronounced over 20–50°N. This study demonstrates that there is strong sensitivity in the behaviour of the resolved and parametrized surface drag over this region.

ADS-R conflict detection and resolution algorithm for general aviation in low altitude flights

The National Airspace System (NAS) will change dramatically when the demand of airspace used by general aviation (GA) as the Small Aircraft Transport System (SATS) provides alternative for air transport. As a result, collision avoidance for GA will become the most concern to aviation safety. The unmanned aerial vehicle (UAV) has rapidly growing into civilian applications. Low altitude airspace becomes more crowded. Varieties of conflict detection and resolution (CD&R) were required under two and three maneuvering dimensions for GA and UAV. This paper focuses on efficient collision avoidance logic for GA and UAV based on ADS-R for NextGen. Heading change of horizontal resolution is more suitable for small GA or UAV flying in low altitude in the proposed CD&R model. Simulations using real ultralight flight data are tested to verify the performance and efficiency of the proposed algorithm.

Hydrostatic and non-hydrostatic simulations of dense waters cascading off a shelf: The East Greenland case

The cascade of dense waters of the Southeast Greenland shelf during summer 2003 is investigated with two very high-resolution (0.5-km) simulations. The first simulation is non-hydrostatic. The second simulation is hydrostatic and about 3.75 times less expensive. Both simulations are compared to a 2-km hydrostatic run, about 31 times less expensive than the 0.5km non-hydrostatic case.Time-averaged volume transport values for deep waters are insensitive to the changes in horizontal resolution and vertical momentum dynamics. By this metric, both lateral stirring and vertical shear instabilities associated with the cascading process are accurately parameterized by the turbulent schemes used at 2-km horizontal resolution. All runs compare well with observations and confirm that the cascade is mainly driven by cyclones which are linked to dense overflow boluses at depth. The passage of the cyclones is also associated with the generation of internal gravity waves (IGWs) near the shelf.Surface fields and kinetic energy spectra do not differ significantly between the runs for horizontal scales L>30km. Complex structures emerge and the spectra flatten at scales L<30km in the 0.5-km runs. In the non-hydrostatic case, additional energy is found in the vertical kinetic energy spectra at depth in the 2km<L<10km range and with frequencies around 7 times the inertial frequency. This enhancement is missing in both hydrostatic runs and is here argued to be due to the different IGW evolution and propagation offshore. The different IGW behavior in the non-hydrostatic case has strong implications for the energetics: compared to the 2-km case, the baroclinic conversion term and vertical kinetic energy are about 1.4 and at least 34 times larger, respectively. This indicates that the energy transfer from the geostrophic eddy field to IGWs and their propagation away from the continental slope is not properly represented in the hydrostatic runs.

Verification of the ECMWF ensemble forecasts of wind speed against analyses and observations

AbstractA framework for the verification of ensemble forecasts of near‐surface wind speed is described. It is based on existing scores and diagnostic tools, though considering observations from synoptic stations as reference instead of the analysis. This approach is motivated by the idea of having a user‐oriented view of verification, for instance with the wind power applications in mind. The verification framework is specifically applied to the case of ECMWF ensemble forecasts and over Europe. Dynamic climatologies are derived at the various stations, serving as a benchmark. The impact of observational uncertainty on scores and diagnostic tools is also considered. The interest of this framework is demonstrated from its application to the routine evaluation of ensemble forecasts and to the assessment of the quality improvements brought in by the recent change in horizontal resolution of the ECMWF ensemble prediction system. The most important conclusions cover (1) the generally high skill of these ensemble forecasts of near‐surface wind speed when evaluated at synoptic stations, (2) the noteworthy improvement of scores brought by the change of horizontal resolution, and, (3) the scope for further improvements of reliability and skill of wind speed ensemble forecasts by appropriate post‐processing. Copyright © 2011 Royal Meteorological Society

Response of Arctic 1000 hPa circulation to changes in horizontal resolution and sea ice forcing in the Community Atmospheric Model

Arctic winter surface atmospheric circulation biases inherent to the Community Atmospheric Model version 3 (CAM3) are examined with emphasis on the influence of horizontal resolution (T42, ∼2.8° versus T85, ∼1.4°). Using geopotential height at 1000 hPa (Z1000) to represent surface circulation, the self‐organizing map technique is used to better understand these biases from a synoptic climatology perspective. The previously documented low sea level pressure (SLP) bias over the Beaufort Sea, Canadian Archipelago, Greenland Sea, Norwegian Sea, and northern Europe and high SLP bias over the northern Pacific and Atlantic oceans are shown to result from an underprediction of weak Aleutian and Icelandic low patterns and an overprediction of strong Arctic low patterns, strong Icelandic low patterns, and patterns with pronounced ridges spanning the Arctic Ocean. Resolution‐caused differences are shown to be the result of CAM3 at T85 resolution predicting fewer strong Icelandic lows, North Atlantic lows, and strong high‐pressure ridges across the Arctic relative to CAM3 at T42 resolution. In addition, CAM3 at T85 resolution predicts more strong Arctic low patterns relative to CAM3 at T42 resolution. The ability of CAM3 at both resolutions to resolve mesoscale features such as polar lows near the sea ice edge is also explored. Despite these biases, CAM3 can still be used experimentally to examine atmospheric response to changing climatic conditions, particularly if results are viewed in the framework of a sensitivity study. With this in mind, the impact of future projections of sea ice on winter Arctic circulation is examined at both model resolutions. It is found that weak Arctic low patterns and strong Icelandic low patterns increase in frequency in the winter, while Aleutian/Icelandic low patterns and North Atlantic low/Pacific high patterns decrease in frequency in the winter with future projections of sea ice.

Sensitivity to Horizontal Resolution in the AGCM Simulations of Warm Season Diurnal Cycle of Precipitation over the United States and Northern Mexico

Abstract This study examines the sensitivity of the North American warm season diurnal cycle of precipitation to changes in horizontal resolution in three atmospheric general circulation models, with a primary focus on how the parameterized moist processes respond to improved resolution of topography and associated local/regional circulations on the diurnal time scale. It is found that increasing resolution (from approximately 2° to ½° in latitude–longitude) has a mixed impact on the simulated diurnal cycle of precipitation. Higher resolution generally improves the initiation and downslope propagation of moist convection over the Rockies and the adjacent Great Plains. The propagating signals, however, do not extend beyond the slope region, thereby likely contributing to a dry bias in the Great Plains. Similar improvements in the propagating signals are also found in the diurnal cycle over the North American monsoon region as the models begin to resolve the Gulf of California and the surrounding steep terrain. In general, the phase of the diurnal cycle of precipitation improves with increasing resolution, though not always monotonically. Nevertheless, large errors in both the phase and amplitude of the diurnal cycle in precipitation remain even at the highest resolution considered here. These errors tend to be associated with unrealistically strong coupling of the convection to the surface heating and suggest that improved simulations of the diurnal cycle of precipitation require further improvements in the parameterizations of moist convection processes.

Representation of Northern Hemisphere winter storm tracks in climate models

Northern Hemisphere winter storm tracks are a key element of the winter weather and climate at mid-latitudes. Before projections of climate change are made for these regions, it is necessary to be sure that climate models are able to reproduce the main features of observed storm tracks. The simulated storm tracks are assessed for a variety of Hadley Centre models and are shown to be well modelled on the whole. The atmosphere-only model with the semi-Lagrangian dynamical core produces generally more realistic storm tracks than the model with the Eulerian dynamical core, provided the horizontal resolution is high enough. The two models respond in different ways to changes in horizontal resolution: the model with the semi-Lagrangian dynamical core has much reduced frequency and strength of cyclonic features at lower resolution due to reduced transient eddy kinetic energy. The model with Eulerian dynamical core displays much smaller changes in frequency and strength of features with changes in horizontal resolution, but the location of the storm tracks as well as secondary development are sensitive to resolution. Coupling the atmosphere-only model (with semi-Lagrangian dynamical core) to an ocean model seems to affect the storm tracks largely via errors in the tropical representation. For instance a cold SST bias in the Pacific and a lack of ENSO variability lead to large changes in the Pacific storm track. Extratropical SST biases appear to have a more localised effect on the storm tracks.

CCSM–CAM3 Climate Simulation Sensitivity to Changes in Horizontal Resolution

Abstract The latest version of the Community Climate System Model (CCSM) Community Atmosphere Model version 3 (CAM3) has been released to allow for numerical integration at a variety of horizontal resolutions. One goal of the CAM3 design was to provide comparable large-scale simulation fidelity over a range of horizontal resolutions through modifications to adjustable coefficients in the parameterized treatment of clouds and precipitation. Coefficients are modified to provide similar cloud radiative forcing characteristics for each resolution. Simulations with the CAM3 show robust systematic improvements with higher horizontal resolution for a variety of features, most notably associated with the large-scale dynamical circulation. This paper will focus on simulation differences between the two principal configurations of the CAM3, which differ by a factor of 2 in their horizontal resolution.

Modelling the UK reduced nitrogen budget

Currently, more deposition of reduced nitrogen species to the UK is measured than can be accounted for from the calculated emissions. A newly developed long-range transport model was used to study this budget. Changes in horizontal resolution of model and an improved parameterization of dry deposition of ammonia gave an improved agreement between measured and modelled deposition. However, the model still estimated only 65% of the measured deposition with a UK emissions term of 283 ktonnes N yr−1. Scaling the emissions by factors of × 1.2 and × 1.3 still resulted in the model underestimating of reduced nitrogen deposition. Modelled aerosol ammonium and ammonia gas concentrations for the base-case were compared with observations. The ammonia gas concentrations appear to be modelled reasonably well but the aerosol ammonium concentrations were underestimated by a factor of at least 2. This is an important finding as this indicates that other parameters related to wet deposition simulation are unlikely to be in gross error, implying that the UK emissions term is too small. It was concluded that an emission source term of the order 400 ktonnes N yr−1 is required to reconcile the model with the measurements.

Validation and Sensitivities of Frontal Clouds Simulated by the ECMWF Model

Abstract Clouds simulated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model are composited to derive the typical organization of clouds surrounding a midlatitude baroclinic system. Comparison of this composite of about 200 cyclones with that based on satellite data reveals that the ECMWF model quite accurately simulates the general positioning of clouds relative to a low pressure center. However, the optical depths of the model’s high/low clouds are too small/large relative to the satellite observations, and the model lacks the midlevel topped clouds observed to the west of the surface cold front. Sensitivity studies with the ECMWF model reveal that the error in high-cloud optical depths is more sensitive to the assumptions applied to the ice microphysics than to the inclusion of cloud advection or a change of horizontal resolution from 0.5625° to 1.69° lat. This reflects the fact that in the ECMWF model gravitational settling is the most rapid process controlling the abundance of ic...

The sensitivity of the Martian surface pressure and atmospheric mass budget to various parameters: A comparison between numerical simulations and Viking observations

The sensitivity of the Martian atmospheric circulation to a number of poorly known or strongly varying parameters (surface roughness length, atmospheric optical depth, CO2 ice albedo, and thermal emissivity) is investigated through experiments performed with the Martian version of the atmospheric general circulation model of Laboratoire de Météorologie Dynamique, with a rather coarse horizontal resolution (a grid with 32 points in longitude and 24 points in latitude). The results are evaluated primarily on the basis of comparisons with the surface pressure records of the Viking mission. To that end, the records are decomposed into long‐period seasonal variations due to mass exchange with the polar caps and latitudinal redistribution of mass, and short‐period variations due to transient longitudinally propagating waves. The sensitivity experiments include a 5‐year control simulation and shorter simulations (a little longer than 1 year) performed with “perturbed” parameter values. The main conclusions are that (1) a change of horizontal resolution (twice as many points in each direction) mostly affects the transient waves, (2) surface roughness lengths have a significant impact on the near‐surface wind and, as a matter of consequence, on the latitudinal redistribution of mass, (3) atmospheric dust optical depth has a significant impact on radiative balance and dynamics, and (4) CO2 ice albedo and thermal emissivity strongly influence mass exchange between the atmosphere and the polar caps. In view of this last conclusion, an automatic procedure is implemented through which the albedo and emissivity of each of the two polar caps are determined, together with the total (i.e., including the caps) atmospheric CO2 content, in such a way as to get the closest fit of the model to the Viking pressure measurements.

The impact of horizontal resolution on moist processes in the ECMWF model

Summer and winter climates simulated with the ECMWF (cycle 33) model at spectral scales T21, T42, T63 and T106 are analyzed to determine the impact of changes in horizontal resolution on atmospheric water vapor, clouds, convection, and precipitation. Qualitative changes in many moist processes occur in the transition from T21 to T42, especially in the tropics; at higher resolutions mostly incremental variations from patterns established at T42 result. Large-scale tropical moist processes are simulated more realistically at T21 than at finer resolutions, possibly reflecting a mismatch between the finer-scale dynamics and the scales at which the underlying assumptions of the physical parameterizations apply. Global precipitation increases monotonically with resolution, as a consequence of increasing convection. Global cloud cover, however, decreases in the transition from T21 to T42 due to drying of the tropics, but then increases slightly at finer resolutions. These small global increases are an outcome of compensating changes in different regions: decreases in cloud cover due to drying of the atmosphere at low latitudes are offset by high-latitude increases resulting from enhanced relative humidity associated with an intensifying atmospheric cold bias at finer resolutions.

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...

Interannual characteristics of an 80 km resolution diagnostic Arctic ice–ocean model

Seasonal simulations with large-scale coupled ice–ocean models have reproduced many features of the ice and ocean circulation of the Arctic Ocean and the Greenland and Norwegian seas (e.g. Hibler and Bryan, 1987; Semtner, 1987). However, the crude resolution and high lateral eddy viscosity used by these models prevent the simulation of many of the smaller-scale seasonal features and tend to produce sluggish circulation. Similarly, the use of a single year’s atmospheric forcing prevents the simulation of features on an interannual time-scale. As an initial step towards addressing these issues, an 80 km diagnostic Arctic ice–ocean model is constructed and integrated over a three-year period using daily atmospheric forcing to drive the model. To examine the effect of topographic resolution and eddy viscosity on model results, similar simulations were performed with a 160 km-resolution model. The results of these simulations are compared with one another, with buoy drift in the Arctic Basin, and with observed ice-edge variations. The model results proved most sensitive to changes in horizontal resolution. The 80 km results provided a more realistic and robust circulation in most areas of the Arctic and improved the modelled ice edge in the Barents Sea, while also successfully simulating the interannual variation in the region. Although it performed better than the 160 km model, the 80 km model still produced too large an ice extent in the Greenland Sea. No significant improvement in the ice-edge prediction was observed by varying the lateral eddy viscosity. The results indicate that problems remain in the vertical resolution in shallow regions, in treating penetrative convection, and in the simulation of inflow into the Arctic Basin through the Fram Strait.

Interannual characteristics of an 80 km resolution diagnostic Arctic ice–ocean model

Seasonal simulations with large-scale coupled ice–ocean models have reproduced many features of the ice and ocean circulation of the Arctic Ocean and the Greenland and Norwegian seas (e.g. Hibler and Bryan, 1987; Semtner, 1987). However, the crude resolution and high lateral eddy viscosity used by these models prevent the simulation of many of the smaller-scale seasonal features and tend to produce sluggish circulation. Similarly, the use of a single year’s atmospheric forcing prevents the simulation of features on an interannual time-scale. As an initial step towards addressing these issues, an 80 km diagnostic Arctic ice–ocean model is constructed and integrated over a three-year period using daily atmospheric forcing to drive the model. To examine the effect of topographic resolution and eddy viscosity on model results, similar simulations were performed with a 160 km-resolution model. The results of these simulations are compared with one another, with buoy drift in the Arctic Basin, and with observed ice-edge variations. The model results proved most sensitive to changes in horizontal resolution. The 80 km results provided a more realistic and robust circulation in most areas of the Arctic and improved the modelled ice edge in the Barents Sea, while also successfully simulating the interannual variation in the region. Although it performed better than the 160 km model, the 80 km model still produced too large an ice extent in the Greenland Sea. No significant improvement in the ice-edge prediction was observed by varying the lateral eddy viscosity. The results indicate that problems remain in the vertical resolution in shallow regions, in treating penetrative convection, and in the simulation of inflow into the Arctic Basin through the Fram Strait.