Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection
Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection
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热带大陆下沉气流特征：中尺度系统与无组织对流
DOI：
10.5194/acp-18-1997-2018
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发表时间：
2017
影响因子：
6.3
通讯作者：
J. D. Neelin
中科院分区：
文献类型：
--
作者：
K. Schiro;J. D. Neelin
摘要。利用美国能源部大气辐射测量（ARM） GoAmazon2014/5项目的多仪器数据，分析了强中尺度对流系统（mcs）和孤立无组织深降水对流的下沉气流和冷池特征。在对流前观察到柱水汽（CWV）的增加，在mcs之前的CWV比孤立的单体要高。对于MCSs和孤立单体，风速的增加、表面湿度和温度的降低以及相对湿度的增加都与系统通道一致。垂直速度资料和雷达风廓线雷达反射率的复合资料表明，与地表等效位温（θe）下降幅度最大相关的下降气流的发生概率随着冰点以下高度的降低而增加。MCSs和无组织对流随高度的平均下降气流大小和概率相似。混合计算表明，平均而言，在高于3 km的高度产生的空气必须经过大量混合，特别是在孤立单元的情况下，以匹配观测到的冷池θe，这意味着典型的低起源水平。以地表等效势温（Δθe）的减少为条件平均的降水表现出很强的关系，因为最负的Δθe值与降水的高概率有关。Δθe对降水的条件平均更具有物理动机，表明θe随降水率的增加而下降趋于平稳，以地表θe的最大值与高空剖面中θe的最小值之间的差为界。在不同尺度和区域观测到的这些统计数据的稳健性表明，它们可能被用作模式诊断工具，用于改进气候模式中的下沉气流参数化。
Abstract. Downdrafts and cold pool characteristics for strong mesoscale convective systems (MCSs) and isolated, unorganized deep precipitating convection are analyzed using multi-instrument data from the DOE Atmospheric Radiation Measurement (ARM) GoAmazon2014/5 campaign. Increases in column water vapor (CWV) are observed leading convection, with higher CWV preceding MCSs than for isolated cells. For both MCSs and isolated cells, increases in wind speed, decreases in surface moisture and temperature, and increases in relative humidity occur coincidentally with system passages. Composites of vertical velocity data and radar reflectivity from a radar wind profiler show that the downdrafts associated with the sharpest decreases in surface equivalent potential temperature (θe) have a probability of occurrence that increases with decreasing height below the freezing level. Both MCSs and unorganized convection show similar mean downdraft magnitudes and probabilities with height. Mixing computations suggest that, on average, air originating at heights greater than 3 km must undergo substantial mixing, particularly in the case of isolated cells, to match the observed cold pool θe, implying a low typical origin level. Precipitation conditionally averaged on decreases in surface equivalent potential temperature (Δθe) exhibits a strong relationship because the most negative Δθe values are associated with a high probability of precipitation. The more physically motivated conditional average of Δθe on precipitation shows that decreases in θe level off with increasing precipitation rate, bounded by the maximum difference between surface θe and its minimum in the profile aloft. Robustness of these statistics observed across scales and regions suggests their potential use as model diagnostic tools for the improvement of downdraft parameterizations in climate models.