Sensitivity of simulated temperature,precipitation, and global radiation to different WRF configurations over the Carpathian Basin for regional climate applications

In this study, the Weather Research and Forecasting (WRF) model is used to produce short-term regional climate simulations with several configurations for the Carpathian Basin region. The goal is to evaluate the performance of the model and analyze its sensitivity to different physical and dynamical settings, and input data. Fifteen experiments were conducted with WRF at 10 km resolution for the year 2013. The simulations differ in terms of configuration options such as the parameterization schemes, the hydrostatic and non-hydrostatic dynamical cores, the initial and boundary conditions (ERA5 and ERA-Interim reanalyses), the number of vertical levels, and the length of the spin-up period. E-OBS dataset 2 m temperature, total precipitation, and global radiation are used for validation. Temperature underestimation reaches 4–7 °C for some experiments and can be reduced by certain physics scheme combinations. The cold bias in winter and spring is mainly caused by excessive snowfall and too persistent snow cover, as revealed by comparison with satellite-based observations and a test simulation without snow on the surface. Annual precipitation is overestimated by 0.6–3.8 mm day⁻¹, with biases mainly accumulating in the period driven by large-scale weather processes. Downward shortwave radiation is underestimated all year except in the months dominated by locally forced phenomena (May to August) when a positive bias prevails. The incorporation of downward shortwave radiation to the validation variables increased the understanding of underlying problems with the parameterization schemes and highlighted false model error compensations.

     

Validation for a tropical belt version of WRF: sensitivity tests on radiation and cumulus convection parameterizations

对基本气候态和降水日变化的分析是检验模式模拟性能、理解模式误差来源的重要手段。为了评估出对热带气候模拟效果较好的物理参数化方案组合,本文应用WRF带状区域模式,主要比较了四种积云对流参数化方案:NewTiedtke、Kain-Fritsch、newSAS、Tiedtke,和两种辐射参数化方案:RRTMG和CAM,对热带带状区域的气候模拟结果。研究表明:使用NewTiedtke积云对流参数化方案和RRTMG辐射方案的试验,表现出对气温、降水及降水日变化等综合性最好的模拟性能;NewTiedtke积云对流参数化方案能模拟出较好的降水空间分布和降水日变化位相分布特征;与RRTMG辐射方案相比,CAM辐射方案会使温度模拟偏低,特别是陆地上更明显,这种陆地上的冷偏差可能主要来源于Tmin的模拟偏冷。     

Atmospheric circulation in regional climate models over Central Europe: links to surface air temperature and the influence of driving data

The study examines simulation of atmospheric circulation, represented by circulation indices (flow direction, strength and vorticity), and links between circulation and daily surface air temperatures in regional climate models (RCMs) over Central Europe. We explore control simulations of five high-resolution RCMs from the ENSEMBLES project driven by re-analysis (ERA-40) and the same global climate model (ECHAM5 GCM) plus of one RCM (RCA) driven by different GCMs. The aims are to (1) identify errors in RCM-simulated distributions of circulation indices in individual seasons, (2) identify errors in simulated temperatures under particular circulation indices, and (3) compare performance of individual RCMs with respect to the driving data. Although most of the RCMs qualitatively reflect observed distributions of the airflow indices, each produces distributions significantly different from the observations. General biases include overestimation of the frequency of strong flow days and of strong cyclonic vorticity. Some circulation biases obviously propagate from the driving data. ECHAM5 and all simulations driven by ECHAM5 underestimate frequency of easterly flow, mainly in summer. Except for HIRHAM, however, all RCMs driven by ECHAM5 improve on the driving GCM in simulating atmospheric circulation. The influence on circulation characteristics in the nested RCM differs between GCMs, as demonstrated in a set of RCA simulations with different driving data. The driving data control on circulation in RCA is particularly weak for the BCM GCM, in which case RCA substantially modifies (but does not improve) the circulation from the driving data in both winter and summer. Those RCMs with the most distorted atmospheric circulation are HIRHAM driven by ECHAM5 and RCA driven by BCM. Relatively strong relationships between circulation indices and surface air temperatures were found in the observed data for Central Europe. The links differ by season and are usually stronger for daily maxima than minima. RCMs qualitatively reproduce these relationships. Effects of the driving model biases were found on RCMs’ performance in reproducing not only atmospheric circulation but also the links to surface temperature. However, the RCM formulation appears to be more important than the driving data in representing the latter. Differences of the circulation-to-temperature links among the RCA simulations are smaller and the links tend to be more realistic compared to the driving GCMs.     

The global scale,distribution and growth of aviation: Implications for climate change

Prior to the COVID-19 crisis, global air transport demand was expected to triple between 2020 and 2050. The pandemic, which reduced global air travel significantly, provides an opportunity to discuss the scale, distribution and growth of aviation until 2018, also with a view to consider the climate change implications of a return to volume growth. Industry statistics, data provided by supranational organizations, and national surveys are evaluated to develop a pre-pandemic understanding of air transport demand at global, regional, national and individual scales. Results suggest that the share of the world’s population travelling by air in 2018 was 11%, with at most 4% taking international flights. Data also supports that a minor share of air travelers is responsible for a large share of warming: The percentile of the most frequent fliers – at most 1% of the world population - likely accounts for more than half of the total emissions from passenger air travel. Individual users of private aircraft can contribute to emissions of up to 7,500 t CO₂ per year. Findings are specifically relevant with regard to the insight that a large share of global aviation emissions is not covered by policy agreements.     

Comparison of vertical mixing schemes embedded in a global ocean GCM. Part II: Large scale circulation and water mass formation

Summary The sensitivity of the circulation and water mass properties in a global ocean circulation model (OGCM) to the stability dependent vertical mixing parameterisations of Pacanowski and Philander (1981) and Henderson-Sellers (1985) is investigated. The work extends a previous study which examined upper ocean charateristics and mixed layer evolution resulting from these schemes incorporated in the OGCM and made a recommendation as to the appropriateness of the latter scheme for global models. Under the assumption of constant vertical eddy coefficients (the control case), the model climatology displays acceptable values of North Atlantic Deep Water formation, Antarctic Circumpolar Current strength, and Indonesian throughflow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that these circulation and water mass properties are sensitive to the choice of parametrisation of vertical mixing and that the two stability dependent schemes are unable to perform satisfactorily over the global domain, instead being better suited to the tropics. Under conditions optimal for representing the tropical current and temperature structure, these schemes result in significant weakening of major currents (particularly, the ACC) and reductions in the rates of deep water formation and poleward heat transports. These deficiencies can only be remedied at the expense of the improvements to the simulation in the tropical part of the domain. The results presented indicate that the suggestions made in the previous study do not extend to situations where the deep ocean, and particularly, the global thermohaline circulation is important.With 8 Figures     

A climatological perspective of water vapor at the UTLS region over different global monsoon regions: observations inferred from the Aura-MLS and reanalysis data

The Aura-MLS observations of eight years from 2004 to 2011 have been utilized to understand the hydration and the dehydration mechanism over the northern and the southern hemispheric monsoon (NH and SH) regions. The monsoon regions considered are the Asian Summer Monsoon, East Asian Summer Monsoon, Arizona Monsoon (AM), North African Monsoon, South American Monsoon and the Australian Monsoon. The annual cycle of water vapor as expected shows maxima over the NH during June–August and during December–February over the SH. The time taken by the air parcels over the NH monsoon regions is found to be different compared to that over the SH monsoon regions. The analysis shows the concentration of water vapor in the upper troposphere and the lower stratosphere (UTLS) has not changed over these eight years in both the hemispheres during their respective monsoon seasons. The present analysis show different processes viz., direct overshooting convection, horizontal advection, temperature and cirrus clouds in influencing the distribution of water vapor to the UTLS over these different monsoon regions. Analysis of the UTLS water vapor with temperature and ice water content shows that the AM is hydrating the stratosphere compared to all the other monsoon regions where the water vapor is getting dehydrated. Thus it is envisaged that the present results will have important implications in understanding the exchange processes across the tropopause over the different monsoon regions and its role in stratosphere chemistry.     

Spatial interpolation of monthly climate data for Finland: comparing the performance of kriging and generalized additive models

The Finnish Meteorological Institute has calculated statistics for the new reference period of 1981–2010. During this project, the grid size has been reduced from 10 to 1 km, the evaluation of the interpolation has been improved, and comparisons between different methods has been performed. The climate variables of interest were monthly mean temperature and mean precipitation, for which the spatial variability was explained using auxiliary information: mean elevation, sea percentage, and lake percentage. We compared three methods for spatial prediction: kriging with external drift (KED), generalized additive models (GAM), and GAM combined with residual kriging (GK). Every interpolation file now has attached statistical key figures describing the bias and the normality of the prediction error. According to the cross-validation results, GAM was the best method for predicting mean temperatures, with only very small differences relative to the other methods. For mean precipitation, KED produced the most accurate predictions, followed by GK. In both cases, there was notable seasonal variation in the statistical skill scores. For the new reference period and future interpolations, KED was chosen as the primary method due to its robustness and accuracy.     

Eddy-covariance CO2 flux measurements using open- and closed-path CO2 analysers: Corrections for analyser water vapour sensitivity and damping of fluctuations in air sampling tubes

Methods of calibrating infrared CO₂ analysers for sensitivity to CO₂ and water vapour are described. Equations to correct eddy covariance CO₂ flux measurements are presented for: (i) analyser cross-sensitivity to water vapour and the effects of density fluctuations arising from atmospheric fluxes of water vapour and sensible heat, (ii) flux losses caused by signal processing and limited instrument frequency response for open- and closed-path CO₂ analysers, and (iii) flux losses resulting from damping of concentration fluctuations in a tube used to sample air for closed-path CO₂ analysers. Examples of flux corrections required for typical instruments are presented.