Paper Reviewed
Abhik, S., Mukhopadhyay, P. and Goswami, B.N. 2014. Evaluation of mean and intraseasonal variability of Indian summer monsoon simulation in ECHAM5: identification of possible source of bias. Climate Dynamics 43: 389-406.

Abhik et al. (2014) write that the Indian Summer Monsoon (ISM) experiences what they refer to as Boreal Summer Intra-Seasonal Oscillations (BSISOs) that manifest themselves "in the form of enhanced (active) and reduced (break) spells of precipitation over the central and northern regions of the Indian subcontinent," as described by Ramamurthy (1969). But they state that "the ability to represent the complexity of the BSISO in the current general circulation models (GCMs) remains a great challenge," citing Waliser et al. (2003), Kim et al. (2008), Lin et al. (2008), Sperber and Annamalai (2008), Goswami (2011), Jiang et al. (2011) and Joseph et al. (2012). In the present study, Abhik et al. attempt to diagnose the BSISO simulation in an AGCM at two different vertical resolutions, choosing for this task the ECHAM5 model described by Roeckner et al. (2003, 2006), because they say that it "shows certain ability to reproduce some of the features of the BSISO" and "is among the few AGCMs which have a reasonable cloud microphysics to include the warm and cold cloud processes."

In describing their findings the three researchers report that the ECHAM5 model has (1) "a significant problem in simulating JJAS mean precipitation and low-level specific humidity," that at both of the vertical resolutions it (2) "overestimates the seasonal mean precipitation over the ocean," but that it (3) "underestimates the precipitation over the Indian landmass," that (4) "moderate rain events are less frequent than the observed," that (5) "over-estimation of the heavy precipitation degrades the skill over the Bay of Bengal," that (6) "overestimation of the lighter rainfall and shallow heating distribution indicate the lack of 'slow moistening processes' in the model," that (7) "lower atmospheric moisture distribution in the model sharply contrasts with observations," that (8) "the model systematically underestimates mean meridional specific humidity gradient," that (9) there is an "erroneous simulation of the mean low-level moisture," that (10) "ECHAM5 produces weaker spectral peaks for both meridional as well as zonally propagating components of the BSISO," that (11) "both northward propagation and eastward propagation appear at lower frequencies than in observations," that (12) "spectral power is about 40% less than [that of] the observations," that (13) ECHAM5 "fails to show the eastward propagation of the convection across the Maritime Continent," that (14) "the model shows unrealistic westward propagation that originates over the tropical western Pacific region," that (15) "the model fails to capture the observed phase-relationship of the dynamical as well as the thermodynamical parameters associated with the northward propagation," that (16) "the model also fails to reproduce the lower atmospheric convective instability that is conducive for triggering the new convection ahead of the existing convection," that (17) there is an "unusual tilting of the rainband" that is different "than the observed," that (18) "the asymmetric meridional moisture distribution is found to be shallower in the model," and that (19) vertical moisture transport "is also simulated weaker than the observed."

This study by Abhik et al. illustrates the fact that in spite of all that the world's climate modelers have accomplished in many different areas, they have only just begun to traverse the long-and-winding road that leads to ultimate climate modeling success; for as they learn ever more about the various facets of Earth's climatic system, they begin to discover there is even more out there in the 'great beyond' that they must successfully reduce to functionally-correct mathematical expressions that can be incorporated into their computer-driven models.

Maybe someday they'll get there. Then again, maybe they won't.

References
Goswami, B.N. 2011. South Asian summer monsoon. In: Lau, W.K.-M. and Waliser, D.E. (Eds.). Intraseasonal Variability of the Atmosphere-Ocean Climate System. Springer, Berlin, Germany, pp. 21-72.

Jiang, X., Waliser, D.E., Li, J.L. and Woods, C. 2011. Vertical cloud structures of the boreal summer intraseasonal variability based on CloudSat observations and ERA-interim reanalysis. Climate Dynamics 36: 2219-2232.

Joseph, S., Sahai, A.K., Goswami, B.N., Terray, P., Masson, S. and Kuo, J.-J. 2012. Possible role of warm SST bias in the simulation of boreal summer monsoon in SINTEX-F2 coupled model. Climate Dynamics 38: 1561-1576.

Kim, H.-M., Kang, I.-S., Wang, B. and Lee, J.-Y. 2008 Interannual variations of the boreal summer intraseasonal variability predicted by ten atmosphere-ocean coupled models. Climate Dynamics 30: 485-496.

Lin, J.L., Weickman, K.M., Kiladis, G.N., Mapes, B.E., Schubert, S.D., Suarez, M.J., Bacmeister, J.T. and Lee, M.I. 2008. Subseasonal variability associated with Asian summer monsoon simulated by 14 IPCC AR4 coupled GCMs. Journal of Climate 21: 4541-4567.

Ramamurthy, K. 1969. Monsoon of India: some aspects of "Break" in the Indian South West Monsoon during July and August (forecasting manual, Part IV.18.3), India Meteorological Department, New Delhi, India.

Roeckner, E., Bauml, G., Bonaventura, L., Brokopf, R., Esch, M., Giorgetta, M., Hagemann, S., Kirchner, I., Kornblueh, L., Manzini, E., Rhodin, A., Schlese, U., Schulzweida, U. and Tompkins, A. 2003. The Atmospheric General Circulation Model ECHAM5. Part I: Model Description. Max-Planck-Institut fur Meteorology Rep 349. Hamburg, Germany, p. 140.

Roeckner, E., Brokopf, R., Esch, M. Giorgetta, M., Hagemann, S., Kornblueh, L., Manzini, E., Schlese, U. and Schulzweida, U. 2006. Sensitivity of simulated climate to horizontal and vertical resolution in the ECHAM5 atmosphere model. Journal of Climate 19: 3771-3791.

Sperber, K.R. and Annamalai, H. 2008. Coupled model simulations of boreal summer intraseasonal (30-50 day) variability, part 1: systematic errors and caution on use of metrics. Climate Dynamics 31: 345-372.

Waliser, D.E., Jin, K., Kang, I.-S., Stern, W.F., Schubert, S.D., Wu, M.L.C., Lau, K.-M., Lee, M.I., Krishnamurthy, V., Kitoh, A., Meehl, G.A., Galin, V.Y., Satyan, V., Mandke, S.K., Wu, G., Liu, Y. and Park, C.-K. 2003. AGCM simulations of intraseasonal variability associated with the Asian summer monsoon. Climate Dynamics: 10.1007/s00382-003-0337-1.