Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection Atmospheric Chemistry and Physics

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. Grant no. NA14OAR4310274

本研究利用美国能源部（Department of Energy, DOE）大气辐射测量（Atmospheric Radiation Measurement, ARM）计划GoAmazon2014/5观测场的多仪器观测数据，对强中尺度对流系统（mesoscale convective systems, MCSs）以及孤立无组织强降水对流的下沉气流与冷池特征展开分析。研究观测到对流发生前柱总水汽（column water vapor, CWV）会出现升高，且中尺度对流系统发生前的柱总水汽增幅高于孤立对流单体。无论是中尺度对流系统还是孤立对流单体，在系统过境时均伴随风速升高、地表湿度与气温降低，以及相对湿度升高的现象。基于雷达风廓线仪获取的垂直速度与雷达反射率合成数据显示，与地表相当位温（surface equivalent potential temperature, θe）骤降相关的下沉气流，其发生概率随冻结层下方高度的降低而升高。中尺度对流系统与无组织对流的平均下沉气流强度及不同高度下的发生概率均表现出相似性。混合过程计算结果表明，平均而言，源自3千米以上高度的空气需要经历充分的混合过程（尤其是孤立对流单体情形），才能匹配观测到的冷池相当位温，这意味着下沉气流的典型源高度较低。以地表相当位温降幅（Δθe）为条件对降水进行平均处理后，二者表现出显著的相关性：Δθe负值最大的情形对应极高的降水发生概率。以降水为条件对Δθe进行更具物理意义的平均处理后发现，θe降幅随降水率升高逐渐趋于平缓，其上限为地表θe与高空廓线中最低θe的最大差值。本研究在不同尺度与区域下观测到的统计结果具有鲁棒性，表明这些结果可作为模式诊断工具，用于改进气候模式中的下沉气流参数化方案。资助编号：NA14OAR4310274