Orographic cloud in a GCM: the missing cirrus

Observations from the International Satellite Cloud Climatalogy Project (ISCCP) are used to demonstrate that the 19-level HadAM3 version of the United Kingdom Met Office Unified Model does not simulate sufficient high cloud over land. By using low-altitude winds, from the European Centre for Medium Range Weather Forecasting (ECMWF) Re-Analysis from 1979 to 1994 (ERA-15) to predict the areas of maximum likelihood of orographic wave generation, it is shown that much of the deficiency is likely to be due to the lack of a representation of the orographic cirrus generated by sub-grid scale orography. It is probable that this is a problem in most GCMs.     

A comparison of MODIS-derived cloud amount with visual surface observations

Two main sources for global cloud climatologies are visual surface observations and observations made by spaceborne sensors. Satellite observations compared with surface data show in most cases differences ranging from − 15% up to − 1%, depending on sensor and observation conditions. These differences are partially controlled by sensors' cloud detection capabilities — a higher number of spectral bands and higher spatial resolution are believed to allow discrimination of clouds from land/ocean/snow background. A Moderate-Resolution Imaging Spectroradiometer (MODIS) produces images of the atmosphere in 36 spectral bands with a spatial resolution of 250–1000 m, thus having a capacity for cloud detection far more advanced than other operating sensors. In this study, instantaneous MODIS cloud observations were compared with surface data for Poland for January (winter) and July (summer) 2004. It was found that MODIS observed 4.38% greater cloud amount in summer conditions and 7.28% in winter conditions. Differences were greater at night (7–8%) than in daytime (0.5–7%) and correlations ranged between 0.577 (winter night) and 0.843 (winter day, summer day and night).     

Model studies on the effect of nitric acid vapour on cirrus cloud activation

The effect of elevated nitric acid concentrations on cloud particle number concentrations and sizes of mixed phase cirrus cloud (containing both supercooled liquid and frozen ice particles) is studied using a detailed multicomponent condensation model. Our model calculations suggest that high nitric acid volume mixing ratios (VMRs), corresponding to upper tropospheric measurements, can increase the ice crystal number concentrations substantially especially in the case of freezing of non-activated solution droplets (as opposed to freezing of supercooled cloud droplets). This is due to increased droplet sizes caused by nitric acid absorption and the resulting uptake of water.     

Lidar remote sounding of cirrus clouds and comparison of simulated fluxes with surface and METEOSAT observations

Three cirrus cloud cases have been remotely sounded near Paris by a ground-based backscatter lidar and broadband radiometers. Some cirrus properties (optical depth, emissivity, height) are derived from these measurements and used to compare radiative transfer calculations to surface and METEOSAT observations of broadband irradiances.For a useful comparison, the three cirrus cases were selected to have different morphologies and optical properties: June 29, 1993—thin cirrus cloud (thickness 1.5 km, optical depth 0.22); September 6, 1993—thick cirrus cloud (thickness 5 km, optical depth 2.7); and November 16, 1993—inhomogeneous and geometrically thick cirrus cloud (thickness 3.5–6.5 km) but optically thin (optical depth 0.82).At surface, the differences between measurements and model range from 1.5 to 4 Wm⁻² for longwave fluxes, and from 20 to 70 Wm⁻² for shortwave fluxes.At the top of the atmosphere, the differences between METEOSAT measurements and model are in fair agreement for longwave fluxes (up to 50 Wm⁻²). However, unexpected high differences are found for shortwave fluxes (up to 144 Wm⁻²) due to cirrus clouds heterogeneities and uncertainties in their microphysical properties and especially the occurrence of high reflectivity due to horizontally oriented ice crystals at the cloud top, which are not taken into account by the Model presently.     

Application of a semi-spectral cloud water parameterization to cooling tower plumes simulations

In order to simulate the plume produced by large natural draft cooling towers, a semi-spectral warm cloud parameterization has been implemented in an anelastic and non-hydrostatic 3D micro-scale meteorological code. The model results are compared to observations from a detailed field experiment carried out in 1980 at Bugey (location of an electrical nuclear power plant in the Rhône valley in East Central France) including airborne dynamical and microphysical measurements. Although we observe a slight overestimation of the liquid-water content, the results are satisfactory for all the 15 different cases simulated, which include different meteorological conditions ranging from low wind speed and convective conditions in clear sky to high wind and very cloudy. Such parameterization, which includes semi-spectral determination for droplet spectra, seems to be promising to describe plume interaction with atmosphere especially for aerosols and cloud droplets.     

毫米波雷达测云个例研究

云参数是影响降水和大气辐射过程的重要因子,但对云参数的遥感探测存在许多困难。利用35GHz的毫米波雷达进行云探测,并进行云参数反演研究,反演了云水含量、冰水含量和云滴有效直径的垂直廓线,得到了6类云况的垂直分布。结果表明：1）不同类型的云具有不同的云参数分布;2）在低于-15dBz的非降水云情况下,反演的云水含量及云滴有效直径较可靠;3）雷达探测的线性退偏振比因子,可以用于判别云中的过冷却水和冰晶,有助于更好了解云的宏微观特征。     

A comparison of AMIP II model cloud layer properties with ISCCP D2 estimates

The cloud amounts and liquid and ice water paths as a function of height in five Atmospheric Model Intercomparison Project (AMIP) II models have been compared to International Satellite Cloud Climatology Project (ISCCP) d2 observations. The model layer data have been transformed to the ISCCP low, mid and height cloud amount and vertically integrated water values. In addition a simple radiative transfer model has been used to transform both model output and ISCCP cloud amount and water contents into top of atmosphere albedos for the low, mid and high cloud fractions. Overall, most models represent moderately well the spatial, seasonal and interannual variability of total cloud albedo, which is largely a function of the total cloud amount. The models also tend to predict moderately well the spatial, seasonal, and interannual variability of cloud fraction, but fail to display the observed spatial, and especially, seasonal and interannual variability in cloud water path. In particular nearly all models have mid and low cloud water path variabilities, which are much larger than those observed in the ISCCP observations. This increased cloud water path variability seems to compensate partially for smaller underestimates of cloud fraction variability in most models. Furthermore, variations in cloud amount and cloud water path are much more often negatively correlated in models than in the observations. A simple estimate of the influence of cloud overlap suggests that monthly mean model cloud layers are less stacked in the vertical in models than in an observational estimate based upon a combination of satellite and ground-based observations.     

Retrieving microphysical properties and air motion of cirrus clouds based on the doppler moments method using cloud radar

Radar parameters including radar reflectivity,Doppler velocity,and Doppler spectrum width were obtained from Doppler spectrum moments.The Doppler spectrum moment is the convolution of both the particle spectrum and the mean air vertical motion.Unlike strong precipitation,the motion of particles in cirrus clouds is quite close to the air motion around them.In this study,a method of Doppler moments was developed and used to retrieve cirrus cloud microphysical properties such as the mean air vertical velocity,mass-weighted diameter,effective particle size,and ice content.Ice content values were retrieved using both the Doppler spectrum method and classic Z-IWC(radar reflectivity-ice water content) relationships;however,the former is a more reasonable method.     

辐射模式对红外冷却率计算的影响

本文以一种精度可与逐线积分相比拟的新的红外透过率模式为基准,系统地研究了不同辐射模式以及不同光谱资料对大气红外冷却率计算的影响.结果发现:光谱资料的不同对大气平流层红外冷却率的影响大于对对流层和地面的影响.同时,我们还发现:比辐射率模式在CO_2和O_3长波冷却率的计算中带来较大的误差;使用CG近似的各种带模式方法都程度不同地带来误差;当用压力换算因子来处理大气非均匀路径时,不但应当对不同气体采用不同的n值,而且在不同的高度范围,同一气体的n值亦应不同.最后,我们表明了当用透过率相乘定律来处理重迭吸收带时,只有那些宽度小于15cm~(-1)的窄带模式才能得到比较满意的结果.     

基于卫星红外窗亮温探测上冲云顶

上冲云顶是卷云砧上的穹顶状突起,表示存在强对流切变和强烈上升气流,是强雷暴的重要指示者.伴随上冲云顶的雷暴经常产生灾害性天气,如航空湍流、强降雨、冰雹、破坏性的风和龙卷等.本文以2010年9月6日发生在安徽北部地区的一次特大暴雨过程为例,用卫星红外窗区通道观测亮温探测上冲云顶,并将高时空分辨率的局地分析预报系统(LAPS)的中尺度分析场资料与用红外窗方法探测到的上冲云顶进行定性比较,结果表明造成此次强降雨过程的对流系统即为一个伴随上冲云顶的强雷暴系统,LAPS中尺度分析场资料客观地验证了上冲云顶的存在.     

水汽红外冷却率的精确计算

本文用一种新的红外透过率模式计算了对流层和平流层水汽的长波(0—2380cm~(-1))冷却率分布.在计算中,我们精确处理了频率积分以及非均匀大气路径效应.结果表明:在近地面层,大气窗区(730—1200cm~(-1))(主要是水汽连续吸收)对总的冷却率贡献很大,在1公里处大约占72％;通常所用的处理非均匀大气路径及漫射辐射的CG近似和1.66漫射因子近似,基本上适用于水汽冷却率的计算.在整个对流层和平流层,它们所带来的最大误差不超过0.16K/日和0.19K/日.同时,我们还考查了水汽吸收系数的温度效应     

An exact calculation of infrared cooling rate due to water vapor

The longwave (0-2380 cm-1) cooling rate due to water vapor in the troposphere and the stratosphere has been calculated by a new infrared transmission model in this paper. An exact scheme is used for treating the integration over wavenumber and the inhomogeneous path in the atmosphere. It is shown that the atmospheric window region (730-1200 cm-1) (mainly water vapor continuum) plays an important role in the total cooling near the surface, about 72% of the total cooling lying in this region at the height of 1 km; the CG approximation used for an inhomogeneous path is fairly applicable for calculating the cooling rate due to water vapor, with a maximum error of 0.16 K/day throughout the troposhere and the stratosphere; on the other hand, the error due to the diffusivity factor of 1.66 appears to be slightly larger near the surface. In this study, the influences on the calculation of above infrared cooling rate, of the temperature-dependence of the absorption coefficients of water vapor, the upper level cutoff and the integration step for altitude, and the substitution of the quasi-grey approximation for the exact integration over wavenumber, are also examined.     

红外高光谱资料模拟大气廓线反演对云的敏感性

用不同云顶高度和不同有效云量时的星载红外高光谱观测值,模拟大气温湿垂直廓线反演对云参数的敏感性。用特征向量统计反演法按照云顶高度和有效云量分类反演了大气温湿廓线,统计了不同云高（200,300,500,700和850hPa）及云高有50hPa误差时温度、水汽混合比反演的均方根误差随有效云量的变化。结果表明,随着有效云量的增大,云顶以下各高度层上的温度、水汽混合比反演误差都明显增大。云高有50hPa误差较准确已知而言,温度和水汽混合比的反演误差增大,但温度反演对云高误差的敏感性比水汽反演要高。     

A numerical comparison study of cloud seeding by silver iodide and liquid carbon dioxide

A comparison study on dynamic and microphysical effects of cloud seeding by silver iodide (AgI) and liquid carbon dioxide (liquid CO₂) was made using a 3D cloud model with seeding processes. The model was initialized based on the rawinsonde sounding taken from Pinliang station located in the western China on 20 April 2001. The sounding air reflects moist and stable characteristics at middle and low layers. The model results show that the seeding by liquid CO₂ and AgI at − 15 to − 20 °C levels of cloud has almost the same dynamic effect on the simulated clouds. The seeding is able to induce the formation of weak convective cells in both seeded and unseeded regions due to latent heat released by the transformation from liquid saturation to ice saturation. However, the initial seeding conducted by liquid CO₂ in the region of maximum supercooled water with temperature of 0 to − 5 °C enable to produce much stronger dynamic effect and precipitation by forming many convective new cells at low levels in the later stage of seeded clouds. The accumulated precipitation at the surface can be increased and redistributed, and more concentrated in the downstream region of seeded clouds.     

Observed trends of heating and cooling degree-days in Xinjiang Province, China

The Mediterranean climate region on the west coast of the United States is characterized by wet winters and dry summers, and by high fire activity. The importance of synoptic-scale circulation patterns (ENSO, PDO, PNA) on fire-climate interactions is evident in contemporary fire data sets and in pre-Euroamerican tree-ring-based fire records. We investigated how interannual variability in two fire weather indices, the Haines index (HI) and the Energy Release Component (ERC), in the Mediterranean region of southern Oregon and northern California is related to atmospheric circulation and fire extent. Years with high and low fire weather index values corresponded to years with a high and low annual area burned, respectively. HI combines atmospheric moisture with atmospheric instability and variation in HI was more strongly associated with interannual variation in wildfire extent than ERC, which is based on moisture alone. The association between fire extent and HI was also higher for fires in southern Oregon than in northern California. In terms of synoptic-scale circulation patterns, years of high fire risk (i.e., increased potential for erratic fire behavior, represented by HI and ERC) were associated with positive winter PNA and PDO conditions, characterized by enhanced regional mid-tropospheric ridging and low atmospheric moisture. The time lag we found between fire risk potential and prior winter circulation patterns could contribute to the development of long-lead fire-climate forecasting.     

Spatial distributions of heating, cooling, and industrial degree-days in Turkey

Summary The degree-day method is commonly used to estimate energy consumption for heating and cooling in residential, commercial and industrial buildings, as well as in greenhouses, livestock facilities, storage facilities and warehouses. This article presents monthly and yearly averages and spatial distributions of heating, cooling, and industrial degree-days at the base temperatures of 18 °C and 20 °C, 18 °C and 24 °C, and 7 °C and 13 °C, respectively; as well as the corresponding number of days in Turkey. The findings presented here will facilitate the estimation of heating and cooling energy consumption for any residential, commercial and industrial buildings in Turkey, for any period of time (monthly, seasonal, etc.). From this analysis it will also be possible to compare and design alternative building systems in terms of energy efficiencies. If one prefers to use set point temperatures to indicate the resumption of the heating season would also be possible using the provided information in this article. In addition, utility companies and manufacturing/marketing companies of HVAC systems would be able to easily determine the demand, marketing strategies and policies based on the findings in this study.     

卷云对青藏高原的射出长波辐射影响的研究

本文叙述了一种求解卷云射出长波辐射的方法。通过建立卷云示迹模式,用累加法进行分析,并通过分析卷云的一些微物理特征,假设卷云含水量垂直分布模式,由三层近似化筒求得射出长波辐射通量密度。根据模式计算得到:1.与卫星观测青藏高原高云地区对应射出长波辐射通量密度基本一致的结果。2.卷云底高度及卷云厚度与射出长波辐射通量密度相关的一些有意义的结果,并讨论了青藏高原上卷云影响的特殊情况。     

A one-dimensional Explicit Time-dependent cloud Model (ETM): Description and validation with a three-dimensional cloud resolving model

An algorithm for a one-dimensional Explicit Time-dependent cloud Model (ETM) that takes into account non-hydrostatic pressure, entrainment, cloud microphysics, lateral and vertical eddy mixing processes is developed and tested against a state-of-the-art cloud resolving three dimensional mesoscale model—the Advanced Regional Prediction System (ARPS). The numerical schemes and sub-grid scale processes are rather similar in both ETM and ARPS, although the dimensionality is different.Results show that the ETM is able to simulate the complete lifecycle for a cloud cell, featuring comparable zones of maximum vertical velocity, and overshooting layers on the cloud top. Heat and moisture fluxes within the cloud column of the ETM occur at the same level as ARPS, giving confidence towards adequate formulations in ETM. However, mass flux fields are not in good agreement; there is significant difference in intensity and the altitude where maxima occur. Sensitivity of the ARPS cloud to the amplitude and depth of the initial thermal bubble was examined; the resulting cloud showed sensitivity to both parameters. The maximum vertical velocity decreases with greater amplitude and occurs earlier. This was used as a tuning parameter to ensure similarity in the lifecycle of ETM and ARPS clouds.     

Effects of radiation models on the calculation of atmospheric infrared cooling rates

Effects of various radiation models and spectral data on the calculation of the atmospheric infrared cool-ing rates (IRCR) have been systematically investigated by contrasting with a model, whose accuracy of IR transmission function is comparable to that of exact line-by-line integration, as a reference model.     

Recurrence method for calculation of atmospheric cooling rate due to infrared radiation

A computational method for the mean slab transmissivities of water vapour and CO2 and a recurrence me-thod for the atmospheric cooling rate due to infrared radiation are developed and discussed in some detail in this paper.At first the spectra of water vapour and CO2 are divided into 10 and 2 basic bands according to the experi-mental data; then an analytic and universal expression of the mean slab transmissivity in those bands is obtained and checked with the experimental data at 220o, 260o and 300oK. Results show that the computational method developed in this paper has quite good accuracy, and therefore it can be used in the GCMs for calculations of the atmospheric cooling rates.The recurrence method for calculations of the upward and the downward fluxes of infrared radiation is de-rived and checked. It is proved that the method has a similar accuracy as others, but saves a lot of computation time. At last a simplified method of the transmissivity calculation, i.e. the tabulation method, is developed and its accuracy is checked as well.