Impact of Initial Soil Wetness on Seasonal Atmospheric Prediction

Abstract

This study investigates the importance of initial soil wetness in seasonal predictions with dynamical models. Two experiments are performed, each consisting of two ensembles of global climate model integrations initialized from early June observed atmospheric states. In each experiment the only difference between the two ensembles is that they are initialized with a different soil wetness. In the first experiment both ensembles are initialized from 1988 observed atmospheric states and use observed 1988 SST; one ensemble is initialized with seasonally varying climatological soil wetness, and the other is initialized with proxy 1988 soil wetness derived from the European Centre for Medium-Range Weather Forecasts analysis-forecast system. In the second experiment the two ensembles are initialized from observed atmospheric states and use observed SST for five different years, and each ensemble is initialized with a different climatological soil wetness. After initialization, a coupled atmosphere-biosphere model determines the evolution of the soil wetness fields in all the integrations.The experiments are analyzed to determine the impact of the initial soil wetness differences. In contrast to several previous studies in which initial soil wetness was prescribed arbitrarily, a somewhat more realistic soil wetness impact is analyzed by comparing integrations initialized with climatological soil wetness to integrations initialized with soil wetness derived from the output of an operational analysis-forecast model. The initial soil wetness impact is found to be largely local and is largest on near-surface fields, in agreement with previous results. Significant impacts were found in several tropical and extratropical regions in both experiments. Almost all the regions that had significant increases (decreases) in initial soil wetness had significant increases (decreases) in seasonal mean evaporation and significant decreases (increases) in seasonal mean surface air temperature. Half of the regions had significant increases (decreases) in seasonal mean precipitation in response to increased (decreased) initial soil wetness, though the response of the precipitation was more variable and was highly dependent on the response of the moisture flux convergence to the initial soil wetness anomaly. In order for an initial soil wetness difference to force a significant seasonal mean precipitation difference in a region, it must effectively alter the mean convective stability of the region and thereby the mean convective precipitation.The strength of the impact of initial soil wetness differences, as well as the nature of the impact on precipitation and other atmospheric fields, depends on several factors. These factors include the areal extent and magnitude of the initial soil wetness difference, the persistence of the soil wetness difference, the strength of the solar forcing, the availability of nearby moisture sources, and the strength of the regional dynamical circulation. The results suggest that seasonal atmospheric predictio...

Current approaches to seasonal to interannual climate predictions

“…Studies focusing on seasonal prediction impacts are suggestive of important land surface initialization impacts under some conditions (Fennessey and Shukla, 1999;Douville and Chauvin, 2000).…”

Seasonal variation of land–atmosphere coupling strength over the West African monsoon region in an atmospheric general circulation model

“…The additional problem of the potential impact of soil moisture on the seasonal evolution of meteorological states has typically been examined with atmospheric general circulation models (AGCMs) (e.g. Shukla and Mintz 1982, Fennessy and Shukla 1999, Douville and Chauvin 2000, Koster et al 2000. Kanae et al (2006) investigated the impact of realistic soil moisture variations on precipitation variability.…”

Assessing GCM Sensitivity to Soil Wetness Using GSWP Data

“…We suspect a priori that there might be a connection between soil moisture conditions and climate fluctuations based on previous experiments (e.g., Shukla and Mintz, 1982;Dirmeyer, 1994;Fennessy and Shukla, 1998). Given it is particularly difficult to distinguish the impact of land surface conditions on interannual climate fluctuations between these two years.…”

“…Soil wetness is treated in one of two ways. In one pair of ensembles, soil wetness is initialized from operational ECMWF analysis values, adjusted to be consistent with SSiB (Fennessy and Shukla, 1998). This initial soil wetness is identical in all three layers, since the vertical structure of the operational ECMWF LSP lacked realism.…”

“…Do the interannual variations in soil moisture lead to significant changes in climate that are comparable to those caused by interannual soil moisture variations in SST? There has been some suggestion that this may be true in the case of the 1988 drought simulation with the COLA GCM (Fennessy and Shukla, 1998).…”