Impact of middle atmospheric humidity on boundary layer turbulence and clouds

Highlights

• Middle atmospheric drying enhances turbulence within the boundary layer.

• Reduction in liquid water path and hence cloud albedo.

• Fluxes derived from aircraft observations were comparable with cloud free LES resolved fluxes.

• More cooling at the cloud top and warming in subcloud layer due to introduction of radiation parametrization in LES.

Abstract

An attempt is made to connect the moisture present above the boundary layer and its association with the cloud development and boundary layer turbulence using Large Eddy Simulation (LES) and aircraft observations. The dry boundary layer (BL) with shallow clouds observed during Cloud-Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the arid Indian peninsula is investigated in this study. The constant altitude aircraft observations at different elevations are used to verify LES derived fluxes and variances. LES simulations with the drier conditions above BL resulted in deeper, warmer and drier BL with an enhanced boundary layer turbulent kinetic energy. A dynamic feedback from dry air intrusion into the BL resulted in energetic BL eddies in the dry conditions and a doubling of moisture exchange coefficients. The liquid water content in cloud updrafts rather than in downdrafts were depleted significantly in dry conditions. LES sensitivity indicates that the middle atmospheric water vapor alone could influence the shallow to deep cumulus cloud transitions in the monsoon regime in a dramatic way, by changing the population of cumulus, congestus or deep cumulus clouds. A 30% drying above the BL could drastically reduce the liquid water path and cloud albedo by 10–15%.

Introduction

Indian peninsular region is dominated by shallow and high clouds during monsoon season (Rajeevan and Shrinivasan, 2000; Balachandran and Rajeevan, 2007). Clouds with a smaller vertical extent (3–5 km) typically exist during the break phases of monsoon (Rajeevan et al., 2012). Wonsick et al. (2009) noted that these shallow clouds can transform into deeper convection and contribute to precipitation under suitable large-scale conditions. Detailed observations of cloud microphysics over this region during Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) show the dominance of low liquid water clouds and these clouds sometimes grow deeper into mixed-phase during the active monsoon periods. The occurrence of fair-weather cumulus clouds with (low liquid water path) during the monsoon break conditions plays an essential role in the radiation balance (Hartmann et al., 1992) of the earth-atmosphere system. Sengupta et al. (2003) have shown that the radiative fluxes are very sensitive to small changes in the low liquid water path clouds and can lead to large nonlinear errors in surface radiation.

The representation of shallow clouds in the numerical models is also known to introduce significant uncertainty in the prediction of diurnal variation of clouds and precipitation (Guichard et al., 2004). Cloud albedo of such clouds is poorly simulated in the General Circulation Models (GCMs), often are underestimated (Zhang et al., 2005; Bender et al., 2006) and remains as one of the biggest uncertainty in the climate models (Bony and Dufresne, 2005; Solomon, 2007). In the GCMs, shallow convection associated with increased vertical shear and relative vorticity over certain regions of India play a decisive role in supporting rainfall after the peak active phase of monsoon (Chattopadhyay et al., 2013). One of the topics that received less attention is the role of middle atmospheric humidity and how the presence of shallow clouds impact boundary layer processes as part of a feedback process. This is especially important as the lower atmosphere transforms into more dry conditions such as the monsoon break periods. This is often the case when the monsoon circulation weakens, and there is less transport of moisture over land (Balaji et al., 2017).

We discuss the active and forced cumulus clouds (Stull 1985) and how the boundary layer properties are modified in response to drying of different degrees above the boundary layer. Some of the early studies have used ARM site observations to simulate shallow clouds and their properties as a classic case (Brown et al., 2002). A long term observational study from the ARM site by Zhang and Klein (2010), have shown that moisture above the boundary layer during early morning hours controls afternoon onset of convection and hence precipitation. The shallow to deep convective clouds transitions are controlled by the availability of free tropospheric moisture, through the action of enhanced buoyancy. The shallow clouds support the growth of deep convective clouds by moistening and destabilizing the free troposphere (Wright et al., 2017). The environmental conditions impact seasonal shallow-deep cloud transitions (Zhuang et al., 2017). The study has also reported an increase in convective available potential energy (CAPE) and destabilization of the upper atmosphere. Zhuang et al. (2017) detailed midlatitude shallow cumulus cases, and their characteristics, which they indicate, may not be applicable to high heat flux regions such as the tropical land regions. It may be noted that there are no LES case studies available from the tropical continental Indian region, especially the peninsular Indian region, characterized by shallow continental clouds driven by the boundary layer forcing. Although the characteristics of these clouds seem similar to the midlatitude cases reported from the ARM site, the middle atmospheric moistening is crucial for the precipitation over arid locations in the southern Indian peninsula. This makes the major difference with existing LES studies.

The presence of Indian summer Monsoon flow makes a significant difference to the middle atmospheric humidity. The study focuses on some of the unique observations conducted during the withdrawal of the monsoon season during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) of boundary layer and middle atmospheric fluxes over the arid region. We consider a non-precipitating, shallow, thin Cumulus case with in situ airborne observations.

The present study investigates drying above the boundary layer noted after the monsoon season. LES simulations are mimicking the withdrawal of monsoon (see Supplementary Fig. showing sample SkewT plots) or the break periods of monsoon. Initial attempt is to investigate suitability of aircraft observations for estimating turbulent parameters. The fluxes and variances computed from observations are compared with LES simulation. The main objectives of the study are to illustrate how the changes in humidity above the boundary layer impact a) the boundary layer turbulent fluxes and turbulent kinetic energy and the exchange between the cloud layer and the boundary layer b) the cloud fraction profiles, cloud liquid water path and cloud albedo over land c) the role of interactive radiation on the cloud and boundary layer turbulence. Section 2 describes data and methods in the study. LES sensitivity experiments, model initialization, aircraft observations, suitability of aircraft data for finding turbulent fluxes etc. are described. Section 3 describes results of the LES in comparison with aircraft fluxes, satellite data and discussion on various sensitivity experiments are detailed. The active and passive cumulus regimes were present during the simulation and model results and corresponding exchange coefficients are also investigated.