A study of vertical cloud structure of the Indian summer monsoon using CloudSat data

Precise specification of the vertical distribution of cloud optical properties is important to reduce the uncertainty in quantifying the radiative impacts of clouds. The new global observations of vertical profiles of clouds from the CloudSat mission provide opportunities to describe cloud structures and to improve parameterization of clouds in the weather and climate prediction models. In this study, four years (2007–2010) of observations of vertical structure of clouds from the CloudSat cloud profiling radar have been used to document the mean vertical structure of clouds associated with the Indian summer monsoon (ISM) and its intra-seasonal variability. Active and break monsoon spells associated with the intra-seasonal variability of ISM have been identified by an objective criterion. For the present analysis, we considered CloudSat derived column integrated cloud liquid and ice water, and vertically profiles of cloud liquid and ice water content. Over the South Asian monsoon region, deep convective clouds with large vertical extent (up to 14 km) and large values of cloud water and ice content are observed over the north Bay of Bengal. Deep clouds with large ice water content are also observed over north Arabian Sea and adjoining northwest India, along the west coast of India and the south equatorial Indian Ocean. The active monsoon spells are characterized by enhanced deep convection over the Bay of Bengal, west coast of India and northeast Arabian Sea and suppressed convection over the equatorial Indian Ocean. Over the Bay of Bengal, cloud liquid water content and ice water content is enhanced by ~90 and ~200 % respectively during the active spells. An interesting feature associated with the active spell is the vertical tilting structure of positive CLWC and CIWC anomalies over the Arabian Sea and the Bay of Bengal, which suggests a pre-conditioning process for the northward propagation of the boreal summer intra-seasonal variability. It is also observed that during the break spells, clouds are not completely suppressed over central India. Instead, clouds with smaller vertical extent (3–5 km) are observed due to the presence of a heat low type of circulation. The present results will be useful for validating the vertical structure of clouds in weather and climate prediction models.     

Comparison of cloud properties between cloudsat retrievals and airplane measurements in mixed-phase cloud layers of weak convective and stratus clouds

Cloud microphysical properties including liquid and ice particle number concentration (NC), liquid water content (LWC), ice water content (IWC) and effective radius (RE) were retrieved from CloudSat data for a weakly convective and a widespread stratus cloud. Within the mixed-phase cloud layers, liquid-phase fractions needed to be assumed in the data retrieval process, and one existing linear (p1) and two exponential (p2 and p3) functions, which estimate the liquid-phase fraction as a function of subfreezing temperature (from -20°C to 0°C), were tested. The retrieved NC, LWC, IWC and RE using p1 were on average larger than airplane measurements in the same cloud layer. Function p2 performed better than p1 or p3 in retrieving the NCs of cloud droplets in the convective cloud, while function p1 performed better in the stratus cloud. Function p3 performed better in LWC estimation in both convective and stratus clouds. The REs of cloud droplets calculated using the retrieved cloud droplet NC and LWC were closer to the values of in situ observations than those retrieved directly using the p1 function. The retrieved NCs of ice particles in both convective and stratus clouds, on the assumption of liquid-phase fraction during the retrieval of liquid droplet NCs, were closer to those of airplane observations than on the assumption of function p1.     

Coupled model simulations of boreal summer intraseasonal (30–50 day) variability, Part 1: Systematic errors and caution on use of metrics

Boreal summer intraseasonal (30–50 day) variability (BSISV) over the Asian monsoon region is more complex than its boreal winter counterpart, the Madden–Julian oscillation (MJO), since it also exhibits northward and northwestward propagating convective components near India and over the west Pacific. Here we analyze the BSISV in the CMIP3 and two CMIP2+ coupled ocean–atmosphere models. Though most models exhibit eastward propagation of convective anomalies over the Indian Ocean, difficulty remains in simulating the life cycle of the BSISV, as few represent its eastward extension into the western/central Pacific. As such, few models produce statistically significant anomalies that comprise the northwest to southeast tilted convection, which results from the forced Rossby waves that are excited by the near-equatorial convective anomalies. Our results indicate that it is a necessary, but not sufficient condition, that the locations the time-mean monsoon heat sources and the easterly wind shear be simulated correctly in order for the life cycle of the BSISV to be represented realistically. Extreme caution is needed when using metrics, such as the pattern correlation, for assessing the fidelity of model performance, as models with the most physically realistic BSISV do not necessarily exhibit the highest pattern correlations with observations. Furthermore, diagnostic latitude-time plots to evaluate the northward propagation of convection from the equator to India and the Bay of Bengal also need to be used with caution. Here, incorrectly representing extratropical–tropical interactions can give rise to “apparent” northward propagation when none exists in association with the eastward propagating equatorial convection. Despite these cautions, the use of multiple cross-checking diagnostics enables the fidelity of the simulation of the BSISV to be meaningfully assessed.     

Seasonal variation and physical properties of the cloud system over southeastern China derived from CloudSat products

Based on the National Centers for Environmental Prediction(NCEP) and Climate Prediction Center(CPC) Merged Analysis of Precipitation(CMAP) data and Cloud Sat products, the seasonal variations of the cloud properties, vertical occurrence frequency, and ice water content of clouds over southeastern China were investigated in this study. In the Cloud Sat data, a significant alternation in high or low cloud patterns was observed from winter to summer over southeastern China. It was found that the East Asian Summer Monsoon(EASM) circulation and its transport of moisture leads to a conditional instability, which benefits the local upward motion in summer, and thereby results in an increased amount of high cloud. The deep convective cloud centers were found to coincide well with the northward march of the EASM, while cirrus lagged slightly behind the convection center and coincided well with the outflow and meridional wind divergence of the EASM. Analysis of the radiative heating rates revealed that both the plentiful summer moisture and higher clouds are effective in destabilizing the atmosphere. Moreover, clouds heat the mid-troposphere and the cloud radiative heating is balanced by adiabatic cooling through upward motion, which causes meridional wind by the Sverdrup balance. The cloud heating–forced circulation was observed to coincide well with the EASM circulation, serving as a positive effect on EASM circulation.     

Some physical aspects of summer monsoon clouds—comparison of cloud model results with observations

The physical characteristics of the summer monsoon clouds were investigated. The results of a simple cloud mod-el were compared with the aircraft cloud physical observations collected during the summer monsoon seasons of 1973,1974,1976 and 1981 in the Deccan Plateau region.The model predicted profiles of cloud liquid water content (LWC) are in agreement with the observed profiles. There is reasonable agreement between the model predicted cloud vertical thickness and observed rainfall.The observed cloud-drop spectra were found to be narrow and the concentration of drops with diameter >20μm is either low or absent on many occasions. In such clouds the rain-formation cannot take place under natural atmos-pheric conditions due to the absence of collision-coalescence process. A comparison of the model predicted and ob-served rainfall suggested that the precipitation efficiency in cumulus clouds of small vertical thickness could be as low as 20 per cent.The clouds forming in the Deccan Plateau region during the summer monsoon are, by and large, cumulus and strato-cumulus type. The vertical thickness of the cumulus clouds is in the range of 1.0-2.0 km. The LWC is found to be more in the region between 1.6-1.9 km A. S. L., which corresponds to the level at almost 3 / 4 th of the total verti-cal thickness of the cloud and thereafter the LWC sharply decreased. Nearly 98 per cent of the tops of the low clouds in the region are below freezing level and the most frequent range of occurrence of these cloud-tops is in the range of 2.0-3.0 km A. S. L.. The dominant physical mechanism of rain-formation in these summer monsoon clouds it the col-lision-coalescence process.     

基于飞机观测的美国落基山地区冬季混合相态层状云与夏季对流云的微物理特征

混合相态层状云与对流云的微物理特征有很大的差异性,但现阶段数值模式中并没有充分考虑两者的区别,这是导致云降水的模拟有较大不确定性的原因之一。为了加深对层状云与对流云的微物理特征差异的理解,并为模式的验证和参数化开发提供支撑,本文基于在中落基山地区进行的Ice in Clouds Experiment—Layer Clouds（ICE-L）项目和High Plain Cumulus（HiCu）项目的飞机观测资料,定量对比分析了该地区大陆性混合相态冬季较浅薄的层状云与较弱及中等强度的夏季对流云的微物理特征。其中,粒子图像和粒子谱通过2D-Cloud和2D-Precipitation探头得到,液态水含量通过热线式King探头测量得到,冰水含量基于粒子谱计算得到。主要结论有:（1）在?30°C～0°C的温度层范围内,夏季对流云内的液态水含量比冬季层状云高一个数量级,冰水含量高一到两个数量级,并且在对流云云顶附近观测到更多的过冷水。此外,夏季对流云中液态水含量在?20°C～0°C上随温度降低而升高,而冬季层状云则相反。夏季对流云中更活跃的冰晶生成和生长过程使得云内液态水质量分数小于层状云。（2）冬季层状云与夏季对流云内相态空间分布极不均匀。随着温度从0°C降低到?30°C,在冬季层状云中冰晶发生贝吉龙过程,云中的过冷水为主的区域向混合相态和冰相转化。而夏季对流云中相态结构更为复杂,体现了对流云中复杂的冰水相互作用。（3）在?30°C～0°C的温度范围内,夏季对流云的粒子谱宽度大于冬季层状云。随着温度的降低,冬季层状云与夏季对流云均存在粒子谱增宽的现象。（4）冬季层状云中,温度低于?20°C时冰晶主要为无规则状,在?20°C～?10°C观测到了辐枝状和无规则状冰晶,在?10°C以上观测到了柱状和无规则状冰晶,说明冰晶的生长主要为凝华增长和碰并增长。而夏季对流云以冻滴、霰粒子与不规则冰晶为主,说明主要为液滴冻结、淞附增长和碰并增长为主。（5）在夏季对流云较强的上升气流中存在较高的液态水含量,但垂直速度与云内冰水含量没有明显的相关性。     

An Attempt to Improve Kessler-Type Parameterization of Warm Cloud Microphysical Conversion Processes Using CloudSat Observations

Improvements to the Kessler-type parameterization of warm cloud microphysical conversion processes (also called autoconversion) are proposed based on a large number of CloudSat observations between Jun...     

河北省层状云降水系统微物理结构的飞机观测研究

对4架次飞行个例的PMS资料进行综合分析,发现河北省春季层状云降水系统存在不均匀性,表现之一为较强降水云带。1991年5月25日的个例在飞机上升和下降过程中两次在2000m左右探测到较强的云内逆温,逆温层顶下方存在云水含量的峰值。对1992年6月20日两次个例的冷云的冰晶尺度、冰水含量(IWC)、冰晶浓度和液水含量(IWC)的垂直分布进行了分析。对两次个例的云中可播性进行判别,发现潜力区占云区的1／2左右,有时存在大片强可播区。     

青藏高原、东亚季风区、西北太平洋地区云系结构及相联加热机制的对比分析

应用7年(2006年5月18日—2013年5月18日)的CloudSat卫星观测资料,对比分析了青藏高原、东亚季风区、西北太平洋地区云发生频率的特征,并利用欧洲中心再分析资料,计算了三个地区的视热源、视水汽汇Q₁、Q₂,分析探讨了三个地区与云发生频率相联系的加热机制。结果表明:青藏高原、东亚季风区、西北太平洋地区云的发生频率分别为35%、22%、27%,其中:青藏高原和东亚季风区的低云频率最大,中云次之;西北太平洋地区的高云和低云的频率大,分别为19%和16%。具体云型来看,青藏高原多高层云、雨层云;东亚季风区多高层云和卷云,夏季深对流云频率增大明显;西北太平洋地区多卷云、深对流云和高层云。三个地区视水汽汇Q₂的垂直分布特征及季节变化与云发生频率对应较好,青藏高原的低云(雨层云)、中云(高层云)形成过程中,凝结释放潜热,加热大气;东亚季风区低云(深对流云)、中云(高层云)对加热大气贡献大;西北太平洋地区大气的主要加热机制是深对流云形成过程中凝结释放潜热以及湿静能涡旋垂直输送。      

A possible new mechanism for northward propagation of boreal summer intraseasonal oscillations based on TRMM and MERRA reanalysis

Boreal summer intraseasonal oscillations (BSISOs) manifest in the active and break spells and act as the primary building block of the Indian summer monsoon. Although recent research has evolved a basic framework for understanding the scale selection and northward propagation of the BSISO, the role of different hydrometeors in modulating these processes remains poorly explored. In this study, TRMM-2A12 retrievals and Modern Era Retrospective-analysis for Research and Applications reanalysis data are examined to establish relationship between cloud hydrometeors and other atmospheric dynamical parameters with the northward propagation of the BSISOs. The study reveals that the cloud liquid water leads the deep convection during the northward propagation of BSISOs in the lower troposphere, while the cloud ice slightly lags the convection. This distribution indicates the occurrence of a possible mechanism of the lower level moistening through the large scale moisture advection in lower atmosphere and boundary layer (PBL) convergence, followed by triggering of the deep convection. The analyses of moisture advection and the dynamical fields with respect to the convection center show that low level moistening is a manifestation of the barotropic vorticity and PBL convergence of moisture anomaly north of the convection center. A new internal dynamical-thermodynamical mechanism is unraveled to understand the reason behind the middle tropospheric heating maximum and its role on the northward propagation. It is shown that the enhanced moisture perturbation in lower levels together with the heat transport by the sub-grid scale eddies within the PBL induces lower level instability required to precondition the lower atmosphere for triggering the deep convection. Vigorous upward motion inside the deep convection uplifts the liquid hydrometeors to upper levels and the formation of precipitable ice leads to the heating maxima in the middle troposphere. To check the robustness of the proposed hypothesis, similar analysis is performed for the weak northward propagating BSISO cases.     

副热带东亚季风区一次穿透性对流过程影响下平流层成分变化的个例分析

穿透性对流是导致北半球夏季平流层低层南亚高压内水汽极值形成的重要机制之一,关于副热带东亚季风区穿透性对流是否对平流层低层水汽等物质分布存在影响目前尚不清楚。本文选取2016年的武汉暴雨事件,采用Cloudsat和Aura Microwave Limb Sounder（MLS）卫星数据,分析了东亚季风区的穿透性对流活动对上对流层/下平流层物质分布的影响。利用CloudSat卫星资料云分类产品和Aura MLS卫星数据联合分析武汉暴雨过程中捕捉到1次穿透性对流事件,该事件发生于2016年7月4日05时（协调世界时）的穿透性对流,中心位于海上梅雨带区域。分析表明,这次对流穿透事件对上对流层/下平流层物质分布有显著影响,穿透性对流活动影响到对流层顶以上的物质分布,具体表现是:首先,穿透性对流显著减少了局地对流层顶附近的臭氧含量,较之气候态对流层顶臭氧含量偏少32.53%;其次,穿透性对流能够增加局地对流层顶附近的水汽混合比含量,它通过更多的云冰粒子蒸发来增强局地平流层水汽含量,同时通过更强的垂直水汽输送来直接加湿平流层。此次穿透性对流事件对水汽变化影响较之对臭氧含量变化的影响更为显著,它使得对流层顶水汽混合比增加近乎一倍（98.15%）。因此,副热带东亚季风区的穿透性对流活动对于对流层向平流层的物质输送起着重要的作用。     

Application of aircraft observations over Beijing in cloud microphysical property retrievals from CloudSat

Cloud microphysical property retrievals from the active microwave instrument on a satellite require the cloud droplet size distribution obtained from aircraft observations as a priori data in the iteration procedure.The cloud lognormal size distributions derived from 12 flights over Beijing,China,in 2008-09 were characterized to evaluate and improve regional CloudSat cloud water content retrievals.We present the distribution parameters of stratiform cloud droplet （diameter ＜500 tm and ＜1500 μm） and discuss the effect of large particles on distribution parameter fitting.Based on three retrieval schemes with different lognormal size distribution parameters,the vertical distribution of cloud liquid and ice water content were derived and then compared with the aircraft observations.The results showed that the liquid water content （LWC） retrievals from large particle size distributions were more consistent with the vertical distribution of cloud water content profiles derived from in situ data on 25 September 2006.We then applied two schemes with different a priori data derived from flight data to CloudSat overpasses in northern China during April-October in 2008 and 2009.The CloudSat cloud water path （CWP） retrievals were compared with Moderate Resolution Imaging Spectroradiometer （MODIS） liquid water path （LWP） data.The results indicated that considering a priori data including large particle size information can significantly improve the consistency between the CloudSat CWP and MODIS CWP.These results strongly suggest that it is necessary to consider particles with diameters greater than 50 tm in CloudSat LWC retrievals.     

基于CloudSat/CALIPSO卫星资料的青藏高原云辐射及降水的研究进展

青藏高原上空的云及其相关联的降水和辐射影响了高原上空非绝热加热的空间结构。2006年卫星发射升空的CloudSat/CALIPSO卫星提供了定量的、完整的云垂直结构信息。本文回顾了国内外基于该资料进行的青藏高原上云宏观和微观结构特征,云与降水相关性,云辐射效应以及模式中的云-辐射问题方面的研究。指出抬升的青藏高原上水汽较少,限制了高原上云的垂直高度,对云层厚度和层数有显著压缩作用。在云量及其季节变化上,单层云的相对贡献大于亚洲季风区的其他区域;夏季对流云比较浅薄,积云发生频率最高,云内滴谱较宽;降水云以积云和卷云为主,云对总降水的贡献随着云层数增多而减小,降水增强时高层冰粒子的密集度趋于紧密;夏季青藏高原地区云的净辐射效应在8 km高度存在一个厚度仅1 km左右但较强的辐射冷却层,而在其下（4~7 km高度之间）为强的辐射加热层。最后展望了未来需要进一步开展的研究。     

亚洲热带气候态夏季风涌传播特征及其对中国降水的影响

基于1979—2020年逐日的NOAA向外长波辐射资料、NCEP/NCAR再分析风场资料,以及全球CMAP再分析降水资料,探讨了气候态亚洲热带夏季风涌的传播过程及与我国夏季相应的降水联系。分析结果表明,主汛期亚洲热带气候态夏季风季节内振荡(CISO)活动是亚洲夏季风活动的主要特征,随时间北传的亚洲热带夏季风CISO称为亚洲热带夏季风涌,主要有南亚夏季风涌和南海夏季风涌。亚洲热带夏季风涌的传播可分为四个阶段。在亚洲热带夏季风涌的发展阶段,印度洋区域低频气旋与对流活跃,孟加拉湾和南海热带区域被低频东风控制,我国大部分地区无降水发生,降水中心位于两广地区。当进入亚洲热带夏季风涌活跃阶段,孟加拉湾和南海热带地区低频气旋和对流活跃,东亚低频“PJ”波列显著,我国降水中心北移到长江以南的附近区域。亚洲热带夏季风涌减弱阶段,孟加拉湾与南海低频气旋消亡,对流减弱,低频西风加强,日本南部附近为低频反气旋控制,我国长江中下游低频南风活跃,降水中心也北移到长江中下游地区,而华南地区已基本无降水,此阶段的大气低频环流场与亚洲热带夏季风涌发展阶段基本相反。进入亚洲热带夏季风涌间歇阶段时,孟加拉湾和南海热带地区低频反气旋活跃,对流不显著,日本南部附近的低频反气旋北移减弱,我国东部基本在低频南风的控制下,降水中心也逐步北移到华北-朝鲜半岛一带,此时的大气低频环流场与亚洲季风涌活跃阶段基本相反。      

中国地区夏季6～8月云水含量的垂直分布特征

基于观测资料的夏季云水含量时空分布情况对于数值天气预报、气候预测以及人工影响天气试验都十分重要。本文利用CloudSat卫星资料, 分析了2006～2008年中国地区夏季月平均云水含量的垂直和区域变化特征。结果显示, 青藏高原地形以及东亚夏季风对月平均云含水量分布具有明显影响。中国中部纬度上对流层中层的月平均液态水含量比南部及北部的量值大。各月平均云液水含量垂直廓线存在两个不同高度上的峰值区, 原因可能主要是受大尺度参数的控制, 以及受到青藏高原和东亚季风环流的影响。平均冰水含量纬向垂直分布的高值区主要在对流层中上部。本文中所揭示的云水含量特征为天气和气候模式改进、人工影响天气及云—辐射相互作用提供了重要的基础信息。     

中国及其周边地区多种水凝物资料的气候态特征比较

对云的水凝物含量进行研究有利于认识云的辐射性质和强迫效应,以及改善模式的预报性能。利用目前几种较为常用的卫星观测资料（ISCCP、MODIS和CloudSat）和再分析资料（CFSR和ERA-Interim）,对中国及其周边地区的多种水凝物变量,包括积分的云水路径、液水路径和冰水路径,以及分层的液态水含量和冰水含量的气候态水平及垂直分布特征进行了比较研究。结果表明,在总的水凝物含量方面,无论是描述整个中国及其周边地区的水平分布特征和主要变化模态,还是不同海陆区域的月变化特点,MODIS、ERA和CFSR三种资料都显示出较高的一致性,而ISCCP的绝对数值和变化幅度与它们均存在一定差异。在液态水含量方面,无论是水平还是垂直分布,ERA-Interim都有最高的数值,作为观测数据的MODIS和ISCCP则显著偏低。对于冰水含量,不同资料间无论是水平和垂直分布形式还是具体数值都存在明显差异。通过分析不同水凝物资料间气候态分布的差异性特征,有利于认识目前常用的几种水凝物资料的“不确定性”程度,从而更好地估计云的辐射效应,以及理解其在气候变化中所扮演的角色。      

Simulation of monsoon intraseasonal variability in NCEP CFSv2 and its role on systematic bias

We have evaluated the simulation of Indian summer monsoon and its intraseasonal oscillations in the National Centers for Environmental Prediction climate forecast system model version 2 (CFSv2). The dry bias over the Indian landmass in the mean monsoon rainfall is one of the major concerns. In spite of this dry bias, CFSv2 shows a reasonable northward propagation of convection at intraseasonal (30–60 day) time scale. In order to document and understand this dry bias over the Indian landmass in CFSv2 simulations, a two pronged investigation is carried out on the two major facets of Indian summer monsoon: one, the air–sea interactions and two, the large scale vertical heating structure in the model. Our analysis shows a possible bias in the co-evolution of convection and sea surface temperature in CFSv2 over the equatorial Indian Ocean. It is also found that the simulated large scale vertical heat source (Q1) and moisture sink (Q2) over the Indian region are biased relative to observational estimates. Finally, this study provides a possible explanation for the dry precipitation bias over the Indian landmass in the simulated mean monsoon on the basis of the biases associated with the simulated ocean–atmospheric processes and the vertical heating structure. This study also throws some light on the puzzle of CFSv2 exhibiting a reasonable northward propagation at the intraseasonal time scale (30–60 day) despite a drier monsoon over the Indian land mass.     

中国东部和印度季风区云辐射特性的比较

基于 ＩＳＣＣＰ和 ＥＱＢＥ资料,本文比较了中国东部和印度季风区的云和云辐射强迫的气候特征。虽然它们同属于亚洲季风区,并且有相似的降水季节特征,但它们各自的云和云辐射强迫特征差异很大。在印度区域,所有的云量有着相同的季节变化,最大云量分布都出现在夏季,且总云量中以高云量为主。而中国东部云量的季节变化都比较复杂,在总云量中以中、低云量为主,最大总云量出现在春季。冬季的总云量和中、低云量要大于夏季。在全球云量分布中,中国东部最典型的特征是：该地区为全球最大的雨层云覆盖区。与云的分布和变化相关,印度季风区最大的负短波云辐射强迫,最大的正的长波辐射强迫和最大的负的净云辐射强迫发生在夏季,而在中国东部,大的负的短波云辐射强迫发生在春夏之交。年平均的负的短波云辐射强迫在中国东部地区明显要大于在印度季风区。     

Cloud effects on air-sea interactions during the 1979 Indian summer monsoon as studied from satellite observations

Summary The Indian summer monsoon, one of the earth's most vigorous and energetic seasonally occurring weather events, influences the global atmospheric circulation. Its onset, duration, and intensity are governed by large- and meso-scale geophysical processes, such as surface solar heating and air-sea interactions. In this paper, using innovative combinations of satellite sensor data, we investigate some of these fundamental processes which are closely tied to clouds and control the monsoon system's evolution. The study, which focuses on the monsoon period of June, 1979, examines the low-frequency variability of clouds and their effects on air-sea processes through an analysis of the complex influence clouds play on the surface heat and water budgets. First, the effects of clouds on both the solar and longwave components of the surface radiation budget are assessed using a cloud radiative forcing parameter. While the effects of clouds on the long-wave irradiance act in a manner opposite to their effects on the shortwave irradiance, only a partial compensation is found to take place and the net effect results in a maximum cloud forcing of 60 Wm^–2 in the southwestern Arabian Sea. Second, employing satellite-derived precipitation and evaporation estimates, the paper analyzes the net surface fresh water budget variability around the monsoon onset. This budget is important in that fresh water affects the upper ocean density distribution and, consequently, the thermohaline circulation. Two regions are found to dominate the analysis: the western Arabian Sea, where evaporation is dominant by more than 10 mm day^–1, and the eastern Arabian Sea, where precipitation is dominant by more than 10 mm day^–1. Thus, a strong zonal gradient of fresh water at the surface is established during the monsoon. The last topic investigated is the intraseasonal variability of convection as analyzed using a cloud parameter indicative of deep convection. Cloud oscillations of 30–50 days, associated with the different phases of the monsoon, are found to propagate northward in the eastern Indian Ocean and eastward in the Bay of Bengal. Our analysis not only supports the hypothesis that the 30–50-day oscillation is driven by deep convection but also, and more importantly, suggests that the ocean thermal forcing is modulated by 30–50-day oscillations through cloud-induced surface radiative forcing. Although the results presented are limited in scope and preliminary because of the diffculty in quantifying the accuracy of the parameters examined, they do demonstrate: 1) the role of clouds in modulating the surface heat and water budgets, 2) the advantage of using combinations of multi-sensor and multi-platform satellite observations to quantify interrelated surface heat/water budget processes, and 3) the potential to examine the intraseasonal variability of air-sea interaction processes associated with the monsoon, even though these processes are not directly measurable from space.With 6 FiguresB. DiJulio passed away in September 1990.     

Evaluation of mean and intraseasonal variability of Indian summer monsoon simulation in ECHAM5: identification of possible source of bias

The performance of ECHAM5 atmospheric general circulation model (AGCM) is evaluated to simulate the seasonal mean and intraseasonal variability of Indian summer monsoon (ISM). The model is simulated at two different vertical resolutions, with 19 and 31 levels (L19 and L31, respectively), using observed monthly mean sea surface temperature and compared with the observation. The analyses examine the biases present in the internal dynamics of the model in simulating the mean monsoon and the evolution of the boreal summer intraseasonal oscillation (BSISO) and attempts to unveil the reason behind them. The model reasonably simulates the seasonal mean-state of the atmosphere during ISM. However, some notable discrepancies are found in the simulated summer mean moisture and rainfall distribution. Both the vertical resolutions, overestimate the seasonal mean precipitation over the oceanic regions, but underestimate the precipitation over the Indian landmass. The performance of the model improves with the increment of the vertical resolution. The AGCM reasonably simulates some salient features of BSISO, but fails to show the eastward propagation of the convection across the Maritime Continent in L19 simulation. The propagation across the Maritime Continent and tilted rainband structure improve as one moves from L19 to L31. The model unlikely shows prominent westward propagation that originates over the tropical western Pacific region. L31 also produces some of the observed characteristics of the northward propagating BSISOs. However, the northward propagating convection becomes stationary in phase 5–7. The simulation of shallow diabatic heating structure and the heavy rainfall activity over the Bay of Bengal indicate the abundance of the premature convection-generated precipitation events in the model. It is found that the moist physics is responsible for the poor simulation of the northward propagating convection anomalies.