亚洲区域气候模式比较计划的进展

区域气候变化的信息是气候变化影响评估的基础。在发展全球气候模式的同时 ,发展高分辨的区域气候模式是获取区域气候变化信息的重要途径之一。作者介绍了由中国科学院大气物理研究所START全球变化东亚区域研究中心发起和主持 ,有中、美、韩、日和澳大利亚 1 0个研究组参加的亚洲区域气候模式比较计划的目的、意义、工作任务和试验设计等主要内容 ,给出了该国际比较计划第 1阶段 (连续 1 8个月积分 )和第 2阶段( 1 988～ 1 998年连续 1 0年积分 )的初步结果 ,讨论了区域气候模式在亚洲应用中的一些共性问题以及进一步研究的方向。     

Tropical cyclone genesis frequency over the western North Pacific simulated in medium-resolution coupled general circulation models

This study examines the tropical cyclone (TC) genesis frequency over the western North Pacific simulated in atmosphere–ocean coupled general circulation models from the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3. We first evaluate performances of eight models with atmospheric horizontal resolution of T63 or T106 by analyzing their daily-mean atmospheric outputs of twentieth-century climate simulations available from the Program for Climate Model Diagnosis and Intercomparison database. The genesis frequency is validated against the best-track data issued by the Japan Meteorological Agency. Five of the eight models reproduce realistic horizontal distribution of the TC genesis with a large fraction over the 10°–20°N, 120°–150°E area. These five high-performance models also realistically simulate the summer–winter contrast of the frequency. However, detailed seasonal march is slightly unrealistic; four of the models overestimate the frequency in the early season (May–June) while all of them underestimate the frequency in the mature season (July–September). Reasons for these biases in the seasonal march for the five high-performance models are discussed using the TC genesis potential (GP) index proposed by Emanuel and Nolan (in Am Meteor Soc, pp 240–241, 2004). The simulated GP has seasonal biases consistent with those of the TC genesis frequency. For all five models, the seasonal biases in GP are consistent with those in environmental lower-tropospheric vorticity, vertical wind shear, and relative humidity, which can be attributed to the simulated behavior of monsoon trough. The observed trough migrates northward from the equatorial region to reach the 10°–20°N latitudinal band during the mature season and contributes to the TC frequency maximum, whereas the simulated trough migrates northward too rapidly and reaches this latitude band in the early season, leading to the overestimation of the TC genesis frequency. In the mature season, the simulated trough reaches as far as 15°–25°N, accompanied by a strong vertical shear south of the trough, providing an unfavorable condition for TC genesis. It is concluded that an adequate simulation of the monsoon trough behavior is essential for a better reproduction of the TC frequency seasonal march.     

Further Characterisation of the 91500 Zircon Crystal

This paper reports the results from a second characterisation of the 91500 zircon, including data from electron probe microanalysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser fluorination analyses. The focus of this initiative was to establish the suitability of this large single zircon crystal for calibrating in situ analyses of the rare earth elements and oxygen isotopes, as well as to provide working values for key geochemical systems. In addition to extensive testing of the chemical and structural homogeneity of this sample, the occurrence of banding in 91500 in both backscattered electron and cathodoluminescence images is described in detail. Blind intercomparison data reported by both LA-ICP-MS and SIMS laboratories indicate that only small systematic differences exist between the data sets provided by these two techniques. Furthermore, the use of NIST SRM 610 glass as the calibrant for SIMS analyses was found to introduce little or no systematic error into the results for zircon. Based on both laser fluorination and SIMS data, zircon 91500 seems to be very well suited for calibrating in situ oxygen isotopic analyses.     

The tropospheric distribution of carbon monoxide as observed during the Tropoz II experiment

As part of the TROPOZ II large-scale measurement campaign in January 1991 we deployed a Four Laser Airborne Infra Red (FLAIR) tunable diode laser spectrometer on board a Caravelle 116 research aircraft. We report here in situ CO measurements which were obtained with one of the four channels of the FLAIR instrument at a time resolution of either one or two minutes. The flight route of the TROPOZ II campaign followed the Atlantic coasts of North America, the Pacific and Atlantic coasts of South America and the Atlantic coasts of West Africa and Europe. A total of 48 CO vertical profiles extending from the surface to 10.5 km altitude were obtained. In the meridional direction adjacent profiles were separated by less than 10° latitude. Polewards of 30°S the CO distribution was very homogeneous with a mean mixing ratio of 55 ppbv. Between 30°S and the equator, the CO mixing ratio above 8 km altitude ranged up to 130 ppbv and was 20–60 ppbv higher than in the mid free troposhere. Three day backward trajectories for these CO rich airmasses originated over Amazonia. Earlier trace gas measurements as well as circulation studies suggested that these airmasses were of Northern Hemispheric origin and had been rapidly convected to the upper troposphere over central South America. The influence of biomass burning is clearly apparent from the measurements performed at 10°N on the African side of the Atlantic with CO mixing ratios being 100–300% higher than on the Central American side. CO mixing ratios further north ranged from 80 to 130 ppbv in the free troposphere and increased to 130–150 ppbv at lower altitudes.     

Accuracy of Sonic Anemometers: Laminar Wind-Tunnel Calibrations Compared to Atmospheric In Situ Calibrations Against a Reference Instrument

Two Gill Solent Ultrasonic anemometers, models 1012R2 and 1210R3, weretested in field parallel measurements against a windvane based hot-film anemometerwith additional sensors for temperature and wet-bulb temperature, the MIUU (MIUU:Meteorology Institute, Uppsala University) instrument. This instrument was shown toretain its precision from laminar wind-tunnel tests when used in atmospheric turbulentflow. This contrasts strongly to the observed results for the two sonic anemometers,which were first calibrated in laminar wind-tunnel flow. Individual three-dimensionalcalibration matrices were constructed, and were shown to reduce the remaining calibration uncertainty for the wind speed to 0.4–0.8% for all azimuths and for angles of attack within ±40°. In the field intercomparison tests of the sonics against the MIUU instrument, it was found that the precision not only of the mean wind speed but of all second-order moments studied (variances and covariances, with and without temperature) deteriorated by a factor of typically three to four. Most of the scatter appears to be random, but in the case of the wind speed, a clear dependence on wind direction is found as well. It is concluded that the correction for the effect of the vertical supporting rods of the R2 and R3 instruments, which gives nearly perfect agreement for laminar flow, does not work entirely satisfactory in the natural turbulent flow. This, in turn, is likely to be so because of high sensitivity of the wake behind the cylindrical supporting rods to the character of the approach flow.     

A multi-model study of the restratification phase in an idealized convection basin

The representation of baroclinic instability in numerical models depends strongly upon the model physics and significant differences may be found depending on the vertical discretization of the governing dynamical equations. This dependency is explored in the context of the restratification of an idealized convective basin with no external forcing. A comparison is made between an isopycnic model including a mixed layer (the Miami Isopycnic Coordinate Ocean Model, MICOM), its adiabatic version (MICOM-ADIAB) in which the mixed layer physics are removed and the convective layer is described by a deep adiabatic layer outcropping at the surface instead of a thick dense mixed layer, and a z-coordinate model (OPA model).In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of this layer. As a result, lateral heat exchanges in the upper layers are limited while mass transfers across the outer boundary of the deep convective mixed layer result in an unrealistic outward spreading of this layer. Such a widespread deep mixed layer maintains a low level of baroclinic instability, and therefore limits lateral heat exchanges in the upper layers over most of the model domain. The behavior of the adiabatic isopycnic model and z-coordinate model is by far more satisfactory although contrasted features can be observed between the two simulations. In MICOM-ADIAB, the more baroclinic dynamics introduce a stronger contrast between the surface and the dense waters in the eddy kinetic energy and heat flux distributions. Better preservation of the density contrasts around the dense water patch maintains more persistent baroclinic instability, essentially associated with the process of dense water spreading. The OPA simulation shows a faster growth of the eddy kinetic energy in the early stages of the restratification which is attributed to more efficient baroclinic instability and leads to the most rapid buoyancy restoring in the convective area among the three simulations. Dense water spreading and warm surface capping occur on fairly similar time scales in MICOM-ADIAB although the former is more persistent that the latter. In this model, heat is mainly transported by anticyclonic eddies in the dense layer while both cyclonic and anticyclonic eddies are involved in the upper layers. In OPA, heat is mainly brought into the convective zone through the export of cold water trapped in cyclonic eddies with a strong barotropic structure. Probably the most interesting difference between the z-coordinate and the adiabatic isopycnic model is found in the temperature distribution ultimately produced by the restratification process. OPA generates a spurious volume of intermediate water which is not seen in MICOM-ADIAB where the volume of the dense water is preserved.     

Multi-model Bayesian assessment of climate change in the northern annular mode

A typical question in climate change analysis is whether a certain observed climate characteristic, like a pronounced anomaly or an interdecadal trend, is an indicator of anthropogenic climate change or still in the range of natural variability. Many climatic features are described by one-dimensional index time series, like for instance the global mean temperature or circulation indices. Here, we present a Bayesian classification approach applied to the time series of the northern annular mode (NAM), which is the leading mode of Northern Hemisphere climate variability. After a pronounced negative phase during the 1950s and 1960s, the observed NAM index reveals a distinct positive trend, which is also simulated by various climate model simulations under enhanced greenhouse conditions. The objective of this study is to decide whether the observed temporal evolution of the NAM may be an indicator of global warming. Given a set of prior probabilities for disturbed and undisturbed climate scenarios, the Bayesian decision theorem decides whether the observed NAM trend is classified in a control climate, a greenhouse-gas plus sulphate aerosol climate or a purely greenhouse-gas induced climate as derived from multi-model ensemble simulations.The three climate scenarios are well separated from each other in terms of the 30-year NAM trends. The multi-model ensembles contain a weak but statistically significant climate change signal in the form of an intensification of the NAM. The Bayesian classification suggests that the greenhouse-gas scenario is the most probable explanation for the observed NAM trend since 1960, even if a high prior probability is assigned to the control climate. However, there are still large uncertainties in this classification result because some periods at the end of the 19th century and during the “warm” 1920s are also classified in an anthropogenic climate, although natural forcings are likely responsible for this early NAM intensification. This demonstrates a basic shortcoming of the Bayesian decision theorem when it is based on one-dimensional index time series like the NAM index.     

Intercomparison of NO_x, SO_2, O_3, and Aromatic Hydrocarbons Measured by a Commercial DOAS System and Traditional Point Monitoring Techniques

A field-based Intercomparison study of a commercial Differential Optical Absorption Spectroscopy (DOAS) instrument (OPSIS AB, Sweden) and different point-sample monitoring techniques (PM, based on an air monitoring station, an air monitoring vehicle, and various chemical methods) was conducted in Beijing from October 1999 to January 2000. The mixing ratios of six trace gases including NO, NO2, SO2, O3, benzene, and toluene were monitored continuously during the four months. A good agreement between the DOAS and PM data was found for NO2 and SO2. However, the concentrations of benzene, toluene, and NO obtained by DOAS were significantly lower than those measured by the point monitors. The ozone levels monitored by the DOAS were generally higher than those measured by point monitors. These results may be attributed to a strong vertical gradient of the NO-O3-NO2 system and of the aromatics at the measurement site. Since the exact data evaluation algorithm is not revealed by the manufacturer of the DOAS sys     

Does CMIP6 Inspire More Confidence in Simulating Climate Extremes over China?

Based on climate extreme indices calculated from a high-resolution daily observational dataset in China during1961–2005, the performance of 12 climate models from phase 6 of the Coupled Model Intercomparison Project(CMIP6),and 30 models from phase 5 of CMIP(CMIP5), are assessed in terms of spatial distribution and interannual variability. The CMIP6 multi-model ensemble mean(CMIP6-MME) can simulate well the spatial pattern of annual mean temperature,maximum daily maximum temperature, and minimum daily minimum temperature. However, CMIP6-MME has difficulties in reproducing cold nights and warm days, and has large cold biases over the Tibetan Plateau. Its performance in simulating extreme precipitation indices is generally lower than in simulating temperature indices. Compared to CMIP5, CMIP6 models show improvements in the simulation of climate indices over China. This is particularly true for precipitation indices for both the climatological pattern and the interannual variation, except for the consecutive dry days. The arealmean bias for total precipitation has been reduced from 127%(CMIP5-MME) to 79%(CMIP6-MME). The most striking feature is that the dry biases in southern China, very persistent and general in CMIP5-MME, are largely reduced in CMIP6-MME. Stronger ascent together with more abundant moisture can explain this reduction in dry biases. Wet biases for total precipitation, heavy precipitation, and precipitation intensity in the eastern Tibetan Plateau are still present in CMIP6-MME, but smaller, compared to CMIP5-MME.     

洞穴地点骨化石铀系年龄可信度的讨论*

骨化石是铀不平衡系测年法广泛应用而又颇有争议的研究对象。文章通过对典型铀加入模式的计算,指出以α能谱的精度,二种铀系年龄差异的显著性表明近期内有较大量铀的迁移,但其在误差范围内的一致不能保证样品构成封闭体系。以往积累的数据表明,大多数洞穴地点骨化石给出了二法一致的铀系年龄,但钙板与下伏骨化石铀系年龄大多差异显著且与地层顺序矛盾,即使被次生碳酸盐岩包裹,多半骨化石的铀系年龄仍显著偏低于其包裹体。此外,相当一部分样品的U-Th同位素比难以用简单的铀后期加入或淋失来解释。基于对次生碳酸盐岩铀系年龄可信度的认识,骨化石总体上不构成封闭体系,所载铀系年代信息只具有限的分辨率。