Summertime inter-annual temperature variability in an ensemble of regional model simulations: analysis of the surface energy budget

The inter-annual variability in monthly mean summer temperatures derived from nine different regional climate model (RCM) integrations is investigated for both the control climate (1961–1990) and a future climate (2071–2100) based on A2 emissions. All regional model integrations, carried out in the PRUDENCE project, use the same boundaries of the HadAM3H global atmospheric model. Compared to the CRU TS 2.0 observational data set most RCMs (but not all) overpredict the temperature variability significantly in their control simulation. The behaviour of the different regional climate models is analysed in terms of the surface energy budget, and the contributions of the different terms in the surface energy budget to the temperature variability are estimated. This analysis shows a clear relation in the model ensemble between temperature variability and the combined effects of downward long wave, net short wave radiation and evaporation (defined as F). However, it appears that the overestimation of the temperature variability has no unique cause. The effect of short-wave radiation dominates in some RCMs, whereas in others the effect of evaporation dominates. In all models the temperature variability and F increase when imposing future climate boundary conditions, with particularly high values in central Europe.     

Temporal trends in the stable isotope composition of precipitation: a comparison between the eastern Mediterranean and central Europe

Stable isotopes in precipitation are recognized as a major tool for tracking the water cycle. The development of gridded datasets is a solution to the need for high-resolution isotopic data. The lack of measurement though, dictates the use of ??long-term average?? isotopic values, usually, calculated from short time series, of unequal durations and, sometimes, not even referring to the same time period. Thus, the influence of possible trends, that might be present in the isotopic data, should be accounted for. In the present work, we investigated the existence of temporal trends in the isotopic composition of precipitation of central Europe and eastern Mediterranean. Nine stations were selected, having time series extending over at least a 20-year-long period and fulfilling certain data completeness criteria. Possible trends were detected for three overlapping 20-year periods (1961?C1980, 1971?C1990, and 1981?C2000) using linear regression, the Mann?CKendall test, and a partial Mann?CKendall test, to compensate for the influence of meteorological parameters. We found that very few cases present statistically significant trends. There is significant variability of the observed trends, both on a seasonal and on a station basis, in the central Europe and the eastern Mediterranean area alike. Overall, the insignificance of the observed isotopic trends, and the lack of any coherent spatial and temporal patterns, seems to be supporting the current practice for estimating the long-term average isotopic composition of precipitation.     

Assessing the role of deep rooted vegetation in the climate system with model simulations: mechanism, comparison to observations and implications for Amazonian deforestation

Deep rooted vegetation (of up to 68?m) has been found in many parts of the tropics. However, models of the general atmospheric circulation (GCMs) typically use rooting depths of less than 2?m in their land surface parametrizations. How does the incorporation of deep roots into such a model affect the simulated climate? We assess this question by using a GCM and find that deeper roots lead to a pronounced seasonal response. During the dry season, evapotranspiration and the associated latent heat flux are considerably increased over large regions leading to a cooling of up to 8?K. The enhanced atmospheric moisture is transported towards the main convection areas in the inner tropical convergence zone where it supplies more energy to convection thus intensifying the tropical circulation patterns. Comparison to different kinds of data reveals that the simulation with deeper roots is much closer to observations. The inclusion of deep roots also leads to a general increased climatic sensitivity to rooting depth change. We investigate this aspect in the context of the climatic effects of large-scale deforestation in Amazonia. Most of the regional and remote changes can be attributed to the removal of deep roots. We conclude that deep rooted vegetation is an important part of the tropical climate system. Without the consideration of deep roots, the present-day surface climate cannot adequately be simulated.     

Validation of the “Lokal-Modell” over heterogeneous land surfaces using aircraft-based measurements of the REEEFA experiment and comparison with micro-scale simulations

Summary An aircraft-based experimental investigation of the atmospheric boundary layer (ABL) structure and of the energy exchange processes over heterogeneous land surfaces is presented. The measurements are used for the validation of the mesoscale atmospheric model “Lokal-Modell” (LM) of the German Weather Service with 2.8 km resolution. In addition, high-resolution simulations using the non-hydrostatic model FOOT3DK with 250 m resolution are performed in order to resolve detailed surface heterogeneities. Two special observation periods in May 1999 show comparable convective boundary layer (CBL) conditions. For one case study vertical profiles and area averages of meteorological quantities and energy fluxes are investigated in detail. The measured net radiation is highly dependent on surface albedo, and the latent heat flux exhibits a strong temporal variability in the investigation area. A reduction of this variability is possible by aggregation of multiple flight patterns. To calculate surface fluxes from aircraft measurements at low altitude, turbulent energy fluxes were extrapolated to the ground by the budget method, which turned out to be well applicable for the sensible heat flux, but not for the latent flux. The development of the ABL is well captured by the LM simulation. The comparison of spatiotemporal averages shows an underestimation of the observed net radiation, which is mainly caused by thin low-level clouds in the LM compared to observed scattered CBL clouds. The sensible heat flux is reproduced very well, while the latent flux is highly overestimated especially above forests. The realistic representation of surface heterogeneities in the investigation area in the FOOT3DK simulations leads to improvements for the energy fluxes, but an overestimation of the latent heat flux still persists. A study of upscaling effects yields more structures than the LM fields when averaged to the same scale, which are partly caused by the non-linear effects of parameter aggregation on the LM scale.     

Long-term simulations of the sulfur concentrations over the China, Japan and Korea: A model comparison study

Three comprehensive acid deposition models were used to simulate the sulfur concentrations over northeast Asia over the period covering entire year of 2002, and discussed the aggregated uncertainties and discrepancies of the three models. The participating models are from the countries participating in the project of Longrange Transboundary Air Pollutants in Northeast Asia (LTP): China, Japan and Korea. The Eulerian Model-3/CMAQ (by China), Regional Air Quality Model (RAQM, by Japan), and Comprehensive Acid Deposition Model (CADM, by Korea) were employed by each country with common emissions data established by the administrative agencies of China, Japan and Korea. The episodic simulation results between 1 to 15, March 2002 are also presented, during which aircraft measurements were carried out over the Yellow sea. The episodic results show both a wide short-term variability in simulations against measurements, and maximum concentration differences of 3～5 times among the three models, requiring that further attention before confidence among the three models can be claimed for short-term simulations. However, the year-long cumulative simulations showed almost the same general features, with lower aggregated uncertainties between the three models, produced by the long term integration over northeast Asia.     

Aircraft measurements and model simulations of stratospheric ozone and N2O: implications for chemistry and transport processes in the models

Airborne measurements of stratospheric ozone and N₂O from the SCIAMACHY (Scanning Imaging Absorption Spectrometer) Validation and Utilization Experiment (SCIA-VALUE) are presented. The campaign was conducted in September 2002 and February–March 2003. The Airborne Submillimeter Radiometer (ASUR) observed stratospheric constituents like O₃ and N₂O, among others, spanning a latitude from 5°S to 80°N during the survey. The tropical ozone source regions show high ozone volume mixing ratios (VMRs) of around 11 ppmv at 33 km altitude, and the altitude of the maximum VMR increases from the tropics to the Arctic. The N₂O VMRs show the largest value of 325 ppbv in the lower stratosphere, indicating their tropospheric origin, and they decrease with increasing altitude and latitude due to photolysis. The sub-tropical and polar mixing barriers are well represented in the N₂O measurements. The most striking seasonal difference found in the measurements is the large polar descent in February–March. The observed features are interpreted with the help of SLIMCAT and Bremen Chemical Transport Model (CTMB) simulations. The SLIMCAT simulations are in good agreement with the measured O₃ and N₂O values, where the differences are within 1 ppmv for O₃ and 15 ppbv for N₂O. However, the CTMB simulations underestimate the tropical middle stratospheric O₃ (1–1.5 ppmv) and the tropical lower stratospheric N₂O (15–30 ppbv) measurements. A detailed analysis with various measurements and model simulations suggests that the biases in the CTMB simulations are related to its parameterised chemistry schemes.     

Projecting South Asian summer precipitation in CMIP3 models: A comparison of the simulations with and without black carbon

Considering the importance of black carbon (BC), this study began by comparing the 20th century simulation of South Asian summer climate in IPCC CMIP3, based on the scenario of models with and without BC. Generally, the multi-model mean of the models that include BC reproduced the observed climate relatively better than those that did not. Then, the 21st century South Asian summer precipitation was projected based on the IPCC CMIP3 projection simulations. The projected precipitation in the present approach exhibited a considerable difference from the multimodel ensemble mean (MME) of IPCC AR4 projection simulations, and also from the MME of the models that ignore the effect of BC. In particular, the present projection exhibited a dry anomaly over the central Indian Peninsula, sandwiched between wet conditions on the southern and northern sides of Pakistan and India, rather than homogeneous wet conditions as seen in the MME of IPCC AR4. Thus, the spatial pattern of South Asian summer rainfall in the future may be more complicated than previously thought.     

Quantifying the role of biosphere-atmosphere feedbacks in climate change: coupled model simulations for 6000 years BP and comparison with palaeodata for northern Eurasia and northern Africa

 The LMD AGCM was iteratively coupled to the global BIOME1 model in order to explore the role of vegetation-climate interactions in response to mid-Holocene (6000 y BP) orbital forcing. The sea-surface temperature and sea-ice distribution used were present-day and CO₂ concentration was pre-industrial. The land surface was initially prescribed with present-day vegetation. Initial climate “anomalies” (differences between AGCM results for 6000 y BP and control) were used to drive BIOME1; the simulated vegetation was provided to a further AGCM run, and so on. Results after five iterations were compared to the initial results in order to identify vegetation feedbacks. These were centred on regions showing strong initial responses. The orbitally induced high-latitude summer warming, and the intensification and extension of Northern Hemisphere tropical monsoons, were both amplified by vegetation feedbacks. Vegetation feedbacks were smaller than the initial orbital effects for most regions and seasons, but in West Africa the summer precipitation increase more than doubled in response to changes in vegetation. In the last iteration, global tundra area was reduced by 25% and the southern limit of the Sahara desert was shifted 2.5 °N north (to 18 °N) relative to today. These results were compared with 6000 y BP observational data recording forest-tundra boundary changes in northern Eurasia and savana-desert boundary changes in northern Africa. Although the inclusion of vegetation feedbacks improved the qualitative agreement between the model results and the data, the simulated changes were still insufficient, perhaps due to the lack of ocean-surface feedbacks. Received: 5 December 1996 / Accepted: 16 June 1997     

The atmospheric boundary layer structure over the open and ice-covered Baltic Sea: in situ measurements compared to simulations with the regional model REMO

The regional model REMO, which is the atmospheric component of the coupled atmosphere–ice–ocean–land climate model system BALTIMOS, is tested with respect to its ability to simulate the atmospheric boundary layer over the open and ice-covered Baltic Sea. REMO simulations are compared to ship, radiosonde, and aircraft observations taken during eight field experiments. The main results of the comparisons are: (1) The sharpness and strength of the temperature inversion are underestimated by REMO. Over open water, this is connected with an overestimation of cloud coverage and moisture content above the inversion. (2) The vertical temperature stratification in the lowest 200 m over sea ice is too stable. (3) The horizontal inhomogeneity of sea ice concentration as observed by aircraft could not be properly represented by the prescribed ice concentration in REMO; large differences in the surface heat fluxes arise especially under cold-air advection conditions. The results of the comparisons suggest a reconsideration of the parameterization of subgrid-scale vertical exchange both under unstable und stable conditions.     

A comparison of 3D model results with observations for an isolated CCOPE thunderstorm

Summary This paper is concerned with the simulation of deep convection for the CCOPE 19 July 1981 case study. Clark's three-dimensional (3D) cloud model modified to use the bulk water parameterization scheme of Lin et al. has been used in the simulation of the CCOPE 19 July 1981 case in coarse mesh, fine mesh, and interactive grid nested schemes, respectively. Comparisons with observations show this 3D grid nested cloud model is capable of both capturing both the dynamic and microphysical properties of the cloud.In the nested grid fine mesh model simulation, the timing and mode of cloud growth, the diameter of liquid cloud, the cloud top rate of rise, the maximum cloud water content, and the altitude of first radar echo are consistent with observations. The simulated thunderstorm begins to dissipate, after precipitation reaches the ground as indicated by the decreasing values of maximum updraft and maximum liquid cloud water content, and ends as a precipitating anvil as was observed in the actual thunderstorm. The model precipitation developed through ice phase processes consistent with the analysis of observations from the actual thunderstorm.Qualitative comparisons of the actual radar RHIs with simulated reflectively patterns from the 3D model show remarkable similarity, especially after the mature stage is reached. Features of the actual RHI patterns, such as the weak echo region, upshear anvil bulge, strong upwind reflectivity gradients, and the upwind outflow region near the surface are reproduced in the simulation. Comparison of the actual radar PPIs with horizontal cross sections of radar reflectivity simulated by the 3D model, however, show modest differences in the storm size with the 3D simulated thunderstorm being 1–2 km longer in the west-east direction than the actual thunderstorm. The model-predicted maximum updraft speed is smaller than the 2D model-predicted maximum updraft speed, but still greater than what was observed.Comparisons among the nested grid fine mesh model (MB), nested grid coarse mesh model (MA), fine mesh model (FM), coarse mesh model (CM), and 2D model results previously published show that the nested grid fine mesh model (MB) gives the best simulation result. The various 3D model simulation results are generally similar to each other except for the difference in the domain maximum values. The domain maximum values in the fine mesh models (MB and FM) are generally higher than the coarse mesh models as a result of averaging over a smaller area.With 7 Figures     

Simulation of surface energy fluxes using high-resolution non-hydrostatic simulations and comparisons with measurements for the LITFASS-2003 experiment

Land-surface heterogeneity effects on the subgrid scale of regional climate and numerical weather prediction models are of vital interest for the energy and mass exchange between the surface and the atmospheric boundary layer. High-resolution numerical model simulations can be used to quantify these effects, and are a tool used to obtain area-averaged surface fluxes over heterogeneous land surfaces. We present high-resolution model simulations for the LITFASS area near Berlin during the LITFASS-2003 experiment, which were carried out using the non-hydrostatic model FOOT3DK of the University of Köln with horizontal resolutions of 1 km and 250 m. The LITFASS-2003 experimental dataset is used for comparison. The screen level quantities show good quality for the simulated pressure, temperature, humidity and wind speed and direction. Averaged over the four week experimental period, simulated surface energy fluxes at land stations show a small bias for the turbulent heat fluxes and an underestimation of the net radiation caused by excessive cloudiness in the simulations. For eight selected days with low cloud amounts, the net radiation bias is close to zero, but the sensible heat flux shows a strong positive bias. Large differences are found for latent heat fluxes over a lake, which are partly due to local effects on the measurements, but an additional problem seems to be the overestimation of the turbulent exchange under stable conditions in the daytime internal boundary layer over the lake. In the area average over the LITFASS area of 20 ×  20 km², again a strong positive bias of 70 W m^?2 for the sensible heat is present. For the low soil moisture conditions during June 2003, the simulation of the turbulent heat fluxes is sensitive to variations in the soil type and its hydrological properties. Under these conditions, the supply of ground water to the lowest soil layer should be accounted for. Different area-averaging methods are tested. The experimental set-up of the LITFASS-2003 experiment is found to be well suited for the computation of area-averaged turbulent heat fluxes.     

A comparison of PMIP2 model simulations and the MARGO proxy reconstruction for tropical sea surface temperatures at last glacial maximum

Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm⁻², which translates to equilibrium climate sensitivity for doubling of atmospheric CO₂ of 2.6–3.1°C.     

Tropospheric NO2 Columns over Northeastern North America： Comparison of CMAQ Model Simulations with GOME Satellite Measurements

We present comparisons of the NO2 regional Chemical Transport Model （CTM） simulations over North-eastern North America during the time period from May to September, 1998 with hourly surface NO2 observations and the NO2 columns retrieved from the GOME （Global Ozone Monitoring Experiment） satellite instrument. The model calculations were performed using the Mesoscale Meteorological Model 5 （MM5）, Sparse Matrix Operator Kernal Emissions （SMOKE）, and Community Multiscale Air Quality （CMAQ） modeling systems, using the emission data from the National Emissions Inventory （NEI） databases of 1996 （U.S.） and 1995 （Canada）. The major objectives were to assess the performance of the CMAQ model and the accuracy of the emissions inventories as they affected the simulations of this important short-lived atmospheric species. The modeled （NcMAQ） and measured （NGOME） NO2 column amounts, as well as their temporal variations, agreed reasonably well. The absolute differences （NcMAQ-NGOME） across the domain were between ±3.0×10^15 molecules cm^-2, but they were less than ±1.0×10^15 molecules cm^-2 over the majority （80%） of the domain studied. The overall correlation coefficient between the measurements and the simulations was 0.75. The differences were mainly ascribed to a combination of inaccurate emission data for the CTM and the uncertainties in the GOME retrievals. Of these, the former were the more easily identifiable.     

An ENSO stability analysis. Part II: results from the twentieth and twenty-first century simulations of the CMIP3 models

In this study, a Bjerknes stability (BJ) index, proposed by Jin et al. (2006), is adopted to assess the overall stability of El Niño and Southern Oscillation (ENSO) in state-of-the-art coupled models. The twentieth and twenty-first century simulations of 12 coupled models among the coupled model intercomparison project phase 3 models used in the intergovernmental panel on climate change forth assessment report demonstrate a significant positive correlation between ENSO amplitude and ENSO stability as measured by the BJ index. The simulations also show a diversity of behavior regarding the ENSO stability among the coupled models, which can be attributed to different mean state and sensitivity of an oceanic and atmospheric response to wind and SST forcing from model to model. When respective components of the BJ index obtained from the coupled models are compared with those from observations, it is revealed that most coupled models underestimate the thermodynamic damping effect and the positive effect of the zonal advective and thermocline feedback. Under increased CO₂ induced warm climate, changes, relative to the twentieth century simulations, in the damping and feedback terms responsible for the ENSO stability measured by the BJ index can be linked to mean state changes and associated atmospheric and oceanic response sensitivity changes. There is a clear multi-model trend in the damping terms and positive zonal advective feedback, thermocline feedback, and Ekman feedback terms under enhanced greenhouse gas conditions. However, the various behavior among the coupled models in competition between the positive feedback and negative damping terms in the BJ index formula prevent the formation of a definitive conclusion regarding future projections of ENSO stability using the current coupled models.