Investigation of surface-cloud coupling over land using ARM observations and model simulations

Planetary boundary layer

planetary boundary layer

The lowest layer of the troposphere where wind is influenced by friction.

Source: National Weather Service Source

(PBL) processes play key roles in the initiation of convection

convection

The rising of warm air and the sinking of cool air. Convection can cause local breezes, winds, and thunderstorms.

Source: NASA earth observatory Source

and boundary-layer low clouds. A large fraction of low clouds is driven by surface fluxes

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

through the conduit of the PBL, especially in a coupled cloud-surface system. Even though surface forcing plays a key role in low clouds, its representation in weather and climate models is still problematic. However, not all low clouds respond to surface forcing. For example, with weak surface forcing over oceans, marine boundary-layer clouds are maintained by radiative cooling at cloud tops. Continental low clouds can also be decoupled from the surface under certain conditions. The contrasting relationships between clouds and surface fluxes would lead to drastic differences in the responses of clouds to PBL thermodynamics

thermodynamics

The study of the effects of work, heat, and energy on a system. Thermodynamics is only concerned with large scale observations.

Source: NASA Source

between different types of clouds. Convective clouds are driven by surface fluxes through the conduit of the PBL over land, forming a coupled cloud-surface system that dictates the development of boundary-layer clouds, especially convective ones. Following our previous studies concerned with the coupling dynamics of boundary-layer clouds over oceans which laid the foundation of this study, but it is not directly applicable to land due to more complex and different land-atmosphere interaction schemes. We have developed a novel method to distinguish coupled and decoupled clouds over land, which will be further tested and optimized for developing the climatology of cloud-surface coupling at different sites by making use of comprehensive and synergistic observations from the Atmospheric Radiation Measurement (ARM) program. We will then examine cloud-surface coupling and its influencing factors using ARM data

ARM data

Continuous field measurements of atmospheric data collected with Atmospheric Radiation Measurement (ARM) instruments at fixed and varying locations worldwide.

Source: Argonne National Laboratory Source

and model simulations. Moreover, we will consider the impacts of cloud-surface coupling on the development of convective clouds. We will use field observations as the tool to investigate cloud-surface coupling at the process level, leading to an improved understanding and quantification of these processes in the climate system. To this end, we will make extensive use of ARM observations from multiple sites that are tailored for tackling our research objectives. Besides using observational data, large-eddy simulations (LESs) is an effective way to unveil the impact of various physical processes (e.g., large-scale meteorology, small-scale motions, and microphysical processes) on cloud evolution. We plan to conduct LES simulations to help (1) understand observation-based findings and (2) reveal underlying physical processes driving the coupling relationship and cloud development. Objectives, Hypotheses and Questions: Our central objectives are to determine the coupling state of the PBL and low clouds and its impact on the development of convective clouds over land using Atmospheric Radiation Measurement (ARM) data and large-eddy simulations (LESs). It is our hypothesis that low-cloud regimes over land are intimately linked with the cloud-surface coupling, having profound impacts on the PBL thermodynamics and development of convective clouds. We will test the hypothesis by addressing the following questions: (1) What are the fundamental processes driving cloud-surface coupling, and how is it determined? (2) How differently do the surface and the PBL affect clouds under coupled and decoupled conditions? (3) How does cloud-surface coupling affect the development of convective clouds over land? Research Tasks: (1) Development of the climatology of PBL height and cloud-surface coupling By virtue of our novel remote sensing

remote sensing

The process of detecting and monitoring the physical characteristics of an area by measuring its reflected and emitted radiation at a distance (typically from satellite or aircraft).

Source: USGS Source

methods, we will develop the climatology of the PBL height (PBLH) and cloud-surface coupling over land using long-term ARM data acquired at multiple sites (e.g., Southern Great Plains, North Slope of Alaska, and GoAmazon Mobile Facility). The PBLH is derived from micropulse lidar measurements, and the cloud-surface coupling is derived from the PBLH and the level of cloud condensation, among others. Disparities between thermodynamic coupling and dynamic coupling will be investigated to better understand different cloud-surface coupling regimes. (2) Investigation of cloud-surface coupling processes over land There are different methods to determine the coupling from thermodynamic and dynamic viewpoints. To illuminate the dynamical coupling, we plan to use the vertical velocity from the Doppler lidar to denote the turbulent exchange between the surface and cloud base. Utilizing profiles of vertical velocity and the hydrometeor mask, we can capture the turbulent mixing processes in the sub-cloud layer, a key process of cloud-surface coupling. The state of thermodynamic and dynamical coupling will be used to investigate the formation, evolution, and transition of cloud-surface coupling. Besides observations, LESs will be used to gain a process-level understanding of cloud-surface coupling processes. (3) Understanding the impacts of cloud-surface coupling on cloud development The cloud-surface coupling and PBLH datasets at ARM sites, together with synergistic products (e.g., surface fluxes, radiation, and meteorology), will be employed to understand cloud-surface coupling and its impacts on clouds. We will use both ARM and our own products to study the breaking up of stratocumulus (Sc) and the transition from cumulus (Cu) to deep convection for different coupling states. Any observation-based findings will be better understood through LESs. Cloud-surface coupling is expected to play a critical role in the transition of cloud regimes.