Diurnal variation appears in cloud-system resolving model and satellite observations

The diurnal variation of deep convection (DC) over southern Africa represented by cloud-system resolving model (NICAM) was compared with TRMM rainfall data and Meteosat-8 cloud data. Diurnal variations of rainfall rate from TRMM/PR and TMI were studied from 12 years 3G68 data with 3 hour temporal resolution. The peak of diurnal variation appears at 14LT by TRMM/PR and at 17LT by TRMM/TMI over land area. The NICAM 3.5km experiment represents diurnal variation of rainfall over land with 17LT peak (3 hours later than TRMM/PR) which is 3 hour earlier than 7km experiment. From the comparison of cumulative percentage for both TBB in Meteosat-8 and OLR in NICAM, DC was defined by 208K TBB and 90W OLR. The diurnal variation of 90W OLR shows peak around 17LT. Cumulonimbus type cloud area classified by the split window shows the peak at the same time of 90W OLR area and 208K cloud area, while 253K or colder area shows the peak 3-6 hours later. This suggests that 253K cloud area includes anvil which appears at later stage of DC life time. Using the 15 minute interval data of Meteosat-8, we studied the diurnal variation of number and area size of cumulonimbus type cloud. The peak time of DC number is corresponding to the TRMM/PR rainfall rate peak time. OLR spatial distribution also shows the smaller size of DC appears at 14LT. This suggests that diurnal variation of rainfall reflect the time when larger number of DC with smaller size, and larger size of DC is not necessarily corresponds to heavier rainfall. We also found semi-diurnal variation with the second peak around 2LT for both number and area size. For both first and second peak, the number precedes to the area size. Further investigation is required for this semi-diurnal variation of DC.