微波成像仪通道对降水云参数响应的数值模拟研究

由于降水云类型和结构等的差异,将导致微波信号出现不同的响应,因此基于星载微波仪器观测结果对降水云参数进行反演仍是目前国际上的难题之一。为了解层云和对流云等两类降水云的TMI微波信号特点,本文以相应的结构模型为基础,利用微波辐射传输模式MWRT分别模拟了不同下垫面和降水云参数条件下的TMI各通道微波亮温,并分析了各种组合通道信号,包括反映发射信息的极化差D(洋面)及反映散射信息的高频极化校正亮温PCT85(洋面)和高低频率垂直极化差VFD(陆面)的变化情况,以此从理论上探讨可能的降水反演方法。研究结果表明:在洋面,D和PCT85均随降水率的升高而减小,其中D_(10)和PCT85都能很好地反映降水率的变化,但分别受地表状况和云体性质(冻结层高度和霰粒大小)的影响较大;D_(19)和D_(37)虽然对下垫面状况和冻结层高度都不甚敏感,但随降水率的增加,存在极化丧失现象,因此不适合反演降水。在陆面,较高频取85 GHz的VFD与降水率有很好的对应关系,随降水率的增长而增大,其中VFD37-85受地表状况影响最小,但受云体性质影响较大;37 GHz对液态水变化敏感,其发射效应易混淆散射信号,故较高频取37 GHz的VFD10-37和VFD19-37不适合研究降水与微波信号之间的关系;层云与对流云存在有无霰粒的差异,会对微波散射性质造成影响,导致PCT85和VED对层云降水率的响应要高于对流云。     

Retrieval of Liquid Water Path Inside Nonprecipitating Clouds Using TMI Measurements

Quantitative estimates of liquid water path （LWP） in clouds using satellite measurements are critical to understanding of cloud properties and the assessment of global climate change. In this paper, the relationship between microwave brightness temperature （TB） and LWP in the nonprecipitating clouds is studied by using satellite microwave measurements from the TRMM Microwave Imager （TMI） onboard the Tropical Rainfall Measuring Mission （TRMM）, together with a radiative transfer model for microwave radiance calculations. Radiative transfer modeling shows that the sensitivity is higher at both 37.0- and 85.5-GHz horizontal polarization channels for the LWP retrievals. Also, the differences between the retrieved values responding to TBs of various channels and the theoretical values are displayed by the model. Based upon above simulations, with taking into account the factor of resolution and retrieval bias for a single,channel, a nonprecipitating cloud LWP in the summer subtropical marine environment retrieval algorithm is formulated by the combination of the two TMI horizontal polarization channels, 37.0 and 85.5 GHz. Moreover,by using TMI measurements （1Bll）, this algorithm is applied to retrieving respectively LWPs for clear sky, nonprecipitating clouds, and typhoon precipitating clouds. In the clear sky case, the LWP cl~anges from -1 to 1 g m-2, and its mean value is about 10^-5 g m^-2. It indicates that, using this combination retrieval algorithm, there are no obvious systemic deviations when the LWP is low enough. The LWP values varying from 0 to 1000 g m^-2 in nonprecipitating clouds are reasonable, and its distribution pattern is very similar to the detected results in the visible channel of Visible and Infrared Scanner （VIRS） on the TRMM. In typhoon precipitating clouds, there is much more proportion of high LWP in the mature phase than the early stage. When surface rainfall rate is lower than 5 mm h^-1, the LWP increases with increasing rainfall rate.     

使用物理方法反演中国陆地雨强和水凝物垂直结构

使用MM5模式模拟了11个降水个例,获得中国陆地的降水廓线,由蒙特卡罗辐射传输模式计算其上行辐射亮温,构成初始云一辐射数据库.采用对照表方法从中获得Db_ml和Db_m2两个数据集,作为GPROF反演算法的先验数据集.对2007年7月的3次强降水过程的地面雨强进行反演,与微波成像仪(TMI)及NESDIS、GSCAT、...     

PCT-SI综合指数法反演陆面雨强

利用TRMM微波成像仪(TMI)数据,采用极化订正温度及散射指数综合指数法(PCT-SI),反演赣州及周边地区2011年11月9日雨强,与同时期的PR星载雷达测雨资料进行对比分析,并用赣州地区47个地面站点的小时降雨对反演结果进行了验证。结果表明:采用K-均值法分类并判别降雨区,可以较好地确定降雨区的范围;TMI各通道亮温与对应时空匹配PR雨强的相关系数不同,利用低频组合拟合85 Hz通道亮温来求得大气散射指数,散射指数越大,雨强越大;PCT-SI综合指数法反演的降水中心的降水强度明显偏小,降水范围扩大,但其反演的雨强与PR反演的雨强基本一致,与地面实际雨量站点雨强相关系数为0.784,表明采用PCT-SI综合指数法反演陆面雨强是合理可信的。     

反演寿县TMI微波频率的地表比辐射率(英)

利用一个微波辐射传输模式,晴天下的大气探测廓线,地面的部分观测资料以及卫星观测的亮温,计算了淮河流域能量和水循环实验(HUBEX)中,寿县地区的热带降雨观测卫星微波成像仪(TRMM/TMI)微波频率的地表比辐射率。通过比较所计算的微波地表比辐射率随地表状况的改变,发现地表微波比辐射率随地表状况的变化存在敏感性,并且其变化是合理的。本文中,地表肤温设定等于地表空气温度,并且仅计算了寿县的地表比辐射率,在HUBEX区域上的地表比辐射率的计算需要更多的观测资料。     

基于TRMM/TMI的亚洲夏季降水研究

利用热带测雨卫星(TRMM)微波成像仪(TMI)的长期观测资料, 对亚洲夏季降水的水平分布特征进行了统计分析, 指出了孟加拉湾北部沿岸, 中国南海南部, 赤道西太平洋暖池三个稳定的强降水中心. 并借助全球降水气候计划(GPCP)地表降水资料, 对亚洲范围内洋面, 陆面及6个典型区域的TMI降水准确性进行了评估, 结果表明利用TMI和GPCP资料对亚洲夏季降水的强弱降水中心及雨带位置的指示基本一致, TMI对陆面降水仍存在普遍的低估, 最大相对偏差在25%左右. 差异水平分布显示出极强的地域性特征, 出现最大差异(>3 mm/d)的区域位于陆地上青藏高原周边(正偏差), 及孟加拉湾北部地区(负偏差). 对产生偏差原因的分析表明, TMI陆面算法强烈依赖于降水云系统上层冰粒子含量的特性是构成其系统性偏低和局部地区对降水高估的主要因素, 而进一步的分析也显示GPCP雨量计极不均匀的分布对差异的产生也有所贡献, 尤其是在雨量计稀少的高原周边地区.     

用TRMM卫星微波成像仪遥感云中液态水

应用热带降雨测量卫星微波成像仪(TRMM/TMI)的被动遥感资料,选用对云中液态水变化非常敏感的85.5 GHz垂直极化通道的亮温信息,通过离散纵坐标矢量辐射传输模式,采取逐步逼近的方法确定出地表的微波比辐射率,并运用迭代方法有效地反演出云中液态水含量及其分布.与对应的卫星红外云图对比结果表明,反演的云中液态水分布是合理和可信的.     

Diurnal rainfall variability over the Upper Blue Nile Basin: A remote sensing based approach

In this study we aim to assess the diurnal cycle of rainfall across the Upper Blue Nile (UBN) basin using satellite observations from Tropical Rainfall Measuring Mission (TRMM). Seven years (2002–2008) of Precipitation Radar (PR) and TRMM Microwave Imager (TMI) data are used and analyses are based on GIS operations and simple statistical techniques. Observations from PR and TMI reveal that over most parts of the basin area, the rainfall occurrence and conditional mean rain rate are highest between mid- and late-afternoon (15:00–18:00 LST). Exceptions to this are the south-west and south-eastern parts of the basin area and the Lake Tana basin where midnight and early morning maxima are observed. Along the Blue Nile River gorge the rainfall occurrence and the conditional mean rain rate are highest during the night (20:00–23:00 LST). Orographic effects by large scale variation of topography, elevation and the presence of the UBN river gorge were assessed taking two transects across the basin. Along transects from north to south and from east to west results indicate increased rainfall with increase of elevation whereas areas on the windward side of the high mountain ranges receive higher amount of rainfall than areas on the leeward side. As such, mountain ranges and elevation affect the rainfall distribution resulting in rain shadow effect in the north-eastern parts of Choke-mountain and the ridges in the north-east of the basin. Moreover, a direct relation between rainfall occurrence and elevation is observed specifically for 17:00–18:00 LST. Further, results indicate that the rainfall distribution in the deeply incised and wide river gorge is affected with relatively low rainfall occurrence and low mean rainfall rates in the gorge areas. Seasonal mean rainfall depth is highest in the south-west area and central highlands of the basin while areas in the north, north-east and along the Blue Nile gorge receive the least amount of rainfall. Statistical results of this work show that the diurnal cycle of rainfall occurrence from TRMM estimates show significant correlation with the ground observations at 95% confidence level. In the UBN basin, the PR conditional mean rain rate estimates are closer to the ground observations than the TMI. Analysis on mean wet season rainfall amount indicates that PR generally underestimates and TMI overestimates the ground observed rainfall.     

Study of the intensity of super cyclonic storm GONU using satellite observations

Satellite microwave measurements show sea surface temperature (SST) increase in advance of significant cyclone intensification. Moreover, cyclone intensification may also be related to the location of high SST. In the present study results indicate pre-existing high sea surface temperature anomaly (SSTA) located at the right side of the storm track for Cyclone GONU. I emphasize that high SST which occurred at the right time and right place was conducive to the cyclone intensification. In particular, high SST in the northeastern quadrant of the storm track induced significant increases in surface latent heat fluxes contributing to the rapid intensification of GONU. The present study also focuses on the air-sea interactions associated with cyclone GONU. Surface latent heat flux (SLHF) and precipitation rate (PR) increase anomalously prior to landfall as compared to when the GONU was at its maximum intensity (category 5). Wind speed and rain-rate data from satellite observations show breakup of the eye-wall and asymmetric structure leading to increased precipitation prior to landfall.     

SATELLITE MICROWAVE REMOTE SENSING OF FLOODING

The ability of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) for flooding and soil wetness detection has been demonstrated in this study.On the basis of TMI measurements,four methods,the classification method,the soil wetness index (SWI) method. the polarization difference index (PDI) method,and the polarization ratio index (PRI) method, were brought out to monitor flooding and study soil wetness in the Changjiang and Huaihe River Basins during the summer 1998.Compared with the images provided by L-band Synthetic Aperture Radar (L-SAR) and Radar Satellite (Radarsat) and the figures derived from daily rainfall data based on the Z-index method,the detection of flooding and soil wetness by TMI was proved to be feasible.