Disappearing drizzle: Evaluating models with observations from ARM

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Description

Drizzle and light precipitation are very common in marine boundary layer (MBL) clouds - about two thirds of MBL clouds observed at the Eastern North Atlantic (ENA) site produce drizzle at cloud base. Evaporation of drizzle not only returns vapour to the sub-cloud layer, but also cools and stabilises it. Thus drizzle evaporation is an important factor in determining the boundary layer state and cloud properties. Global models have a hard time representing the properties of the MBL well: Systematic biases in cloud water content and radiative effects are common, and most models produce light surface precipitation too frequently. When attempting to bring the model's cloud properties and surface precipitation rate closer to observed values, other aspects, such as the boundary layer temperature, humidity and wind can deteriorate. The rate of drizzle evaporation, which simultaneously impacts the water and energy budgets of the MBL, is currently poorly constrained in models. It is highly sensitive to assumptions about how humidity varies horizontally within the model column, or which part of the column the drizzle falls into as this determines the amount of sub-saturation encountered. Fall speed and drop size also impact the rate at which the drizzle evaporates. The recent development of improved drizzle retrievals from the ARM ground-based instrument observations can provide crucial information to constrain model parameterisations. A quantitative measure of drizzle evaporation within the MBL can be used in conjunction with existing cloud retrieval products (e.g. liquid water path, fractional cover) and high (temporal) frequency observations of the boundary layer state (temperature, humidity) to assess and improve the parametrization of drizzle evaporation in models. We illustrate this approach using a stratocumulus case observed at the ENA site and Single Column Model (SCM) simulations with the ECMWF Integrated Forecast System (IFS). Improvements to the microphysical parametrizations of the autoconversion and accretion rates, drizzle drop size distribution, fall speeds and evaporation rate that alter the drizzle production and enhance the evaporation beneath cloud base are evaluated, leading to a better understanding of the representation of drizzling boundary layer cloud processes in models for weather and climate prediction.