The Global Coupled 3 (GC3) configuration of the Met Office Unified Model is presented. Among other applications, GC3 is the basis of the United Kingdom's submission to the Coupled Model ...Intercomparison Project 6 (CMIP6). This paper documents the model components that make up the configuration (although the scientific descriptions of these components are in companion papers) and details the coupling between them. The performance of GC3 is assessed in terms of mean biases and variability in long climate simulations using present‐day forcing. The suitability of the configuration for predictability on shorter time scales (weather and seasonal forecasting) is also briefly discussed. The performance of GC3 is compared against GC2, the previous Met Office coupled model configuration, and against an older configuration (HadGEM2‐AO) which was the submission to CMIP5. In many respects, the performance of GC3 is comparable with GC2, however, there is a notable improvement in the Southern Ocean warm sea surface temperature bias which has been reduced by 75%, and there are improvements in cloud amount and some aspects of tropical variability. Relative to HadGEM2‐AO, many aspects of the present‐day climate are improved in GC3 including tropospheric and stratospheric temperature structure, most aspects of tropical and extratropical variability and top‐of‐atmosphere and surface fluxes. A number of outstanding errors are identified including a residual asymmetric sea surface temperature bias (cool northern hemisphere, warm Southern Ocean), an overly strong global hydrological cycle and insufficient European blocking.
Key Points
Description of the Global Coupled 3 (GC3) configuration of the Met Office Unified Model
A cross‐time‐scale evaluation of the GC3 configuration is presented
Overall, GC3 is an improvement on previous configurations
The representation of the Northern Hemisphere (NH) storm tracks and jet streams and their response to climate change have been evaluated in climate model simulations from Phases 3, 5, and 6 of the ...Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6, respectively). The spatial patterns of the multimodel biases in CMIP3, CMIP5, and CMIP6 are similar; however, the magnitudes of the biases in the CMIP6 models are substantially lower. For instance, the multimodel mean RMSE of the North Atlantic storm track for the CMIP6 models (as measured by time‐filtered sea‐level pressure variance) is over 50% smaller than that of the CMIP3 models in both winter and summer, and over 40% smaller for the North Pacific. The magnitude of the jet stream biases is also reduced in CMIP6, but by a lesser extent. Despite this improved representation of the current climate, the spatial patterns of the climate change response of the NH storm tracks and jet streams remain similar in the CMIP3, CMIP5, and CMIP6 models. The SSP2‐4.5 scenario responses in the CMIP6 models are substantially larger than in the RCP4.5 CMIP5 models, which is consistent with the larger climate sensitivities of the CMIP6 models compared to CMIP5.
Plain Language Summary
The ability of the climate models used in the CMIP (Coupled Model Intercomparison Project) models to represent the Northern Hemisphere storm tracks and jet streams is evaluated. The newest models (CMIP6) are found to have a much better representation, especially for the North Atlantic. The response of the storm tracks and jet streams to climate change is also assessed. The spatial pattern of the response in the CMIP3, CMIP5, and CMIP6 models is found to be similar. The response of the new CMIP6 models to the SSP2‐4.5 scenario is found to be larger than that of the very similar RCP4.5 scenario in the CMIP5 models, suggesting the CMIP6 models are more sensitive to the forcing from climate change.
Key Points
The spatial patterns of the historical CMIP3, CMIP5, and CMIP6 biases are similar, but their magnitudes are substantially smaller in CMIP6
The spatial patterns of the climate change responses in the NH storm tracks and jet streams are similar in the CMIP3, CMIP5, and CMIP6 models
The SSP2‐4.5 response in CMIP6 is larger than the RCP4.5 CMIP5 response, consistent with the larger climate sensitivities in CMIP6
Western disturbances (WDs) are midtropospheric to upper‐tropospheric mesoscale vortices, which typically propagate along the subtropical westerly jet stream and bring heavy rainfall to Pakistan and ...northern India during boreal winter. They are dynamically similar to Tibetan Plateau vortices (TPVs), which affect southwest China during spring and summer and emanate from the Tibetan Plateau. Here we propose that their similarity implies the existence of a more general group of upper‐tropospheric vortices featuring interactions with the orography of the Hindu Kush‐Himalaya‐Tibetan Plateau region. Using existing track databases for WDs and TPVs derived from ERA‐Interim reanalysis, we show that their respective occurrence frequencies are highly anticorrelated with each other through the seasonal cycle, yet both are strongly correlated with jet latitude. Our findings imply that the incidence of hazards due to WDs and TPVs is correlated on intra‐annual and interannual time scales, particularly through upper‐level baroclinicity.
Plain Language Summary
North India and south China (including Tibet) experience seasonal midlevel cyclonic systems, referred to as western disturbances (WDs) and Tibetan Plateau vortices (TPVs), respectively. These systems can be responsible for heavy, even catastrophic, rainfall in the areas associated with them, and thus improving prediction and risk correlation is vital. In this study, we show that: first, previous literature alludes to the two types of system bearing similar structures; second, the mean annual cycles of both are strongly affected by the subtropical westerly jet; third, the interannual populations of each are related; and fourth, this relationship can be explained by considering features of the jet, such as strength and position. We thus conclude that it is likely that WDs and TPVs are specific examples of a more general class of synoptic‐scale vortex.
Key Points
Western disturbances (WDs) and Tibetan Plateau vortices (TPVs) are tracked in ERA‐Interim reanalysis
WD and TPV frequencies are anticorrelated during the annual cycle, controlled by jet latitude
Interannual variability of WD and TPV occurrence is strongly controlled by upper‐level baroclinicity
The time scales of the Paris Climate Agreement indicate urgent action is required on climate policies over the next few decades, in order to avoid the worst risks posed by climate change. On these ...relatively short time scales the combined effect of climate variability and change are both key drivers of extreme events, with decadal time scales also important for infrastructure planning. Hence, in order to assess climate risk on such time scales, we require climate models to be able to represent key aspects of both internally driven climate variability and the response to changing forcings. In this paper we argue that we now have the modeling capability to address these requirements—specifically with global models having horizontal resolutions considerably enhanced from those typically used in previous Intergovernmental Panel on Climate Change (IPCC) and Coupled Model Intercomparison Project (CMIP) exercises. The improved representation of weather and climate processes in such models underpins our enhanced confidence in predictions and projections, as well as providing improved forcing to regional models, which are better able to represent local-scale extremes (such as convective precipitation). We choose the global water cycle as an illustrative example because it is governed by a chain of processes for which there is growing evidence of the benefits of higher resolution. At the same time it comprises key processes involved in many of the expected future climate extremes (e.g., flooding, drought, tropical and midlatitude storms).
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Recently, much attention has been devoted to better understand the internal modes of variability of the climate system. This is particularly important in mid-latitude regions like the North-Atlantic, ...which is characterized by a large natural variability and is intrinsically difficult to predict. A suitable framework for studying the modes of variability of the atmospheric circulation is to look for recurrent patterns, commonly referred to as Weather Regimes. Each regime is characterized by a specific large-scale atmospheric circulation pattern, thus influencing regional weather and extremes over Europe. The focus of the present paper is the study of the Euro-Atlantic wintertime Weather Regimes in the climate models participating to the PRIMAVERA project. We analyse here the set of coupled historical simulations (hist-1950), which have been performed both at standard and increased resolution, following the HighresMIP protocol. The models’ performance in reproducing the observed Weather Regimes is assessed in terms of different metrics, focussing on systematic biases and on the impact of resolution. We also analyse the connection of the Weather Regimes with the Jet Stream latitude and blocking frequency over the North-Atlantic sector. We find that—for most models—the regime patterns are better represented in the higher resolution version, for all regimes but the NAO-. On the other side, no clear impact of resolution is seen on the regime frequency of occurrence and persistence. Also, for most models, the regimes tend to be more tightly clustered in the increased resolution simulations, more closely resembling the observed ones. However, the horizontal resolution is not the only factor determining the model performance, and we find some evidence that biases in the SSTs and mean geopotential field might also play a role.
The dependence of the annual mean tropical precipitation on horizontal resolution is investigated in the atmospheric version of the Hadley Centre General Environment Model. Reducing the grid spacing ...from about 350 km to about 110 km improves the precipitation distribution in most of the tropics. In particular, characteristic dry biases over South and Southeast Asia including the Maritime Continent as well as wet biases over the western tropical oceans are reduced. The annual-mean precipitation bias is reduced by about one third over the Maritime Continent and the neighbouring ocean basins associated with it via the Walker circulation. Sensitivity experiments show that much of the improvement with resolution in the Maritime Continent region is due to the specification of better resolved surface boundary conditions (land fraction, soil and vegetation parameters) at the higher resolution. It is shown that in particular the formulation of the coastal tiling scheme may cause resolution sensitivity of the mean simulated climate. The improvement in the tropical mean precipitation in this region is not primarily associated with the better representation of orography at the higher resolution, nor with changes in the eddy transport of moisture. Sizeable sensitivity to changes in the surface fields may be one of the reasons for the large variation of the mean tropical precipitation distribution seen across climate models.
Modelling spatial covariance is an essential part of all geostatistical methods. Traditionally, parametric semivariogram models are fit from available data. More recently, it has been suggested to ...use nonparametric correlograms obtained from spatially complete data fields. Here, both estimation techniques are compared. Nonparametric correlograms are shown to have a substantial negative bias. Nonetheless, when combined with the sample variance of the spatial field under consideration, they yield an estimate of the semivariogram that is unbiased for small lag distances. This justifies the use of this estimation technique in geostatistical applications. Various formulations of geostatistical combination (Kriging) methods are used here for the construction of hourly precipitation grids for Switzerland based on data from a sparse realtime network of raingauges and from a spatially complete radar composite. Two variants of Ordinary Kriging (OK) are used to interpolate the sparse gauge observations. In both OK variants, the radar data are only used to determine the semivariogram model. One variant relies on a traditional parametric semivariogram estimate, whereas the other variant uses the nonparametric correlogram. The variants are tested for three cases and the impact of the semivariogram model on the Kriging prediction is illustrated. For the three test cases, the method using nonparametric correlograms performs equally well or better than the traditional method, and at the same time offers great practical advantages. Furthermore, two variants of Kriging with external drift (KED) are tested, both of which use the radar data to estimate nonparametric correlograms, and as the external drift variable. The first KED variant has been used previously for geostatistical radar-raingauge merging in Catalonia (Spain). The second variant is newly proposed here and is an extension of the first. Both variants are evaluated for the three test cases as well as an extended evaluation period. It is found that both methods yield merged fields of better quality than the original radar field or fields obtained by OK of gauge data. The newly suggested KED formulation is shown to be beneficial, in particular in mountainous regions where the quality of the Swiss radar composite is comparatively low. An analysis of the Kriging variances shows that none of the methods tested here provides a satisfactory uncertainty estimate. A suitable variable transformation is expected to improve this.
The coarse spacing of automatic rain gauges complicates near‐real‐time spatial analyses of precipitation. We test the possibility of improving such analyses by considering, in addition to the in situ ...measurements, the spatial covariance structure inferred from past observations with a denser network. To this end, a statistical reconstruction technique, reduced space optimal interpolation (RSOI), is applied over Switzerland, a region of complex topography. RSOI consists of two main parts. First, principal component analysis (PCA) is applied to obtain a reduced space representation of gridded high‐resolution precipitation fields available for a multiyear calibration period in the past. Second, sparse real‐time rain gauge observations are used to estimate the principal component scores and to reconstruct the precipitation field. In this way, climatological information at higher resolution than the near‐real‐time measurements is incorporated into the spatial analysis. PCA is found to efficiently reduce the dimensionality of the calibration fields, and RSOI is successful despite the difficulties associated with the statistical distribution of daily precipitation (skewness, dry days). Examples and a systematic evaluation show substantial added value over a simple interpolation technique that uses near‐real‐time observations only. The benefit is particularly strong for larger‐scale precipitation and prominent topographic effects. Small‐scale precipitation features are reconstructed at a skill comparable to that of the simple technique. Stratifying the reconstruction method by the types of weather type classifications yields little added skill. Apart from application in near real time, RSOI may also be valuable for enhancing instrumental precipitation analyses for the historic past when direct observations were sparse.
The purpose of this prospective study was to update epidemiological data on cutaneous larva migrans (CLM) and to assess the therapeutic efficacy of ivermectin. We performed the study between June ...1994 and December 1998 at our travel clinic. Ivermectin (a single dose of 200 g/kg) was offered to all the patients with CLM, and its efficacy and tolerability were assessed by a questionnaire. Sixty-four patients were enrolled. All were European and had stayed in tropical areas. After the patients had returned from their destinations, 55% had lesions occur within a mean of 16 days (range, 1-120 days; >1 month in 7 patients). The initial diagnosis was wrong in 55% of patients. The mean number of lesions was 3 (range, 1-15), and the main sites were the feet (48%) and buttocks (23%). The cure rate after a single dose of ivermectin was 77%. In 14 patients, 1 or 2 supplementary doses were necessary, and the overall cure rate was 97%. The median time required for pruritus and lesions to disappear was 3 and 7 days, respectively. No systemic adverse effects were reported. Physicians' knowledge of CLM, which can have a long incubation period, is poor. Single-dose ivermectin therapy appears to be effective and well tolerated, even if several treatments are sometimes necessary.
The Coupled Model
Intercomparison Project phase 6 (CMIP6) HighResMIP is a new experimental design for global
climate model simulations that aims to assess the impact of model horizontal
resolution on ...climate simulation fidelity. We describe a hierarchy of global
coupled model resolutions based on the Hadley Centre Global Environment Model 3 – Global Coupled vn 3.1 (HadGEM3-GC3.1) model that ranges from
an atmosphere–ocean resolution of 130 km–1∘ to 25 km–1∕12∘, all using the same forcings and initial conditions. In
order to make such high-resolution simulations possible, the experiments
have a short 30-year spinup, followed by at least century-long simulations
with constant forcing to assess drift. We assess the change in model biases as a function of both atmosphere and
ocean resolution, together with the effectiveness and robustness of this new
experimental design. We find reductions in the biases in top-of-atmosphere
radiation components and cloud forcing. There are significant reductions in
some common surface climate model biases as resolution is increased,
particularly in the Atlantic for sea surface temperature and precipitation,
primarily driven by increased ocean resolution. There is also a reduction in
drift from the initial conditions both at the surface and in the deeper
ocean at higher resolution. Using an eddy-present and eddy-rich ocean
resolution enhances the strength of the North Atlantic ocean circulation
(boundary currents, overturning circulation and heat transport), while an
eddy-present ocean resolution has a considerably reduced Antarctic
Circumpolar Current strength. All models have a reasonable representation of El Niño–Southern Oscillation. In general, the biases present after 30 years of simulations do not change character markedly over longer timescales,
justifying the experimental design.
PDF
