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

Abstract

In simulations of the boreal summer Asian monsoon, generations of climate models show a persistent climatological wet bias over the tropical western Indian Ocean and a dry bias over South Asia. Here, focusing on the monsoon developing stages (May–June), process-based diagnostics are first applied to a suite of NCAR models and reanalysis products. Two primary factors are identified for the initiation and maintenance of the wet bias over the northwestern Indian Ocean (NWIO; 5°–15°N, 52°–67°E): (i) excessive tropospheric moisture and (ii) restrained horizontal advection of the 1000–800-hPa levels cold–dry air couplet that originates offshore of Somalia. Second, guided by the diagnostics, we hypothesized that insufficient dilution of convective updrafts is one possible candidate for model bias and performed a series of enhanced entrainment sensitivity experiments with NCAR CAM4. Over the NWIO, the results suggest that globally increasing the maximum entrainment rate εmax leads to a drier free troposphere, arrests the vertical extension of clouds, and weakens moisture–convection and cloud–radiation feedbacks; each factor contributes to a reduced wet bias. Moreover, a higher εmax leads to a reduced dry bias over South Asia through changes in the local circulation features. In CAM4, improved precipitation climatology due to increased εmax suggests that insufficient dilution is one factor, but not the only one, that contributes to systematic errors. Rather, realistic representation of boundary layer processes in climate models arising out of local ocean–atmosphere interaction processes off Somalia’s coast deserves attention in reducing the NWIO wet bias.

Sources of water vapor and their effects on water isotopes in precipitation in the Indian monsoon region: a model-based assessment

“…1 e). In general, simulated seasonal precipitation rates are in agreement with the observations and previous model simulations 34 .

Figure 1 Modeled and observational precipitation in mm/day and 850 hPa winds over India in the JJAS and OND seasons.

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Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: A test for future predictions

“…While various downscaling approaches and parameterizations have been proposed to compensate for these issues, they have been met with limited success; Indian precipitation biases remain (62), even within the core monsoon zone using the latest (CMIP6) multimodel analyses (63). Simulating synoptic-scale convective responses to large-scale forcing remains a fundamental challenge in climate models (64). Second, these idealized precession and obliquity experiments do not incorporate IV or GHG as boundary conditions, both of which significantly lag NH summer-insolation forcing at the obliquity and precession bands.…”

Intraseasonal, Seasonal, and Interannual Characteristics of Regional Monsoon Simulations in CESM2

“…It has been speculated that the systematic error for the South Asian monsoon of too much precipitation too far west in the western Indian Ocean could be affected by an overly strong cold tongue in the Pacific and deficient precipitation over the Maritime Continent (e.g., Meehl et al, 2012), or an overly strong Bjerknes' feedback along the equatorial Indian Ocean (Annamalai et al, 2017), or limitations in representing regional air-sea interactions off the Somali-Oman coasts and associated atmospheric boundary-layer processes (Hanf & Annamalai, 2020). Figure 8 shows annual mean SST and surface wind stress for observations, CESM2, CESM1, differences from observations, and the difference, CESM2 minus CESM1 (seasonal means show similar systematic error patterns).…”