Lifecycle Characteristics of Drizzle Cells in the Southeast Pacific

Shallow marine stratocumulus precipitation and deep convective precipitation appear to exhibit several analogous scale interactions among cloud-scale and mesoscale processes. The widespread stratocumulus cloud deck in the Southeast Pacific offers a large, relatively uniform region to study how these shared convective processes relate to precipitation cell lifecycle in an environment that is relatively isolated from synoptic events.

We used shipboard in situ data and C-band radar data collected from the NOAA Ship Ronald H. Brown obtained during VOCALS Rex in 2008 to examine more than 5000 drizzle cell tracks. Typical drizzle cells have maximum intensity ranges of 10-35 dBZ, areal coverage ranges between 1-100 km2, and cell lifetimes between 15-60 min. Ensemble track statistics show that the majority of cells experience an increase in size (~0.2 km2/min) and median intensity (~0.2 dB/min) over the first 15-30 minutes of their lifecycle. After this initial growth, the majority of cells experience a similarly paced decline over the next 15-30 minutes. A small portion (1-2%) of cells last much longer (90+ min). The longest cell tracks have similar behavior to shorter tracks early in their life cycles with a significantly slower, sometimes pulsed, decline period at the end of their life cycle. This common initial development time, followed by variable rates of decay is consistent with cloud-scale mechanisms initiating precipitation and mesoscale processes sustaining it. Cell lifecycle characteristics are examined with respect to mesoscale parameters such as cloud coverage and proximity to other cells.

A simplified conceptual model for drizzle cell maintenance longer than 60 min is proposed that includes surface cold pool generation, environmental shear, and dynamic lifting. Despite the disparities in cell intensity and vertical extent, this conceptual model for drizzle cell maintenance has many similar characteristics to those of deep-convective cell maintenance.