Observations Pertaining to Precipitation within the Northeast Pacific Stratocumulus-to-Cumulus Transition
Observations Pertaining to Precipitation within the Northeast Pacific Stratocumulus-to-Cumulus Transition
复制标题
有关东北太平洋层积云到积云过渡期间降水的观测
作者：
M. Sarkar;P. Zuidema;B. Albrecht;V. Ghate;J. Jensen;Johannes Mohrmann;R. Wood
记录了在云系统演化交易 (CSET) 活动期间采样的三个真正的层积云到积云的转变。重点是原位降水的拉格朗日演化，由于米氏散射，被认为超过了雷达/激光雷达检索值。三个初始层积云案例中有两个是原始的[云滴数浓度 (Nd) 约为 22 cm−3]，但占据了不同深度的边界层，而第三个是被污染的 (Nd ~ 225 cm−3)。沿拉格朗日轨迹每小时由卫星得出的云分数表明，边界层加深的速度越快，往往会更快地转变为更强烈但更偶然的降水。这些转变要么在早上要么在下午晚些时候开始，这表明前晚的过程可以预先准备或延迟不可避免的转变。随着边界层加深，降水在整个过渡过程中转向更大的液滴尺寸，气溶胶浓度仅在三种情况中的两种情况下减少。超洁净（Nd < 1 cm−3）积云由原始层积云演变而来，具有异常高的降水率，占据浅且混合良好的边界层。简单的一维蒸发模型和雷达/激光雷达检索的结果表明，在整个转变过程中，亚云蒸发可能会增加。再加上能够降低潜在冷却剖面的较大液滴尺寸，有助于过渡到更多表面驱动的对流。边界层深度和降水之间的关联并没有就降水对转变速度的孤立影响提供明确的结论。三个示例的初始条件之间的差异为进一步的建模研究提供了机会。
Three genuine stratocumulus-to-cumulus transitions sampled during the Cloud System Evolution over the Trades (CSET) campaign are documented. The focus is on Lagrangian evolution of in situ precipitation, thought to exceed radar/lidar retrieved values because of Mie scattering. Two of the three initial stratocumulus cases are pristine [cloud droplet number concentrations (Nd) of ~22 cm−3] but occupied boundary layers of different depths, while the third is polluted (Nd ~ 225 cm−3). Hourly satellite-derived cloud fraction along Lagrangian trajectories indicate that more quickly deepening boundary layers tend to transition faster, into more intense but more occasional precipitation. These transitions begin either in the morning or late afternoon, suggesting that preceding night processes can precondition or delay the inevitable transition. The precipitation shifts toward larger drop sizes throughout the transition as the boundary layers deepen, with aerosol concentrations only diminishing in two of the three cases. Ultraclean (Nd < 1 cm−3) cumulus clouds evolved from pristine stratocumulus cloud with unusually high precipitation rates occupying a shallow, well-mixed boundary layer. Results from a simple one-dimensional evaporation model and from radar/lidar retrievals suggest subcloud evaporation likely increases throughout the transition. This, coupled with larger drop sizes capable of lowering the latent cooling profile, facilitates the transition to more surface-driven convection. The coassociation between boundary layer depth and precipitation does not provide definitive conclusions on the isolated effect of precipitation on the pace of the transition. Differences between the initial conditions of the three examples provide opportunities for further modeling studies.