Mechanisms for a remote response to Asian aerosol emissions in boreal winter

Asian emissions of anthropogenic aerosols have increased rapidly since 1980, with half of the increase since the pre-industrial era occurring in this period. Transient experiments with the HadGEM3-GC2 coupled model were designed to isolate the impact of Asian aerosols on global climate. In boreal winter, it is found that this increase has resulted in local circulation changes, which in turn have driven increases in temperature and decreases in precipitation over China, alongside an intensification of the offshore monsoon flow. Over India, the opposite response is found, with decreasing temperatures and increasing precipitation. The dominant feature of the local circulation changes is an increase in low-level convergence, ascent, and precipitation over the Maritime continent, which forms part of a tropical-Pacific-wide La-Nina-like response. HadGEM3-GC2 also simulates pronounced far-field responses. A decreased meridional temperature gradient in the North Pacific leads to a positive-Pacific-North-American circulation pattern, with associated temperature anomalies over the North Pacific and North America. An anomalous anticyclonic circulation over the North Atlantic, and an anomalous cyclonic circulation over the Mediterranean drive advection of cold air into Europe, causing cooling in this region. Using a steady-state primitive equation model, LUMA, we demonstrate that these far-field midlatitude response arise primarily as a result of Rossby waves generated over China, rather than in the Equatorial Pacific

The roles of the atmosphere and ocean in driving Arctic warming due to European aerosol reductions

Clean air policies can have significant impacts on climate in remote regions. Previous modeling studies have shown that the temperature response to European sulfate aerosol reductions is largest in the Arctic. Here, we investigate the atmospheric and ocean roles in driving this enhanced Arctic warming using a set of fully‐coupled and slab‐ocean simulations (specified ocean heat convergence fluxes) with the Norwegian Earth system model (NorESM), under scenarios with high and low European aerosol emissions relative to year 2000. We show that atmospheric processes drive most of the Arctic response. The ocean pathway plays a secondary role inducing small temperature changes mostly in the opposite direction of the atmospheric response. Important modulators of the temperature response patterns are changes in sea‐ice extent and subsequent turbulent heat flux exchange, suggesting that a proper representation of Arctic sea‐ice and turbulent changes are key to predicting the Arctic response to mid‐latitude aerosol forcing

Effective radiative forcing and adjustments in CMIP6 models

The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (&plusmn;0.23) W&thinsp;m&minus;2, comprised of 1.81 (&plusmn;0.09) W&thinsp;m&minus;2 from CO2, 1.08 (&plusmn; 0.21) W&thinsp;m&minus;2 from other well-mixed greenhouse gases, &minus;1.01 (&plusmn; 0.23) W&thinsp;m&minus;2 from aerosols and &minus;0.09 (&plusmn;0.13) W&thinsp;m&minus;2 from land use change. Quoted uncertainties are 1&nbsp;standard deviation across model best estimates, and 90&thinsp;% confidence in the reported forcings, due to internal variability, is typically within 0.1 W&thinsp;m&minus;2. The majority of the remaining 0.21&thinsp;W&thinsp;m&minus;2 is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol&ndash;cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from &minus;0.63 to &minus;1.37&thinsp;W&thinsp;m&minus;2, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4&times;CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming. </div

A Simple Method for Calculations of Wake Effects in Wind Farms with Influence of Atmospheric Stability

Vindturbiner har vanligtvis byggts i kust- och jordbruksområden, men med en planerad utbyggnad av vindkraften i Sverige kommer nya platser behöva övervägas. En vindkraftpark placerad i inlandet kommer att påverkas av ett annorlunda vindklimat än en vindkraftpark i kustnära områden, främst på grund av högre turbulensintensitet och kraftigare vindskjuvning. Dessutom varierar atmosfärens stabilitet snabbare i inlandet. Nedströms vindturbiner uppstår så kallade vakar. En vindturbinvak är ett område med lägre vindhastighet och högre turbulensintensitet. I vindkraftparker orsakar vakar effektbortfall vid tillfällen då turbiner befinner sig i vakar från turbiner uppströms. Atmosfärens stabilitet påverkar vakdiffusionen, instabil skiktning leder till en snabbare återhämtning till omgivande vindhastighet medan stabil skitning leder till att vaken får en större utbredning. En ny metod för beräkningar av vakeffekter i vindkraftparker har utvecklats som bygger på en modell som tar hänsyn till atmosfärisk stabilitet, men i modellen fanns endast uttryck för neutral skiktning. Den centrala parametern i modellen är den karaktäristiska transporttiden som definierar vakens utbredning. Transporttid används tillsammans med Taylors hypotes som koordinat i medelvindens riktning. Genom att tillämpa Monin-Obukhov similaritetsteori kunde ett nytt uttryck, innehållandes stabilitet och turbulensintensitet, härledas för den karaktäristiska transporttiden. Med detta uttryck hittades ett sätt att hantera växelverkan mellan vakar genom att använda turbulensintensitet. Modellen användes för att skapa ett datorprogram för att beräkna vakeffekter för varierande atmosfärisk skiktning. I programmet läggs uttrycken för vakarna in i ett stationärt vindfält. För varje vindriktning beräknas effektutvinningen för olika atmosfäriska förhållanden. För att utvärdera modellen har den jämförts med mätningar från en småskalig vindkraftpark. Från tidigare undersökningar fanns värden på den karaktäristiska transporttiden för stabil, neutral, instabil skiktning och dubbelvakar i neutral skiktning. Med programmet kunde dessa värden fås fram. Den relativa effektutvinningen beräknades för varierande atmosfärisk stabilitet och jämfördes med uppmätt effektutvinning. Modellen visade ganska god överensstämmelse medmätningarna. Stabil skiktning orsakade högre effektbortfall medan instabil skiktning minskade effektbortfallet. Dessutom undersöktes den relativa effektutvinningen för ett antal turbiner i rad. Modellen visade sig återge det jämviktsläge i effektutvinningen som ofta har observerats.Traditionally, wind turbines have been built in farmlands and coastal areas. However, with plans ahead of a large expansion of the installed wind power in Sweden new sites have to be found. A wind farm located in the inland will experience a wind climate different from a wind farm close to the coast, mainly due to higher turbulence intensity and larger wind shear. In addition, the atmospheric stability will vary more rapidly. Wakes will appear downstream wind turbines. A wake is a region of lower wind speed and higher turbulence intensity. In a wind farm wakes cause power losses in situations when turbines operate in the wakes of turbines upstream. The wake diffusion process is dependent on the atmospheric stability, unstable stratification leads to a faster wind speed recovery while the wake will expand further downstream in stable stratification. A new method for calculations of wake effects has been developed based on a model where atmospheric stability is included, but only expressions for a neutral atmosphere were provided. The main feature of the model is the characteristic transport time, defining the expansion of the wake. The transport time is used together with Taylor’s hypothesis as the coordinate in the mean wind direction. By implementing Monin-Obukhov similarity theory a new expression including stability and turbulence intensity was derived for the characteristic transport time. With the turbulence intensity appearing in the new expression a way of handling wake interaction could be found. The model was used to create a computer program for calculations of wake effects in different atmospheric stratifications. In the program the wake expressions are superimposed in a stationary wind field. In each wind direction the power output is calculated for varying atmospheric conditions. For validation the model has been compared with measurements from a small wind farm. From earlier investigations values of the characteristic transport time were provided for stable, neutral, unstable stratification and double wakes in neutral conditions. With the method proposed these values could be reproduced. The relative power output was calculated for different stratifications and compared with measured power output. The model showed a fairly good agreement with the measured data. Stable stratification led to higher power losses while unstable stratification reduced power losses. Moreover, the relative power in a row of turbines was investigated and the model was found to reproduce the equilibrium in power production often observed

Surface temperature response to regional black carbon emissions: do location and magnitude matter?

Aerosol radiative forcing can influence climate both locally and far outside the emission region. Here we investigate black carbon (BC) aerosols emitted in four major emission areas and evaluate the importance of emission location and magnitude as well as the concept of the absolute regional temperature-change potentials (ARTP). We perform simulations with a climate model (NorESM) with a fully coupled ocean and with fixed sea surface temperatures. BC emissions for year 2000 are increased by factors of 10 and 20 in South Asia, North America, and Europe, respectively, and by 5 and 10 in East Asia (due to higher emissions there). The perturbed simulations and a reference simulation are run for 100 years with three ensemble members each. We find strikingly similar regional surface temperature responses and geographical patterns per unit BC emission in Europe and North America but somewhat lower temperature sensitivities for East Asian emissions. BC emitted in South Asia shows a different geographical pattern in surface temperatures, by changing the Indian monsoon and cooling the surface. We find that the ARTP approach rather accurately reproduces the fully coupled temperature response of NorESM. Choosing the highest emission rate results in lower surface temperature change per emission unit compared to the lowest rate, but the difference is generally not statistically significant except for the Arctic. An advantage of high-perturbation simulations is the clearer emergence of regional signals. Our results show that the linearity of normalized temperature effects of BC is fairly well preserved despite the relatively large perturbations but that regional temperature coefficients calculated from high perturbations may be a conservative estimate. Regardless of emission region, BC causes a northward shift of the ITCZ, and this shift is apparent both with a fully coupled ocean and with fixed sea surface temperatures. For these regional BC emission perturbations, we find that the effective radiative forcing is not a good measure of the climate response. A limitation of this study is the uncertainties in BC–cloud interactions and the amount of BC absorption, both of which are model dependent

Local and remote temperature response of regional SO2 emissions

Abstract. Short-lived anthropogenic climate forcers (SLCFs), such as sulfate aerosols, affect both climate and air quality. Despite being short-lived, these forcers do not affect temperatures only locally; regions far away from the emission sources are also affected. Climate metrics are often used in a policy context to compare the climate impact of different anthropogenic forcing agents. These metrics typically relate a forcing change in a certain region with a temperature change in another region and thus often require a separate model to convert emission changes to radiative forcing (RF) changes. In this study, we used a coupled Earth system model, NorESM (Norwegian Earth System Model), to calculate emission-to-temperature-response metrics for sulfur dioxide (SO2) emission changes in four different policy-relevant regions: Europe (EU), North America (NA), East Asia (EA) and South Asia (SA). We first increased the SO2 emissions in each individual region by an amount giving approximately the same global average radiative forcing change (−0.45 Wm−2). The global mean temperature change per unit sulfur emission compared to the control experiment was independent of emission region and equal to ∼0.006 K(TgSyr−1)−1. On a regional scale, the Arctic showed the largest temperature response in all experiments. The second largest temperature change occurred in the region of the imposed emission increase, except when South Asian emissions were changed; in this experiment, the temperature response was approximately the same in South Asia and East Asia. We also examined the non-linearity of the temperature response by removing all anthropogenic SO2 emissions over Europe in one experiment. In this case, the temperature response (both global and regional) was twice that in the corresponding experiment with a European emission increase. This non-linearity in the temperature response is one of many uncertainties associated with the use of simplified climate metrics