Assessing the Aerosols, Clouds and Their Relationship Over the Northern Bay of Bengal Using a Global Climate Model. Issue 2 (21st February 2023)

• Record Type: Journal Article
• Authors: Sharma, Puneet; Ganguly, Dilip; Sharma, Amit Kumar; Kant, Sunny; Mishra, Shiwansha
• Is Part Of: Earth and space science. Volume 10:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-21
• Subjects: aerosol indirect effect -- CAM5 -- nudging -- cloud susceptibility; Space sciences -- Periodicals; Geophysics -- Periodicals; 500.5
• DOI: 10.1029/2022EA002706
• Languages: English
• ISSNs: 2333-5084

Abstract

Abstract: Comprehensive evaluation of aerosol‐cloud interactions (ACI) simulated by climate models using observations is crucial for advancing model development. Here, we use Moderate Resolution Imaging Spectroradiometer (MODIS) data to evaluate aerosol and cloud properties obtained from Community Atmosphere Model 5 (CAM5) over northern Bay of Bengal during winter season. We conduct simulations for default model setup as well as for prescribed, and nudged meteorology using Goddard Earth Observing System (GEOS5) reanalysis dataset in order to study the impact of meteorological parameters on simulated ACI. CAM5 captures the spatial variability of cloud optical depth ( τ c ), cloud droplet number concentration ( N c ), and liquid water path (LWP), although the values are overestimated in the model. Default model strongly simulates observed negative cloud effective radius ( r e )‐ N c susceptibility but fails to reproduce the observed positive LWP‐ N c susceptibility possibly due to evaporative cooling of the large number of smaller droplets. Compared to default and prescribed meteorology simulations, nudging specific humidity ( Q ) at 6‐hr relaxation time scale leads to a positive LWP‐ N c susceptibility, and an overall improved simulation of aerosol indirect effects. Increasing the relaxation time scale beyond 6‐hr degrade the simulation of indirect effects suggesting high sensitivity of indirect effects to Q and serious deficiencies in Q simulated by the model. Improvement in simulation of aerosol and cloud characteristics are also noted when winds (UV) are nudged but it worsens some of the simulated ACI sensitivities due to increased transport of absorbing aerosols over the study region and a dominant semi‐direct effect in the model. Plain Language Summary: Reliable projections of future climate change demand simulation of aerosol impact on clouds and radiation with a certain degree of accuracy. Incomplete understanding about the role of meteorological variables such as winds and moisture transport vis‐a‐vis aerosols in governing the cloud characteristics presents a formidable challenge to the scientific community in quantifying the perturbations caused by aerosols through small‐scale cloud processes on the Earth's energy budget. This study evaluates the ability of the Community Earth System Model (CESM1.2.2) in simulating the different indirect effects of aerosols on clouds under freely evolving and constrained meteorology setup over northern Bay of Bengal during winter season. We show that constraining the model simulated winds to match reanalysis data leads to an overall degradation in the simulated aerosol‐cloud interaction (ACI). On the other hand, the inability of default model with freely evolving meteorology to correctly simulate the ACI relationships owing to systematic bias in simulated moisture availability in the boundary layer is partially overcome through frequently nudging the model simulated specific humidity toward reanalysis data. The results in our study highlight a compelling need to improve model simulated low level moisture transport in order to accurately simulate ACI relationships as compared to satellite observations. Key Points: The wintertime ACI over the NBOB simulated by CAM5 is sensitive to model simulated Q and UV through different pathways Nudging UV impacts transport of absorbing aerosols whereas nudging Q improves the simulation of moisture in CAM5 model Serious issues exist in simulation of Q in CAM5 which is adversely affecting the simulated ACI sensitivities