SST Structure and MJO Rainfall Errors as Observed and Compared to Highly Parameterized Global Models

 SST structure and MJO rainfall errors as observed and compared to highly parameterized global models

R. E. Carbone¹ and Yanping Li²

National Center for Atmospheric Research¹

University of Saskatchewan, Saskatoon²

                A strong association between ordinary mesoscale SST gradients and the occurrence of tropical rainfall has been observed from GHRSST and CMORPH data in the eastern hemisphere (Li and Carbone, 2012).  Under slow  wind speed conditions, lower boundary forcing results from the convergent Laplacian of SST (LSSTc), which induces ascent in the marine atmospheric boundary layer, MABL.  When such forcing exceeds convective inhibition (CIN) in the lower troposphere, moist convection can be triggered, the depth and intensity of which is dependent on deep tropospheric convective available potential energy, CAPE. 

                 Averaged over a 4 year timeseries (GHRSST, CMORPH), LSSTc is equally divided between convergent and divergent mesoscale (~100km)dipoles, yet 3 of 4 rainfall events have their origin over LSSTc in the western Pacific. The ensuing correlation of LSSTc with rainfall event frequency and cumulative rainfall is especially prominent in the active phase of MJOs and across the eastern hemisphere more generally.  

                Parameterizations of tropical convection in global models are heavily influenced by SST.   However, the influence of mesoscale LSSTc forcing is rarely, if ever, represented in highly parameterized global models, potentially leading to systematic rainfall errors irrespective of SST.  Preliminary results will be presented to quantify the statistical relationship between strength of LSSTc forcing, rainfall event frequency, and known systematic rainfall errors in a few global models (e.g. NCAR CESM, Hadley Center, ECMWF, GFDL)  (e.g. GHRSST, CMORPH).

References:

Yanping Li, Richard E. Carbone, 2012: Excitation of rainfall over the tropical western Pacific. Journal of Atmospheric Science, Vol. 69, No. 10, 2983–2994.

Carbone R. E., Yanping Li, 2015: Tropical Oceanic Rainfall and Sea Surface Temperature Structure: Parsing Causation from Correlation in the MJO. Journal of Atmospheric Science, Vol. 72, No. 7, 2703–2718.