Impacts of Assimilating AMMA Soundings on ECMWF Analyses and Forecasts
Impacts of Assimilating AMMA Soundings on ECMWF Analyses and Forecasts
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DOI：
10.1175/2010waf2222370.1
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发表时间：
2010-08-01
影响因子：
2.9
通讯作者：
Thorncroft, Chris
中科院分区：
文献类型：
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作者：
Agusti-Panareda, Anna;Beljaars, Anton;Thorncroft, Chris
非洲季风多学科分析项目在2006年季风雨季进行的实地试验提供了西非地区数量空前的无线电探空仪/下投式探空仪数据。本文探讨了这一宝贵的数据集在欧洲中期天气预报中心的分析和预测的使用和影响。这些探测是唯一能够提供西非大陆上空对流层低层热力学和动力学结构三维信息的数据来源。它们对位于北纬12度至20度之间的萨赫勒区域特别重要，该区域的特点是对流层低层温度和湿度梯度大。通过AMMA前和AMMA前探空站网的同化对比试验表明，AMMA附加探空资料对萨赫勒地区的低层温度和非洲东风急流的结构有显著的分析影响。但AMMA资料对预报的影响在24 h后消失。探测结果显示，在萨赫勒地区北方和东部边界层温度的大模型偏差，这是一致的云，降雨和辐射的知名模型偏差。温度的大分析增量会导致发散和下沉的增量，从而抑制对流。因此，分析增量似乎对云和温度模型偏差具有不期望的反馈。AMMA探测对非洲东风急流的影响是加强和延长急流条纹至东经15度，即朝向萨赫勒东部。在东经15度以东的非洲东风急流水平上没有观测资料被同化，以支持更远的东部急流增强。与独立的大气云运动矢量的比较表明，非洲东风急流的分析是太弱，在这个数据稀疏的区域。这可能对模型预测中非洲东风波的发展产生影响。通过同化大气运动矢量的进一步实验（目前没有使用）可以解决这个问题。
The field experiment of the African Monsoon Multidisciplinary Analysis (AMMA) project during the 2006 wet monsoon season provided an unprecedented amount of radiosonde/dropsonde data over the West African region. This paper explores the usage and impacts of this invaluable dataset in the European Centre for Medium-Range Weather Forecasts analyses and forecasts. These soundings are the only source of data that can provide 3D information on the thermodynamic and dynamic structures of the lower troposphere over continental West Africa. They are particularly important for the Sahel region located between 12 degrees and 20 degrees N, which is characterized by large gradients in temperature and moisture in the lower troposphere. An assimilation experiment comparison between the pre-AMMA and AMMA radiosonde networks shows that the extra AMMA soundings have a significant analysis impact on the low-level temperature over the Sahel and on the structure of the African easterly jet. However, the impacts of the extra AMMA data on the forecast disappear after 24 h. The soundings reveal large model biases in boundary layer temperature over the northern and eastern Sahel, which are consistent with the well-known model biases in cloud, rainfall, and radiation. Large analysis increments in temperature lead to increments in divergence and subsidence, which act to suppress convection. Thus, the analysis increments appear to have an undesirable feedback on the cloud and temperature model biases. The impact of the AMMA soundings on the African easterly jet is to enhance and extend the jet streak to 15 degrees E, that is, toward the eastern part of the Sahel. No observations are assimilated east of 15 degrees E at the level of the African easterly jet to support the jet enhancement farther east. Comparisons with independent atmospheric cloud motion vectors indicate that the African easterly jet in the analysis is too weak over this data-sparse region. This could have implications for the development of African easterly waves in the model forecast. Further experimentation by assimilating atmospheric motion vectors-currently not used-could address this problem.