۱- Introduction

۲- Data and Methodology

۳- General Conditions

۴- Drizzle Shaft Properties

۵- Turbulence Properties

۶- Summary, Discussion, and Conclusion

References

Abstract

Drizzle is ubiquitous in marine boundary layer stratocumulus clouds with much of it evaporating before reaching the surface. Ten days of observations made at the Atmospheric Radiation Measurement’s Eastern North Atlantic site during closed cellular stratocumulus cloud conditions are used to characterize drizzle below the cloud base and its impact on the boundary layer turbulence. Cloud and drizzle microphysical and macrophysical properties were retrieved by combining the data from vertically pointing Doppler cloud radar, ceilometer, and microwave radiometer. On average, the drizzle shafts were 28.14 km wide, with cloud base rain rate and modal diameter of 0.98 mm/day and 138.62 μm, respectively. The rain rate at the surface was negligible yielding an average diabatic cooling of −۲۸٫۶۸ W/m2 in the subcloud layer. The liquid water path and turbulence within the boundary layer increased with an increase in the cloud top radiative cooling; however, none of these variables exhibited any relationship with cloud base rain rate. For a similar amount of radiative cooling at the cloud top, the average variance of vertical velocity in the subcloud layer was about 16% lower during strongly precipitating conditions as compared to lightly precipitating conditions. The reduction in the variance of vertical velocity due to drizzle evaporation was primarily confined to the upper half of the subcloud layer and was due to reduction in the strengths of the downdrafts. Collectively, our results show substantial impact of drizzle evaporation on turbulence below stratocumulus clouds, necessitating its accurate representation in the Earth system models.

Introduction

Marine boundary layer stratocumulus clouds cover vast areas of Eastern subtropical oceans and persist for long timescales (Klein & Hartmann, 1993). These clouds reflect greater amount of solar radiation back to space compared to the ocean surface, causing net cooling of the Earth’s surface. Hence, these clouds are an important component of the Earth’s radiation budget and need to be accurately represented in the Earth system models (ESMs) aimed at predicting the future climate. Stratocumulus clouds are intimately coupled to the turbulence in the boundary layer that is primarily maintained by radiative cooling at the cloud top and is modulated by entrainment, surface turbulent fluxes, wind shear, and precipitation (Wood, 2012). Precipitation is known to be ubiquitous in these clouds with much of it evaporating before reaching the surface (Wood et al., 2015; Zhou et al., 2015 etc.). Marine boundary layer stratocumulus clouds occur in distinct mesoscale organizations named closed cellular (completely overcast) and open cellular (Sharon et al., 2006; Stevens et al., 2005), with precipitation being one of the key factors causing boundary layer decoupling and the transition between these organizations (Feingold et al., 2015; Rapp, 2016; Yamaguchi et al., 2017). Several parameterizations have been proposed to represent stratocumulus clouds in ESM as they occur at spatial scales smaller than the model grid spacing. The most sophisticated parameterizations use higher‐ order moments of the joint probability distribution functions of total water mixing ratio, liquid water potential temperature, and vertical air motion (e.g., Bogenschutz et al., 2013; Golaz et al., 2002). Recently, there have been attempts to represent these clouds in the ESM in a unified way by coupling these higher‐order cloud schemes to bin microphysics schemes (Gettelman & Morrison, 2015; Morrison & Gettelman, 2008). The model simulations made in such a setup showed a stable cold moist layer near the surface caused by evaporation of the drizzle in the subsaturated subcloud layer. This stable layer inhibited replenishing of cloud water by shutting off boundary layer turbulence thereby causing spurious oscillations in cloud cover (Zheng et al., 2016, 2017). These and other studies have called for observational studies that quantify and characterize drizzle and its evaporation in these systems and assess the impact of drizzle evaporation‐ induced cooling on the boundary layer turbulence (Ahlgrimm & Forbes, 2014; Wood et al., 2016).