Figure 15. Precipitation (averaged from 5S–5N) from TRMM
(left), S-SHiELD without MLO (center), and S-SHiELD with MLO (right),
for initializations at (top) 15 October (middle) 8 November and (bottom)
6 December 2011.
Klingaman and DeMott (2020) found that climate models exaggerate the
effect of ocean coupling on the MJO by over-intensifying the MJO in El
Niño years. S-SHiELD does not have a coupled dynamical ocean and nudges
towards climatology, and so can only represent the El Niño-Southern
Oscillation (ENSO) state at initialization; indeed, the DYNAMO period
was during a La Niña event (see
https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php
). Hence, this ENSO contamination of the link between ocean coupling and
the MJO is not present in S-SHiELD.
The diurnal cycle of precipitation is another challenge for climate
models. Covey et al (2016) found
nearly all climate models, even the 30-km resolution GFDL HiRAM,
struggle with both the phase and amplitude of the diurnal cycle,
especially over land and during boreal summer. Figure 16 presents the
JJA diurnal cycle from a 10-year S-SHiELD simulation with MLO SSTs
nudged towards climatology, with results from 13-km SHiELD hindcasts
shown for reference. We find that the observed phase of the diurnal
cycle is beautifully matched by S-SHiELD, over both land and ocean. Most
notably the CONUS evening maximum of precipitation is reproduced.
However, the amplitude of the cycle is biased low over land areas,
possibly due to the inability of S-SHiELD’s 25-km grid to produce the
propagating mesoscale convective systems characteristic of heavier
warm-season precipitation events. This appears to be a resolution effect
as 13-km SHiELD reproduces both the correct phase and amplitude of
precipitation. We also find that the majority of precipitation in
S-SHiELD (55% globally and 80% between 20S and 20N) is from the SAS
convective scheme, although this does not adversely affect the phase of
the diurnal cycle. S-SHiELD does have the correct phase and amplitude
(albeit slightly too high) of the diurnal cycle of 2-m temperature over
land (Supplemental Figure S3).