Effects of environment forcing on marine boundary layer cloud‐drizzle processes

Abstract

Determining the factors affecting drizzle formation in marine boundary layer (MBL) clouds remains a challenge for both observation and modeling communities. To investigate the roles of vertical wind shear and buoyancy (static instability) in drizzle formation, ground‐based observations from the Atmospheric Radiation Measurement Program at the Azores are analyzed for two types of conditions. The type I clouds should last for at least 5 h and more than 90% time must be nondrizzling and then followed by at least 2 h of drizzling periods, while the type II clouds are characterized by mesoscale convection cellular structures with drizzle occur every 2 to 4 h. By analyzing the boundary layer wind profiles (direction and speed), it was found that either directional or speed shear is required to promote drizzle production in the type I clouds. Observations and a recent model study both suggest that vertical wind shear helps the production of turbulent kinetic energy (TKE), stimulates turbulence within cloud layer, and enhances drizzle formation near the cloud top. The type II clouds do not require strong wind shear to produce drizzle. The small values of lower tropospheric stability (LTS) and negative Richardson number (Ri) in the type II cases suggest that boundary layer instability plays an important role in TKE production and cloud‐drizzle processes. By analyzing the relationships between LTS and wind shear for all cases and all time periods, a stronger connection was found between LTS and wind directional shear than that between LTS and wind speed shear.

Relationships Between Precipitation Properties and Large‐Scale Conditions During Subsidence at the Eastern North Atlantic Observatory

“…Of broader interest here is identifying other factors that could drive precipitation variability, which may also further help explain the differences in precipitation characteristics observed between PCF and non‐PCF subsidence conditions. Monotonic relationships are identified between a number of large‐scale parameters and rain properties: Higher sea‐air temperature contrast and higher surface horizontal wind speed are both associated with higher RCF CB and (RR CB ), higher EIS is found to relate to higher RCF CB (as speculated by Kubar et al, 2009) and, higher Estimated Inversion Strength (EIS) is also to some extent found to relate to lower RR CB (in general agreement with Wu et al, 2017), although higher M parameter (also a measure of atmospheric stability) was found to have a much more consistent relationship. Lower correlations between both cloud and rain properties with EIS perhaps relate to the fact that some of the observed clouds are advected rather than locally formed.…”

“…i Higher sea-air temperature contrast and higher surface horizontal wind speed are both associated with higher RCF CB and (RR CB ), ii higher EIS is found to relate to higher RCF CB (as speculated by Kubar et al, 2009) and, iii higher Estimated Inversion Strength (EIS) is also to some extent found to relate to lower RR CB (in general agreement with Wu et al, 2017), although higher M parameter (also a measure of atmospheric stability) was found to have a much more consistent relationship. Lower correlations between both cloud and rain properties with EIS perhaps relate to the fact that some of the observed clouds are advected rather than locally formed.…”

On the role of aerosols, humidity, and vertical wind shear in the transition of shallow-to-deep convection at the Green Ocean Amazon 2014/5 site

“…For example, VWS influences the rainfall and total condensation within developing convection (Weisman and Rotunno, 2004), slantwise ascent of the parcel (Moncrieff, 1978), storm rotation, maintenance, vorticity, updraft speed (Weisman and Rotunno, 2000), and lifetime (Chakraborty et al, 2016). Though detailed microphysical properties are not considered in our simple plume calculations, it is worth noting that a recent study by Wu et al (2017) found that lower tropospheric wind shear promotes the droplet collision and growth inside the shallow clouds by the production of turbulent kinetic energy. However, Weisman and Rotunno (2004) using a two-dimensional vorticity simulation model found that increasing vertical wind shear depth from surface −3 km (low) to surface −10 km (deep) decreases the overall condensation and rainfall output.…”

Scaling of Drizzle Virga Depth With Cloud Thickness for Marine Stratocumulus Clouds

“…Stratocumulus clouds cover large areas in subtropical and tropical ocean regions and play an important role in Earth's radiation budget (Lamb & Verlinde, 2011). Drizzle drops, generating light precipitation, are frequently observed below marine stratocumulus clouds (e.g., Glienke et al, 2017;Leon et al, 2008;Rémillard et al, 2012;Wu et al, 2017). The initiation, growth, and depletion of drizzle play crucial roles in the stratocumulus-topped boundary layer and have been studied extensively (Wood, 2012).…”