scholarly journals Convectively Coupled Equatorial Waves in High-Resolution Hadley Centre Climate Models

2009 ◽  
Vol 22 (8) ◽  
pp. 1897-1919 ◽  
Author(s):  
Gui-Ying Yang ◽  
Julia Slingo ◽  
Brian Hoskins
Keyword(s):  
High Resolution ◽  
Energy Conversion ◽  
Phase Speed ◽  
Atmospheric Model ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Near Surface ◽  
Model Version ◽  
Convectively Coupled Equatorial Waves ◽  
Hadley Centre

Abstract A methodology for diagnosing convectively coupled equatorial waves is applied to output from two high-resolution versions of atmospheric models, the Hadley Centre Atmospheric Model, version 3 (HadAM3), and the new Hadley Centre Global Atmospheric Model, version 1 (HadGAM1), which have fundamental differences in dynamical formulation. Variability, horizontal and vertical structures, and propagation characteristics of tropical convection and equatorial waves, along with their coupled behavior in the models, are examined and evaluated against a previous comprehensive study of observed convectively coupled equatorial waves using the 15-yr ECMWF Re-Analysis (ERA-15) and satellite observed data. The extent to which the models are able to represent the coupled waves found in real atmospheric observations is investigated. It is shown that, in general, the models perform well for equatorial waves coupled with off-equatorial convection. However, they perform poorly for waves coupled with equatorial convection. Convection in both models contains much-reduced variance in equatorial regions, but reasonable off-equatorial variance. The models fail to simulate coupling of the waves with equatorial convection and the tendency for equatorial convection to appear in the region of wave-enhanced near-surface westerlies. In addition, the simulated Kelvin wave and its associated convection generally tend to have lower frequency and slower phase speed than that observed. The models are also not able to capture the observed vertical tilt structure and signatures of energy conversion in the Kelvin wave, particularly in HadAM3. On the other hand, models perform better in simulating westward-moving waves coupled with off-equatorial convection, in terms of horizontal and vertical structures, zonal propagation, and energy conversion signals. In most cases both models fail to simulate well a key picture emerging from the observations, that some wave modes in the lower troposphere can act as a forcing agent for equatorial convection, and that the upper-tropospheric waves generally appear to be forced by the convection both on and off the equator.

scholarly journals Convectively Coupled Equatorial Waves. Part I: Horizontal and Vertical Structures

2007 ◽  
Vol 64 (10) ◽  
pp. 3406-3423 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian Hoskins ◽  
Julia Slingo
Keyword(s):  
Composite Structures ◽  
Wave Structure ◽  
Wave Theory ◽  
Western Hemisphere ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Ambient Flow ◽  
Convectively Coupled Equatorial Waves ◽  
Eastern Hemisphere ◽  
Equatorial Wave

Abstract Multilevel 15-yr ECMWF Re-Analysis (ERA-15) and satellite-observed brightness temperature (Tb) data for the period May–October 1992 are used to examine the horizontal and vertical structures of convectively coupled equatorial waves. Dynamical waves are isolated using a methodology developed previously. Composite structures of convectively coupled equatorial waves are obtained using linear regression/correlation between convection (Tb) and dynamical structures. It is found that the relationship depends on the ambient flow and the nature of the convective coupling, and varies between off-equatorial- and equatorial-centered convection, different hemispheres, and seasons. The Kelvin wave structure in the Western Hemisphere is generally consistent with classic equatorial wave theory and has its convection located in the region of low-level convergence. In the Eastern Hemisphere the Kelvin wave tends to have convection in the region of enhanced lower-tropospheric westerlies and a tilted vertical structure. The Kelvin wave also tends to have a third peak in zonal wind amplitude at 500 hPa and exhibits upward propagation into the lower stratosphere. Lower-tropospheric westward-moving mixed Rossby–gravity (WMRG) and n = 1 Rossby (R1) wave structures and their relationship with convection are consistent with classic equatorial wave theory and the implied lower-tropospheric convergences. In the Eastern Hemisphere the WMRG and R1 waves have first baroclinic mode structures in the vertical. However, in the Western Hemisphere, the R1 wave has a barotropic structure. In the Eastern Hemisphere the R1 wave, like the Kelvin wave, tends to have equatorial convection in the region of enhanced lower-level westerlies, suggesting that enhanced surface energy fluxes associated with these waves may play an important organizing role for equatorial convection in this warm-water hemisphere. In the upper troposphere, eastward-moving Rossby–gravity (EMRG) and n = 1 gravity waves are found in the Eastern Hemisphere, and eastward-moving WMRG and R1 waves are found in the Western Hemisphere, suggestive of Doppler shifting of waves by the ambient flow.

scholarly journals Modelling the climate and surface mass balance of polar ice sheets using RACMO2, part 2: Antarctica (1979–2016)

2017 ◽  
Author(s):  
Jan Melchior van Wessem ◽  
Willem Jan van de Berg ◽  
Brice P. Y. Noël ◽  
Erik van Meijgaard ◽  
Gerit Birnbaum ◽  
...  
Keyword(s):  
High Resolution ◽  
Mass Balance ◽  
Climate Model ◽  
Ice Sheet ◽  
Climate Scenario ◽  
Surface Mass Balance ◽  
Near Surface ◽  
Surface Mass ◽  
Model Version ◽  
Surface Climate

Abstract. We evaluate modelled Antarctic ice sheet (AIS) near-surface climate, surface mass balance (SMB) and surface energy balance (SEB) from the updated polar version of the regional atmospheric climate model RACMO2 (1979–2016). The updated model, referred to as RACMO2.3p2, incorporates upper-air relaxation, a revised topography, tuned parameters in the cloud scheme to generate more precipitation towards the AIS interior, and modified snow properties reducing drifting snow sublimation and increasing surface snowmelt. Comparisons of RACMO2 model output with several independent observational data show that the existing biases in AIS temperature, radiative fluxes and SMB components are further reduced with respect to the previous model version. The model integrated annual average SMB for the ice sheet including ice shelves (minus the Antarctic Peninsula (AP)) now amounts to 2229 Gt y-1, with an interannual variability of 109 Gt y-1. The largest improvement is found in modelled surface snowmelt, that now compares well with satellite and weather station observations. For the high-resolution (~ 5.5 km) AP simulation, results remain comparable to earlier studies. The updated model provides a new, high-resolution dataset of the contemporary near-surface climate and SMB of the AIS; this model version will be used for future climate scenario projections in a forthcoming study.

Overlapping Interests: The Impact of Geographic Coordinate Assumptions on Limited-Area Atmospheric Model Simulations

2013 ◽  
Vol 141 (6) ◽  
pp. 2120-2127 ◽  
Author(s):  
Andrew J. Monaghan ◽  
Michael Barlage ◽  
Jennifer Boehnert ◽  
Cody L. Phillips ◽  
Olga V. Wilhelmi
Keyword(s):  
Land Use ◽  
High Resolution ◽  
Input Data ◽  
Atmospheric Model ◽  
Earth Model ◽  
Limited Area ◽  
Near Surface ◽  
Geographic Coordinate System ◽  
Earth Models ◽  
The Impact

Abstract There is growing use of limited-area models (LAMs) for high-resolution (<10 km) applications, for which consistent mapping of input terrestrial and meteorological datasets is critical for accurate simulations. The geographic coordinate systems of most input datasets are based on spheroid-shaped (i.e., elliptical) Earth models, while LAMs generally assume a perfectly sphere-shaped Earth. This distinction is often neglected during preprocessing, when input data are remapped to LAM domains, leading to geolocation discrepancies that can exceed 20 km at midlatitudes. A variety of terrestrial (topography and land use) input dataset configurations is employed to explore the impact of Earth model assumptions on a series of 1-km LAM simulations over Colorado. For the same terrestrial datasets, the ~20-km geolocation discrepancy between spheroidal-versus-spherical Earth models over the domain leads to simulated differences in near-surface and midtropospheric air temperature, humidity, and wind speed that are larger and more widespread than those due to using different topography and land use datasets altogether but not changing the Earth model. Simulated differences are caused by the shift of static fields with respect to boundary conditions, and altered Coriolis forcing and topographic gradients. The sensitivity of high-resolution LAM simulations to Earth model assumptions emphasizes the importance for users to ensure terrestrial and meteorological input data are consistently mapped during preprocessing (i.e., datasets share a common geographic coordinate system before remapping to the LAM domain). Concurrently, the modeling community should update preprocessing systems to make sure input data are correctly mapped for all global and limited-area simulation domains.

scholarly journals BAYWRF: a high-resolution present-day climatological atmospheric dataset for Bavaria

2020 ◽  
Vol 12 (4) ◽  
pp. 3097-3112
Author(s):  
Emily Collier ◽  
Thomas Mölg
Keyword(s):  
Climate Change ◽  
High Resolution ◽  
Large Scale ◽  
Wrf Model ◽  
Atmospheric Model ◽  
Climate Impact ◽  
Reanalysis Data ◽  
Near Surface ◽  
Local Climate ◽  
The Impact

Abstract. Climate impact assessments require information about climate change at regional and ideally also local scales. In dendroecological studies, this information has traditionally been obtained using statistical methods, which preclude the linkage of local climate changes to large-scale drivers in a process-based way. As part of recent efforts to investigate the impact of climate change on forest ecosystems in Bavaria, Germany, we developed a high-resolution atmospheric modelling dataset, BAYWRF, for this region over the thirty-year period of September 1987 to August 2018. The atmospheric model employed in this study, the Weather Research and Forecasting (WRF) model, was configured with two nested domains of 7.5 and 1.5 km grid spacing centred over Bavaria and forced at the outer lateral boundaries by ERA5 reanalysis data. Using an extensive network of observational data, we evaluate (i) the impact of using grid analysis nudging for a single-year simulation of the period of September 2017 to August 2018 and (ii) the full BAYWRF dataset generated using nudging. The evaluation shows that the model represents variability in near-surface meteorological conditions generally well, although there are both seasonal and spatial biases in the dataset that interested users should take into account. BAYWRF provides a unique and valuable tool for investigating climate change in Bavaria with high interdisciplinary relevance. Data from the finest-resolution WRF domain are available for download at daily temporal resolution from a public repository at the Open Science Framework (Collier, 2020; https://doi.org/10.17605/OSF.IO/AQ58B).

scholarly journals Convectively Coupled Equatorial Waves. Part III: Synthesis Structures and Their Forcing and Evolution

2007 ◽  
Vol 64 (10) ◽  
pp. 3438-3451 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian Hoskins ◽  
Julia Slingo
Keyword(s):  
Lower Troposphere ◽  
Wave Activity ◽  
Equatorial Region ◽  
Western Hemisphere ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Upper Troposphere ◽  
Convectively Coupled Equatorial Waves ◽  
Eastern Hemisphere ◽  
Equatorial Wave

Abstract Building on Parts I and II of this study, the structures of eastward- and westward-moving convectively coupled equatorial waves are examined through synthesis of projections onto standard equatorial wave horizontal structures. The interaction between these equatorial wave components and their evolution are investigated. It is shown that the total eastward-moving fields and their coupling with equatorial convection closely resemble the standard Kelvin wave in the lower troposphere, with intensified convection in phase with anomalous westerlies in the Eastern Hemisphere (EH) and with anomalous convergence in the Western Hemisphere (WH). However, in the upper troposphere, the total fields show a mixture of the Kelvin wave and higher (n = 0 and 1) wave structures, with strong meridional wind and its divergence. The equatorial total fields show what may be described as a modified first internal Kelvin wave vertical structure in the EH, with a tilt in the vertical and a third peak in the midtroposphere. There is evidence that the EH midtropospheric Kelvin wave is closely associated with SH extratropical eastward-moving wave activity, the vertical velocity associated with the wave activity stretching into the equatorial region in the mid–upper troposphere. The midtropospheric zonal wind and geopotential height show a pattern that may be associated with a forced wave. The westward-moving fields associated with off-equatorial convection show very different behaviors between the EH midsummer and the WH transition seasons. In the EH midsummer, the total fields have a baroclinic structure, with the off-equatorial convection in phase with relatively warm air, suggesting convective forcing of the dynamical fields. The total structures exhibit a mixture of the n = 0, 1 components, with the former dominating to the east of convection and the latter to the west of convection. The n = 0 component is found to be closely connected to the lower-level n = 1 Rossby (R1) wave that appears earlier and seems to provide organization for the convection, which in turn forces the n = 0 wave. In the WH transition season the total fields have a barotropic structure and are dominated by the R1 wave. There is evidence that this barotropic R1 wave, as well as the associated tropical convection, is forced by the NH upper-tropospheric extratropical Rossby wave activity. In the EH, westward-moving lower-level wind structures associated with equatorial convection resemble the R1 wave, with equatorial westerlies in phase with the intensified convection. However, westward-moving n = −1 and n = 0 structures are also involved.

scholarly journals Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models. Part I: Convective Signals

2006 ◽  
Vol 19 (12) ◽  
pp. 2665-2690 ◽  
Author(s):  
Jia-Lin Lin ◽  
George N. Kiladis ◽  
Brian E. Mapes ◽  
Klaus M. Weickmann ◽  
Kenneth R. Sperber ◽  
...  
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Intraseasonal Variability ◽  
Phase Speed ◽  
General Circulation Models ◽  
Climate Simulation ◽  
Equatorial Waves ◽  
Intergovernmental Panel ◽  
Convectively Coupled Equatorial Waves ◽  
Wide Range

Abstract This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO) simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail. The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal variability. The total intraseasonal (2–128 day) variance of precipitation is too weak in most of the models. About half of the models have signals of convectively coupled equatorial waves, with Kelvin and MRG–EIG waves especially prominent. However, the variances are generally too weak for all wave modes except the EIG wave, and the phase speeds are generally too fast, being scaled to excessively deep equivalent depths. An interesting result is that this scaling is consistent within a given model across modes, in that both the symmetric and antisymmetric modes scale similarly to a certain equivalent depth. Excessively deep equivalent depths suggest that these models may not have a large enough reduction in their “effective static stability” by diabatic heating. The MJO variance approaches the observed value in only 2 of the 14 models, but is less than half of the observed value in the other 12 models. The ratio between the eastward MJO variance and the variance of its westward counterpart is too small in most of the models, which is consistent with the lack of highly coherent eastward propagation of the MJO in many models. Moreover, the MJO variance in 13 of the 14 models does not come from a pronounced spectral peak, but usually comes from part of an overreddened spectrum, which in turn is associated with too strong persistence of equatorial precipitation. The two models that arguably do best at simulating the MJO are the only ones having convective closures/triggers linked in some way to moisture convergence.

scholarly journals Process-Oriented Diagnosis of Tropical Cyclones in High-Resolution GCMs

2018 ◽  
Vol 31 (5) ◽  
pp. 1685-1702 ◽  
Author(s):  
Daehyun Kim ◽  
Yumin Moon ◽  
Suzana J. Camargo ◽  
Allison A. Wing ◽  
Adam H. Sobel ◽  
...  
Keyword(s):  
Heat Flux ◽  
High Resolution ◽  
Tropical Cyclones ◽  
Surface Heat Flux ◽  
Surface Wind ◽  
Atmospheric Model ◽  
Surface Heat ◽  
The Other ◽  
Near Surface ◽  
Surface Latent Heat Flux

This study proposes a set of process-oriented diagnostics with the aim of understanding how model physics and numerics control the representation of tropical cyclones (TCs), especially their intensity distribution, in GCMs. Three simulations are made using two 50-km GCMs developed at NOAA’s Geophysical Fluid Dynamics Laboratory. The two models are forced with the observed sea surface temperature [Atmospheric Model version 2.5 (AM2.5) and High Resolution Atmospheric Model (HiRAM)], and in the third simulation, the AM2.5 model is coupled to an ocean GCM [Forecast-Oriented Low Ocean Resolution (FLOR)]. The frequency distributions of maximum near-surface wind near TC centers show that HiRAM tends to develop stronger TCs than the other models do. Large-scale environmental parameters, such as potential intensity, do not explain the differences between HiRAM and the other models. It is found that HiRAM produces a greater amount of precipitation near the TC center, suggesting that associated greater diabatic heating enables TCs to become stronger in HiRAM. HiRAM also shows a greater contrast in relative humidity and surface latent heat flux between the inner and outer regions of TCs. Various fields are composited on precipitation percentiles to reveal the essential character of the interaction among convection, moisture, and surface heat flux. Results show that the moisture sensitivity of convection is higher in HiRAM than in the other model simulations. HiRAM also exhibits a stronger feedback from surface latent heat flux to convection via near-surface wind speed in heavy rain-rate regimes. The results emphasize that the moisture–convection coupling and the surface heat flux feedback are critical processes that affect the intensity of TCs in GCMs.

Convectively Coupled Equatorial Waves. Part II: Propagation Characteristics

2007 ◽  
Vol 64 (10) ◽  
pp. 3424-3437 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian Hoskins ◽  
Julia Slingo
Keyword(s):  
Rossby Wave ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Equatorial Waves ◽  
Propagation Characteristics ◽  
Upper Troposphere ◽  
Ambient Flow ◽  
Vertical Coupling ◽  
Convectively Coupled Equatorial Waves ◽  
Coherent Wave

Abstract Following the description of the horizontal and vertical structures of convectively coupled equatorial waves presented in Part I, here their propagation characteristics are investigated. Linear lagged regressions are used to produce their composite evolution, and the Radon transform technique is used to calculate their phase speeds. It is shown that coherent wave structures with convective coupling generally exist for about 1–2 weeks. Typical zonal wavenumbers are 6–8, wavelengths are 42°–64° of longitude, and typical periods are 4–8 days. The eastward phase speed of convectively coupled Kelvin waves is between 10 and 17 m s−1. The westward phase speed of the coupled mixed Rossby–gravity wave is between 10 and 15 m s−1, and the westward phase speed of the coupled n = 1 Rossby wave is between 7 and 9 m s−1. It is found that convection can produce stronger vertical coupling of phase speeds, and Doppler shifting by the ambient flow can modify phase speeds. There is further evidence that some waves tend to act as forcing agents for convection whereas others tend to be forced by convection. Eastward propagation of some n = 0 and 1 modes in the upper troposphere is also examined.

scholarly journals Modelling the climate and surface mass balance of polar ice sheets using RACMO2 – Part 2: Antarctica (1979–2016)

2018 ◽  
Vol 12 (4) ◽  
pp. 1479-1498 ◽  
Author(s):  
Jan Melchior van Wessem ◽  
Willem Jan van de Berg ◽  
Brice P. Y. Noël ◽  
Erik van Meijgaard ◽  
Charles Amory ◽  
...  
Keyword(s):  
High Resolution ◽  
Mass Balance ◽  
Climate Model ◽  
Ice Sheet ◽  
Surface Mass Balance ◽  
Data Set ◽  
Near Surface ◽  
Surface Mass ◽  
Model Version ◽  
Surface Climate

Abstract. We evaluate modelled Antarctic ice sheet (AIS) near-surface climate, surface mass balance (SMB) and surface energy balance (SEB) from the updated polar version of the regional atmospheric climate model, RACMO2 (1979–2016). The updated model, referred to as RACMO2.3p2, incorporates upper-air relaxation, a revised topography, tuned parameters in the cloud scheme to generate more precipitation towards the AIS interior and modified snow properties reducing drifting snow sublimation and increasing surface snowmelt. Comparisons of RACMO2 model output with several independent observational data show that the existing biases in AIS temperature, radiative fluxes and SMB components are further reduced with respect to the previous model version. The model-integrated annual average SMB for the ice sheet including ice shelves (minus the Antarctic Peninsula, AP) now amounts to 2229 Gt y−1, with an interannual variability of 109 Gt y−1. The largest improvement is found in modelled surface snowmelt, which now compares well with satellite and weather station observations. For the high-resolution (∼ 5.5 km) AP simulation, results remain comparable to earlier studies. The updated model provides a new, high-resolution data set of the contemporary near-surface climate and SMB of the AIS; this model version will be used for future climate scenario projections in a forthcoming study.

Improvements to an intermediate complexity atmospheric model for high-resolution downscaling in very complex terrain.

2021 ◽  
Author(s):  
Dylan Reynolds ◽  
Bert Kruyt ◽  
Ethan Gutmann ◽  
Tobias Jonas ◽  
Michael Lehning ◽  
...  
Keyword(s):  
High Resolution ◽  
Complex Terrain ◽  
Large Scale ◽  
Atmospheric Model ◽  
Horizontal Resolution ◽  
Surface Flow ◽  
Near Surface ◽  
Precipitation Patterns ◽  
Intermediate Complexity ◽  
Nwp Model

<p>            Snow deposition patterns in complex terrain are heavily dependent on the underlying topography. This topography affects precipitating clouds at the kilometer-scale and causes changes to the wind field at the sub-kilometer scale, resulting in altered advection of falling hydrometeors. Snow particles are particularly sensitive to changes in the near-surface flow field due to their low density. Atmospheric models which run at the kilometer scale cannot resolve the actual heterogeneity of the underlying terrain, resulting in precipitation maps which do not capture terrain-affected precipitation patterns. Thus, snow-atmosphere interactions such as preferential deposition are often not resolved in precipitation data used as input to snow models. To bridge this spatial gap and resolve snow-atmosphere interactions at the sub-kilometer scale, we couple an intermediate complexity atmospheric model (ICAR) to the COSMO NWP model. Applying this model to sub-kilometer terrain (horizontal resolution of 50 and 250 m) required changes to ICAR’s computational grid, atmospheric dynamics, and boundary layer flow. As a result, the near-surface flow now accounts for surface roughness and topographically induced speed up. This has been achieved by using terrain descriptors calculated once at initialization which consider a point’s exposure or sheltering relative to surrounding terrain. In particular, the use of a 3-dimensional Sx parameter allows us to simulate areas of stagnation and recirculation on the lee of terrain features. Our approach maintains the accurate large-scale precipitation patterns from COSMO but resolves the dynamics induced by terrain at the sub-kilometer scale without adding additional computational burden. We find that solid precipitation patterns at the ridge scale, such as preferential deposition of snow, are better resolved in the high-resolution version of ICAR than the current ICAR or COSMO models. This updated version of ICAR presents a new tool to dynamically downscale NWP output for snow models and enables future studies of snow-atmosphere interactions at domain scales of 100’s of kilometers.</p>

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