Cumulus transport of chemical tracers: 1. Cloud-resolving model simulations
Cumulus transport of chemical tracers: 1. Cloud-resolving model simulations
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化学示踪剂的积云传输：1.云解析模型模拟
DOI：
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发表时间：
2000
期刊：
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通讯作者：
A. Arakawa
中科院分区：
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作者：
R. Lu;Chichung Lin;R. Turco;A. Arakawa
热带和中纬度地区的对流输送是导致化学示踪剂在整个低层大气中垂直并最终水平再分布的关键机制。因此，更好地理解和准确地表示对流输送是必不可少的，在模拟示踪剂的分布和远程扩散的化学输送模式。我们采用加州大学洛杉矶分校的云解析模式（CRM）模拟5天期间的GATE第三阶段实验，在此期间观察到几个中尺度对流系统。这些事件的模拟版本的动力学结构和热力学平衡比较有利的观测。计算和分析了六种不同来源的示踪剂在大气中的分布。这是第一次在一个高分辨率的模式中使用多个示踪剂在多天的时间内对区域扩散进行深入分析。结果发现，影响个别云示踪剂输送的主要过程涉及对流上升气流和相关的下降气流。不同层次的卷吸、砧云出流和卷吸都对对流层的不同区域有重要影响。除了对流尺度的输送外，深对流引起的中尺度沉降能够在单个深对流事件期间将化学示踪剂向下移动多达数百hPa，这可以显著地改变对流层中下部的示踪剂分布。根据我们的分析，我们得出结论，深对流深刻地影响对流层化学示踪剂的分布至少在三个基本方面：（1）通过快速和直接地将示踪剂从边界层输送到对流层上部，（2）通过加速化学示踪剂从对流层上部向下输送到地面，既在局部的下降气流中，也通过沉降在更广泛的区域上，（3）引起对流层上部和平流层下部之间的空气交换。
Convective transport in the tropics and middle latitudes is a critical mechanism leading to the vertical and, eventually, the horizontal redistribution of chemical tracers throughout the lower atmosphere. Accordingly, a better understanding and accurate representation of convective transport are essential in simulating tracer distributions and long-range dispersion in chemical transport models. We employ the UCLA cloud-resolving model (CRM) to simulate a 5-day period during the GATE phase III experiment, during which several mesoscale convective systems were observed. The dynamical structures and thermodynamic balances of the simulated versions of these events compare favorably with observations. The distributions of six tagged tracers originating at different levels in the atmosphere are calculated and analyzed. This represents the first in-depth analysis of regional dispersion over a multi-day period utilizing multiple tracers in a high-resolution model. It is found that the dominant process affecting tracer transport by individual clouds involves convective updrafts and related downdrafts. Entrainment, anvil cloud outflow, and detrainment at various levels all have important effects in different regions of the troposphere. In addition to the convective-scale transport, mesoscale subsidence induced by deep convection is capable of moving chemical tracers downward as much as several hundred hPa during a single deep convective event, which can substantially modify tracer distributions in the middle and lower troposphere. On the basis of our analysis we conclude that deep convection profoundly affects the distribution of tropospheric chemical tracers in at least three fundamental ways: (1) by quickly and directly transporting tracers from the boundary layer to the upper troposphere, (2) by accelerating downward transport of chemical tracers from the upper troposphere to the surface, both locally in downdrafts and over broader areas via subsidence, and (3) by inducing the exchange of air between the upper troposphere and the lower stratosphere.