1. Cloud Diurnal Variations in the Maritime Continent During Boreal Winter
Ming-Dah Chou
Department of Atmospheric Sciences
National Taiwan University
Presentation at the Department of Atmospheric Sciences, National Central University, January 2, 2008.

2. Importance of Cloud Diurnal Variation
Net solar and IR radiative heat balance. Strong diurnal solar heating.
A fundamental mode of variability of the global climate system
A key test of physical parameterization in a global circulation model
An accurate representation of the diurnal cycle over land and ocean provides a key test of many aspects of the physical parametrizations in a climate model.
Diurnal variations of cloud fractional coverage and solar flux are the main factors of the cloud diurnal contributions to shortwave (SW) Bur, although the diurnal variation of cloud type is also important. The cloud diurnal contribution to longwave (LW) flux at the top of the atmosphere (TOA) is produced by diurnal variations of cloud fractional coverage, cloud-top height, and surface temperature. The cloud diurnal contribution to LW flux at the surface is produced by diurnal variations of cloud fractional coverage and cloud-base height. Cloud diurnal contributions to SW fluxes at the surface and TOA are much larger than the contribution to SW atmospheric absorption. The contribution to radiative heating in the atmosphere is concentrated inside the cloud layer. Its vertical profile changes sign, so the cloud diurnal contribution to atmospheric energetics is significantly larger than is implied by the column average.

3. Objectives of the Study
To understand the diurnal variations of convection over different regions and seasons
To understand the impact of diurnal radiative heating and cooling on convection and clouds

4. Possible mechanisms Land Ocean
Low-level thermodynamical response to radiative heating cycle.
Local topography
Land
Direct radiation convection interaction (Kraus 1963, Randall et al. 1991) [Nighttime enhanced convection; daytime depressed convection]
Cloud versus cloud-free radiation difference in horizontal (Gray and Jacobson 1977) [Nighttime enhanced convection; daytime depressed convection]
Thermodynamic (RH: Tao et al.(1996); APW: Sui et al.(1997); Chen and Houze 1997) [nighttime enhanced convection]
Diurnal cycle of SST under weak wind clear sky. (Halpern and Reed 1976; Stramma et al. 1986) [Late afternoon enhanced convection during clear calm day]
Ocean
1.Land: daytime 結果是 對流降水最大發生在傍晚 晚上 增加穩度 壓制對流 而導致對流最小在清晨
2. For deep oceanic convection, the situation is much more complicated, and there may be several mechanisms responsible for the behavior of the diurnal cycle.
3. Liu and Moncrieff (1998) concluded that this diurnal variation was primarily controlled
by the direct radiation–convection interaction; the cloud versus cloud-free radiation difference was of
secondary importance.

5. Japan’s Geostationary Satellite-5 (GMS-5)
Spectral Band & Resolution
VIS (0.55 mm~0 .90 mm) ~1.25 km
IR1(10.5 mm~11.5 mm) ~5 km
IR2(11.5 mm~12.5 mm) ~5 km
IR3(6.5 mm~7.0 mm) ~5 km
Temporal resolution: 1 hour
Period of this study: 3 boreal winters
DJF Dec 1997-Dec 2000
The brightness temperature, Tbb, of IR2 was used to infer clouds.
A smaller Tb indicates a higher cloud fractional cover and a higher cloud height.
Definition of Tbb?

6. Seasonal Mean Brightness Temperature
September February 2003 K
GMS data from Central Weather Bureau

7. Standard Deviation of Daily-Mean Brightness TemperatureK
Figure 5. Seasonal mean standard deviation (K) of Tbb derived from Equation (2) based on hourly GMS data.

9. Percentage Contribution (%) of Different Time Scales to the Total Variance

10. Local Time of Tbb Maximum
Local Hours
(1) The maximum Tbb in most of the open oceanic regions occurs in daytime hours.
(2) There are no open oceanic regions where the maximum Tbb occurs in the early hours after midnight from.

11. Local Time of Tbb Minimum
Over dry regions centered at equator, the minimum Tbb occurs from midnight to early morning (0:00-6:00 LT).
Over humid regions centered at ~5ºN and south of 10ºS, the minimum Tbb occurs in late afternoon and early evening (14:00-18:00 LT).

12. Classification into Active and Inactive Phases
Synoptic scale
Classification into Active and Inactive Phases
Criterion 1:
Standard deviation (SD) of the whole 3-year DJF
Active events: Tb-ave(Tb)< -0.5*SD
Inactive events: Tb-ave(Tb)> +0.5*SD
Criterion 2: The above conditions continued for at least 4 days

13. Synoptic scale
Borneo ( yr mean SD) New Guinea ( yr mean SD)
Inactive 59 days
active 72 days
Inactive 52 days
active 60 days

14. Diurnal Variations of Tbb in Boreal Winters
Local Hours
Units: K

15. Diurnal Variations of Tbb in Boreal Winter
Oceanic Regions
Local Hours
Units: K

16. Diurnal Variations of Tbb in Boreal Winter
Units: K
Local Hours

17. Local Time of Tbb Maximum
Local Hours
(1) The maximum Tbb in most of the open oceanic regions occurs in daytime hours.
(2) There are no open oceanic regions where the maximum Tbb occurs in the early hours after midnight from.

18. The 850-hPa Streamline and Wind Speed
m s-1

19. Mean Diurnal Cycle of the Tbb Anomaly
Along the 140E Meridian
The ordinate is for local hours.

20. Conclusions The land-sea contract of cloud variation is apparent
Over land, the diurnal cloud variation contributes significantly to the total variance
Over oceans, cloud variations with a time scale > one day contribute significantly to the total variance
The large-scale circulation affects the magnitude, but not the phase, of the cloud diurnal variation

21. Conclusions (continued)
Three major thermodynamic processes govern the diurnal
variation of clouds:
In convective regions, the solar heating causes a cloud maximum in the afternoon.
In convection-suppressed oceanic regions, the cloud-top IR cooling induces a cloud maximum in the early hours after midnight.
In oceanic regions adjacent to land, land-sea breezes and prevailing winds cause a phase propagating several hundred kilometers offshore.