South Asian summer monsoon breaks: Process‐based diagnostics in HIRHAM5

Abstract

This study assesses the ability of a high‐resolution downscaling simulation with the regional climate model (RCM) HIRHAM5 in capturing the monsoon basic state and boreal summer intraseasonal variability (BSISV) over South Asia with focus on moist and radiative processes during 1979–2012. A process‐based vertically integrated moist static energy (MSE) budget is performed to understand the model's fidelity in representing leading processes that govern the monsoon breaks over continental India. In the climatology (June–September) HIRHAM5 simulates a dry bias over central India in association with descent throughout the free troposphere. Sources of dry bias are interpreted as (i) near‐equatorial Rossby wave response forced by excess rainfall over the southern Bay of Bengal promotes anomalous descent to its northwest and (ii) excessive rainfall over near‐equatorial Arabian Sea and Bay of Bengal anchor a “local Hadley‐type” circulation with descent anomalies over continental India. Compared with observations HIRHAM5 captures the leading processes that account for breaks, although with generally reduced amplitudes over central India. In the model too, anomalous dry advection and net radiative cooling are responsible for the initiation and maintenance of breaks, respectively. However, weaker contributions of all adiabatic MSE budget terms, and an inconsistent relationship between negative rainfall anomalies and radiative cooling reveals shortcomings in HIRHAM5's moisture‐radiation interaction. Our study directly implies that process‐based budget diagnostics are necessary, apart from just checking the northward propagation feature to examine RCM's fidelity to simulate BSISV.

Representation of Ocean-Atmosphere Processes Associated with Extended Monsoon Episodes over South Asia in CFSv2

“…Given that monsoon region is data sparse and model prejudices influence reanalysis thermodynamical variables, results from CFSR too need to be verified with sustained observations. Overall, our budget analysis, consistent with the earlier works (Prasanna and Annamalai, 2012;Hanf et al, 2017;Mohan and Annamalai, under review), provide robustness in precursor signals and false alarms in key ocean and atmospheric variables during the evolution of extended monsoon episodes.…”

Projection of precipitation extremes over South Asia from CMIP6 GCMs

“…A corresponding distribution pattern for RR10mm and RR20mm in the FF period under a high emission scenario (SSP5-8.5) is observed (Fig. 8b and c); however, there is a distinguished shift in curves to the right side and the curves become somehow flattened for RR20mm, showing an increase in the intensity of precipitation but a decrease in the frequency of precipitation, which may lead to the intensification of the water cycle in the future (Hanf et al, 2017). Under the SSP5-8.5 scenario, the RR50mm curves shift to the right in the FF period, and the simulated maximum precipitation value reaches up to 0.04 mm (Fig.…”

Systematic Errors in South Asian Monsoon Precipitation: Process-Based Diagnostics and Sensitivity to Entrainment in NCAR Models

“…Therefore, a residual component arises in both reanalysis and modeled MSE budgets. However, based on earlier studies (e.g., Neelin and Su 2005;Maloney 2009;Annamalai et al 2014;Hanf et al 2017), the leading processes identified are expected to be robust and reproducible. To assess model biases, we point out errors in sources and sinks of column moisture or MSE.…”