Ice Nucleation of Cirrus Clouds Related to the Transported Dust Layer Observed by Ground-Based Lidars over Wuhan,China

Cirrus clouds related to transported dust layers were identified on 22 occasions with ground-based polarization lidar from December 2012 to February 2018 over Wuhan (30.5°N, 114.4°E), China. All the events occurred in spring and winter. Cirrus clouds were mostly located above 7.6 km on top of the aloft dust layers. In-cloud relative humidity with respect to ice (RH_i) derived from water vapor Raman lidar as well as from ERA5 reanalysis data were used as criteria to determine the possible ice nucleation regimes. Corresponding to the two typical cases shown, the observed events can be classified into two categories: (1) category A (3 cases), in-cloud peak RH_i ≥ 150%, indicating competition between heterogeneous nucleation and homogeneous nucleation; and (2) category B (19 cases), in-cloud peak RH_i < 150%, revealing that only heterogeneous nucleation was involved. Heterogeneous nucleation generally took place during instances of cirrus cloud formation in the upper troposphere when advected dust particles were present. Although accompanying cloud-top temperatures ranged from –51.9°C to –30.4°C, dust-related heterogeneous nucleation contributed to primary ice nucleation in cirrus clouds by providing ice nucleating particle concentrations on the order of 10^?3 L^?1 to 10² L^?1. Heterogeneous nucleation and subsequent crystal growth reduced the ambient RH_i to be less than 150% by consuming water vapor and thus completely inhibited homogeneous nucleation.     

Impact of Long-range Desert Dust Transport on Hydrometeor Formation over Coastal East Asia

Model simulations and hydrological reanalysis data for 2007 are applied to investigate the impact of long-range desert dust transport on hydrometeor formation over coastal East Asia.Results are analyzed from Hong Kong and Shanghai,which are two representative coastal cities of East Asia.Long-range desert dust transport impacts mainly spring and summer clouds and precipitation over coastal East Asia.In spring,clouds and precipitation come mainly from large-scale condensation and are impacted mainly by dust from the Gobi,Sahara,and Thar deserts.These desert dusts can participate in the precipitation within and below the clouds.At lower latitudes,the dust particles act mainly as water nuclei.At higher latitudes,they act as both water nuclei and ice nuclei.The effect of Gobi,Sahara,and Thar dust on large-scale clouds and precipitation becomes stronger at higher latitudes.In summer,clouds and precipitation over coastal East Asia come mainly from convection and are impacted mainly by dust from the Taklamakan,Arabian,and Karakum-Kavir deserts.Most Taklamakan dust particles can participate in precipitation within convective clouds as ice nuclei,while Arabian and Karakum-Kavir dust particles participate only as water nuclei in precipitation below the clouds.The effect of Taklamakan dust on convective clouds and precipitation becomes stronger at lower latitudes.Of all the desert dusts,that from the Gobi and Taklamakan deserts has the relatively largest impact.Gobi dust impacts climate change in coastal East Asia by affecting spring water clouds at higher latitudes.     

Numerical simulations of hailstorm feeder‐cloud initiation

Abstract

A simple, moist two‐dimensional numerical model is developed and used to study the mechanisms for the formation of hailstorm feeder clouds. By using high spatial resolution, the model is able to simulate low‐level storm outflows and reproduce the observed separation between hailstorm feeder clouds. The cold outflow emanating from the storm is induced by evaporation of precipitation. The simulations show that feeder clouds develop in response to the gravity waves generated by wind shear that is inherent in the ambient environment and/or created by the interaction between the low‐level inflow and the storm outflow. The subcloud moisture also plays an important role in the formation of a wave‐like line of feeder clouds. It contributes in the cloud development as well as in the enhancement of gravity waves.     

A Lee wave cloud climatology for pincher creek,Alberta

Abstract

The annual, monthly and diurnal frequencies and durations of wave clouds are summarized from twelve years of observations at Pincher Creek, Alberta. Wave clouds occur on an average of 141 days annually. There is a distinct winter maximum anda summer minimum of occurrence. Wave clouds occur on 50% or more of the days during October and February, the most active months. Wave clouds persist for longer periods in the cooler half of the year. There appears to be a marked diurnal variation in wave cloudfrequency with maxima in the afternoon, however, this may be due to decreased visibility at sunrise and sunset.     

Measurements of bubble plumes and turbulence from a submarine

Abstract

An experiment using turbulence probes and an array of side‐scan and vertically pointing pencil beam sonars mounted on the U.S. submarine Dolphin was carried out to measure turbulence in near‐surface regions of acoustic scattering, in particular, those caused by subsurface bubbles produced by breaking wind waves. The dataset collected during winds of 5–9 m s^?1 reveals the banded patterns of bubbles associated with Langmuir circulation, even though no surface manifestations were visible.

A forward‐pointing side‐scan sonar determined the “age” of bubble clouds after their generation by breaking waves. There is enhanced turbulent dissipation in the bubble clouds, and the dissipation rate close to the surface exceeds that predicted using conventional calculations based on the law of the wall and buoyancy flux. The correspondence between bubbles and turbulence implies a horizontally patchy turbulent structure near the surface. Below the base of the bubble clouds the distance between turbulent patches increases and is much greater than that of the bubble clouds. The submarine provides an excellent platform for multi‐sonar near‐surface studies.     

Application of the delta‐Eddington method to the absorption of solar radiation in the atmosphere

Abstract

A model to compute rapidly the absorption of solar radiation in the atmosphere is described. The model is based partially on the parameterization of Lacis and Hansen and also makes use of the delta‐Eddington method. In addition to absorption by ozone and water vapour, and scattering by air molecules and clouds, the mode1 includes absorption and scattering by aerosols. Good agreement is found in comparison with the Lacis and Hansen parameterization in the absence of clouds and aerosol. The present model represents an improvement in the treatment of scattering by clouds. Its main advantage though, is in its flexibility in allowing for interactions with the atmospheric aerosol.     

合肥上空卷云和沙尘气溶胶退偏振比的激光雷达探测

研制了一台L300偏振激光雷达,用于卷云和沙尘气溶胶后向散射光退偏振比的探测研究.介绍了偏振激光雷达的探测原理,叙述了L300偏振激光雷达的结构、技术参数、测量方法和数据处理方法.初步的探测结果表明,合肥西郊上空高度在10 km左右的卷云的退偏振比在0.4～0.5之间,沙尘气溶胶的退偏振比在0.2～0.3范围内,但是剧烈沙尘暴的气溶胶的退偏振比可达到0.4左右.     

An Overview of Passive and Active Dust Detection Methods Using Satellite Measurements

In this paper, the methods to detect dust based on passive and active measurements from satellites have been summarized. These include the visible and infrared (VIR) method, thermal infrared (TIR) method, microwave polarized index (MPI) method, active lidar-based method, and combined lidar and infrared measurement (CLIM) method. The VIR method can identify dust during daytime. Using measurements at wavelengths of 8.5, 11.0, and 12.0 μm, the TIR method can distinguish dust from other types of aerosols and cloud, and identify the occurrence of dust over bright surfaces and during night. Since neither the VIR nor the TIR method can penetrate ice clouds, they cannot detect dust beneath ice clouds. The MPI method, however, can identify about 85% of the dust beneath ice clouds. Meanwhile, the active lidar-based method, which uses the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data and five-dimensional probability distribution functions, can provide very high-resolution vertical profiles of dust aerosols. Nonetheless, as the signals from dense dust and thin clouds are similar in the CALIOP measurements, the lidar-based method may fail to distinguish between them, especially over dust source regions. To address this issue, the CLIM method was developed, which takes the advantages of both TIR measurements (to discriminate between ice cloud and dense dust layers) and lidar measurements (to detect thin dust and water cloud layers). The results obtained by using the new CLIM method show that the ratio of dust misclassification has been significantly reduced. Finally, a concept module for an integrated multi-satellites dust detection system was proposed to overcome some of the weaknesses inherent in the single-sensor dust detection.     

Discrimination and Validation of Clouds and Dust Aerosol Layers over the Sahara Desert with Combined CALIOP and IIR Measurements

This study validates a method for discriminating between daytime clouds and dust aerosol layers over the Sahara Desert that uses a combination of active CALIOP（Cloud-Aerosol Lidar with Orthogonal Polarization） and passive IIR（Infrared Imaging Radiometer） measurements;hereafter,the CLIM method.The CLIM method reduces misclassification of dense dust aerosol layers in the Sahara region relative to other techniques.When evaluated against a suite of simultaneous measurements from CALIPSO（Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations）,CloudSat,and the MODIS（Moderate-resolution Imaging Spectroradiometer）,the misclassification rate for dust using the CLIM technique is 1.16%during boreal spring 2007.This rate is lower than the misclassification rates for dust using the cloud aerosol discriminations performed for version 2（V2-CAD;16.39%） or version 3（V3-CAD;2.01%） of the CALIPSO data processing algorithm.The total identification errors for data from in spring 2007 are 13.46%for V2-CAD,3.39%for V3-CAD,and 1.99%for CLIM.These results indicate that CLIM and V3-CAD are both significantly better than V2-CAD for discriminating between clouds and dust aerosol layers.Misclassifications by CLIM in this region are mainly limited to mixed cloud-dust aerosol layers.V3-CAD sometimes misidentifies low-level aerosol layers adjacent to the surface as thin clouds,and sometimes fails to detect thin clouds entirely.The CLIM method is both simple and fast,and may be useful as a reference for testing or validating other discrimination techniques and methods.     

NOAA卫星沙尘暴光谱特征分析及信息提取研究

通过对2000—2002年多次沙尘暴过程NOAA卫星AVHRR资料的分析,研究了沙尘、云、沙漠、戈壁、积雪、裸地、植被等不同目标物的光谱特性,发现沙尘暴在AVHRR-2中各通道均有不同程度的反映。1,2通道中沙尘的反射率较高(介于云和沙漠之间);4,5通道的亮温低于晴空地表高于云;在3通道中沙尘表现的很独特,其亮度温度为所有研究目标物中最高的,表明通道3包含较多的沙尘信息,AVHRR-3取代AVHRR-2后对监测沙尘有不利影响。在此基础上提出定量提取沙尘信息的两种沙尘判识指数,并利用沙尘判识指数成功地提取多次沙尘暴过程的沙尘信息。结果表明：利用多通道组合沙尘判识指数能够对沙尘信息进行有效提取。     

GATE A/B‐scale budget analysis

Abstract

Using GATE A/B‐scale U.S.S.R. ship data, heat and moisture budget analyses have been carried out for the three‐day period: 0000 GMT, 7 September to 2400 GMT, 9 September, 1974. The period has been subdivided into an undisturbed period (0000 GMT, 7 September ‐1800 GMT, 8 September) and a disturbed period (1800 GMT, 8 September ‐ 2400 GMT, 9 September) based on surface precipitation and satellite cloud observations. During the undisturbed period, precipitation was very light (1–3 mm day^‐1). On the other hand, the precipitation rate became well over 10 mm day^‐1 during the disturbed period. A/B‐scale heat and moisture budget results for both periods are presented.

It is found that during the undisturbed period, cumulus clouds have heating and moistening effects in the lower troposphere below 700 mb, and cooling and moistening effects in the upper troposphere above 600mb. In the disturbed period, clouds have strong heating and drying effects throughout the entire cloud layer.

Using the diagnostic scheme developed by Cho (1977), the collective properties of cumulus clouds for both the undisturbed and the disturbed periods are also determined. During the undisturbed period, some clouds reached as high as the 300‐mb level, however, little precipitation was produced. The total cloud mass flux is found to be negative in the upper troposphere and can be attributed to downdrafts induced by the evaporation of cloud liquid water. On the other hand, total cloud mass flux for the disturbed period is positive throughout the entire troposphere.     

The delineation of rain areas from visible and IR satellite data for GATE and mid‐latitudes

Abstract

Two‐dimensional pattern matching has been used to delineate raining areas of clouds from GATE and Montreal GOES visible and IR satellite data, with radar as ground truth. For the cases examined, the cloud cover was of the order of 4 times larger than the rain area, requiring skill to separate out low‐thick or high‐thin non‐precipitating clouds from cumulus systems, which is difficult using a single threshold. The more flexible approach described here has allowed useful rain maps to be generated for all the types of weather systems examined. The optimum boundary separating raining from non‐raining areas is relatively insensitive to diurnal and day‐to‐day variations, but is different for the tropical Atlantic and for Montreal.     

The dynamics of moist frontogenesis in a semi‐geostrophic model

Abstract

The effects of condensational heating on the semi‐geostrophic dynamics of frontogenesis are studied using a two‐dimensional deformation model. The model includes water vapour and allows the formation of stratiform clouds. Analysis and numerical results show that heating due to stratiform clouds has the effect of reducing stability to slantwise convection, as found in previous studies (Thorpe and Emanuel, 1985). In addition, heating‐induced potential vorticity and temperature anomalies play a very important role in the frontal circulation. The ageostrophic flow induced by these anomalies tends to reinforce the effect of heating and increases the strength of frontal cloud. The model is also able to produce the low‐level jet maximum ahead of a cold front at an elevated level, in agreement with observations, owing to the explicit condensation scheme used in the model.     

Cloud mapping from the earth's surface using infrared radiance contrasts*

Abstract

Radiance criteria for distinguishing low, middle, and high clouds in the 9.5–11.5 μm band of the infrared are developed and used to produce local cloud maps. The performance of this radiance contrast method for mapping clouds from the earth's surface is evaluated with a view to using the technique for objective observation of cloud amount and distribution in the sky hemisphere.

Discrimination radiance formulas are developed using a multilayer, wavenumber‐specific infrared radiative transfer model including cloud parameters measured by other workers and atmospheric conditions measured by radiosonde. The clear sky radiance (N₀) is the dominant independent variable in the discrimination formulas. The variation of N₀ with time (primarily due to changes in atmospheric water vapour content) and zenith angle are found to be important in distinguishing cirriform clouds from clear sky and other clouds.

The local cloud maps are produced by applying the discrimination radiances (in voltage form) to the output from a narrow‐view infrared radiometer pointed at a sky‐scanning mirror. It is necessary to assume that the radiance from a cloud observed at the surface decreases unambiguously with an increase in cloud base height. Cloud maps for five days in July 1978 indicate the mapping technique shows promise under a wide range of sky conditions. Cloud motions rapidly degrade the maps’ quality over time‐scales that are much less than the current manual cloud sampling period of 1 h.     

一次沙尘暴过程中沙尘气溶胶对云物理量和辐射强迫的影响

利用MOD IS Aqua卫星反演和装载在Aqua上的CERES仪器观测资料,讨论了我国北方地区2004年3月26~28日沙尘暴过程中沙尘气溶胶对云物理特性和辐射强迫的影响。初步结果表明,沙尘气溶胶明显改变了云的物理特性,使云滴变小,含水量及光学厚度减少,减弱了云的净辐射强迫,云的冷却效应受到了抑制,相当于大气层顶沙尘气溶胶的增温作用。     

A proposal for a new statistic and technique development for the design and evaluation of cloud seeding experiments

Abstract

The use of short‐term predictions of rain flux from isolated showers is discussed in the context of the design and evaluation of cloud‐seeding experiments. It is found that for a sample of 85 seeded convective clouds a seeding effect of 25% could be detected at the 10% significance level.     

2006年两次影响北京的沙尘天气对比分析

利用卫星、再分析(NECP)、地面观测等多种资料,针对2006年3和4月影响北京的两次沙尘天气过程,就天气过程形势、云系演变以及沙尘传播和沉降进行分析.前者对北京上空的沙尘天气一扫而过,后者对北京的影响则是遍地黄沙.对比分析表明:1)强沙尘天气过程是蒙古气旋所引起的,沙尘天气起源于中蒙边境、内蒙古西部,在河套地区北部鄂尔多斯高原以及浑善达克沙地得到加强,并随着高空西北风将沙尘输送到华北、渤海、黄海北部一带.卫星遥感监测技术不仅可以有效地监测到沙尘天气的发展演变过程,也能有效地监测到沙尘的覆盖范围和沉降区;2)导致4月16～17日华北东部严重沙尘沉降现象的原因是地面冷锋静止于华北中部地区,距地面1 km以上的浮尘层稳定覆盖华北东部和渤海等广大区域的主要原因,是边界层中尺度涡旋对低空冷锋的阻挡作用;冷锋云系在东移发展过程中,受到日本海上空涡旋云系的顶推阻碍,其结构发生变型瓦解.     

中国沙漠沙尘气溶胶对沙漠源区及北太平洋地区大气辐射加热的影响

针对2001年春季中国沙漠和北太平洋上空沙尘气溶胶的空间分布情况,利用辐射传输模式,分别计算了沙尘气溶胶对沙漠和海洋大气的辐射加热（冷却）率,并讨论了低云、中云、高云对辐射加热率的影响。结果表明:春季,位于中国沙漠和太平洋上空的沙尘层对大气具有明显的加热作用。当沙漠上空光学厚度为1.0,海洋上空光学厚度为0.3时,取春季平均太阳高度角,沙尘层对应的净辐射加热率分别为2.8 K/d和0.4 K/d。由于WMO推荐的沙尘模型比东亚沙尘模型对太阳辐射吸收强,采用该模型计算得到的中国沙漠和海洋上空的加热率比采用东亚沙尘模型分别高1.5 K/d和0.2 K/d。沙尘对大气的加热率很大程度上依赖于沙尘的大气载荷。这种依赖性首先受太阳高度角的影响, 其次也受地表反照率的影响。云对沙尘层辐射加热（冷却）率的影响与云的高度和厚度有关。低云能够加热沙漠和海洋上空的沙尘大气,冷却地面和洋面。中、高云冷却沙漠上空的沙尘层。在海洋上空,中云对云层以上的沙尘层有加热作用,对云层以下的沙尘层有冷却作用。高云对海洋上空沙尘层的辐射加热（冷却）率的影响比较小,加热还是冷却,取决于云的厚度,当云层较薄时,加热沙尘层,而当云层较厚的时候,有可能冷却沙尘层。     

Parhelia in the planetary boundary layer at newmarket,Ontario: Note

Abstract

Observations of parhelia at Newmarket, Ontario on December 10, 1977 support an earlier suggestion regarding observations made in Toronto on January 22 of the same year. Parhelia were found to occur in the planetary boundary layer (PBL), particularly during the morning hours under blowing snow conditions on clear, cold (ca —17°C) winter days. Thus parhelia which occur in southern Ontario during the winter months can develop in the PBL without the presence of cirrostratus‐type clouds.     

Examining the aerosol indirect effect over China using an SO2 emission inventory

The relationship between SO₂ emissions and the effective particle radius of low-level water clouds (r_e) over China was investigated to determine anthropogenic effects on clouds. Sulfur dioxide emission values were obtained from a statistical inventory, and r_e origins were retrieved by satellite remote sensing on a 0.5° grid. Comparisons between annual mean SO₂ emissions and r_e showed generally decreasing r_e values, explained by the Twomey effect. The existence of the Twomey effect is supported by comparisons with simulated aerosol optical depths. Results further suggest that clouds over land show sensitivity to the Twomey effect as well as clouds over the ocean.