FORECASTS OF HYDROLOGICAL PARAMETERS OVER THE MACKENZIE RIVER BASIN -SENSITIVITY TO INITIAL CONDITIONS, HORIZONTAL RESOLUTION AND FORECASTRANGE

As part of the Global Energy and Water Cycle Experiment, Canadian glob al spectral forecast model predictions of surface water and energy flu xes over the Mackenzie River basin are examined. Two nine-member ensem ble forecasts of one month duration are produced with the operational model, for a spring and a summer case, at a horizontal resolution of a bout 100 km (T95). The sensitivity to initial conditions is measured b y the degree to which the individual forecasts in the ensembles vary o ne from another. The evolution in lime of this estimated error (ensemb le standard deviation) is determined for the surface energy and water accumulations, averaged over the basin. For comparison the calculation s are repeated for the Mississippi basin and over North America. The g reatest sensitivity is found for the net accumulation of precipitation minus evaporation. The spring ensemble is redone at a coarser horizon tal resolution (T47), and the results are similar. The forecast uncert ainty (ensemble standard deviation) of the area-averages over the basi n appear to be unaffected by this change, although the ensemble mean v alues are sensitive to the change in resolution. The ensemble standard deviation makes a significant, abrupt increase reward the end of the second week into the forecasts. This investigation suggests a need for an improved model, if the forecasts' useful range is to extend to one month. Available upgrades to the land-surface, precipitation and evap oration schemes will be used in subsequent work, and the forecasts rep orted here will serve as a baseline for comparison.