Model Errors and QPF in the Tropics on Diurnal to Seasonal TimescalesJulia Slingo, Centre for Global Atmospheric Modelling

Systematic errors in climate models present major barriers to achieving successful seasonal forecasts. These errors are often characterised by serious deficiencies in the distribution of rainfall through the season, which can have a substantial impact on the agricultural and hydrological applications.

Key References:•Inness and Slingo, 2002: Simulation of the MJO in a coupled GCM. I: The importance of ocean-atmosphere interaction. J. Clim. (in press).•Neale and Slingo, 2002: The Maritime Continent and its role in the global circulation: A GCM study. J. Clim. (in press).•Yang and Slingo, 2001:The diurnal cycle in the tropics. Mon. Weath. Rev., 129, 784 - 801.

Seasonal CycleThe management of crops, particularly in seasonally arid climates, depends crucially on the onset of the rains and the progression of the wetting front. Over West Africa, for example, the onset of the rains occurs too early, by up to 2 months (Fig.1). Consequently the seasonal cycle surface temperature is also disrupted (Fig. 2), another major factor in crop management.

Intraseasonal OscillationThe distribution of rainfall through the growing season (Fig. 3) is strongly modulated by the Intraseasonal/Madden Julian Oscillation (MJO).

Changes in seasonality may also be an important aspect of a changing climate.

Figure 1: Phase of seasonal cycle in rainfall – month of maximum

Observations

Figure 2: Phase of seasonal cycle in 2m temperature – month of maximum

Model: HadAM3

Diurnal CycleThe diurnal cycle dominates the subseasonal variability in precipitation over land in the tropics. GCMs have major difficulty in capturing the correct phase of the diurnal cycle (Fig. 5), which has major implications for the energy and hydrological budgets of tropical land areas.

Suppressed phases of the MJO lead to prolonged dry spells and high temperatures. These can affect the development of a crop if heat or water stress occurs at particular phenological stages. Synoptic systems, embedded within the active phase of the MJO, often lead to extreme precipitation events and flooding.

Figure 3: Major break in All India Rainfall during 2002 associated with the MJO.

GCMs have considerable difficulty in simulating the MJO and its organised convection.

Observations and modelling studies suggest that air-sea interaction is important for the MJO (Fig.4)

Figure 5: Phase (local time of maximum) in diurnal harmonic in precipitation from satellite observations (upper) and HadAM3 (lower). Note observed cycle shows an evening maximum whereas the GCM rains before noon.

Figure 4: MJO in HadCM3