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...

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Systematic biases in U.S. summer integrations with the Center for Ocean‐Land‐Atmosphere Studies (COLA) atmospheric general circulation model (GCM) have been identified and analyzed. Positive surface air temperature biases of 2°–4°K occurred over the central United States. The temperature biases were coincident with the agricultural region of the central United States, where negative precipitation biases also occurred. The biases developed in June and became very significant during July and August. The impact of the crop area vegetation and soil properties on the biases was investigated in a series of numerical experiments. The biases were largely caused by the erroneous prescription of crop vegetation phenology in the surface model of the GCM. The prescribed crop soil properties also contributed to the biases. On the basis of these results the crop model has been improved and the systematic errors in the U.S. summer simulations have been reduced. The numerical experiments also revealed that land surface effects on the atmospheric variables at and near the surface during the North American summer are very pronounced and persistent but are largely limited to the area of the anomalous land surface forcing. In this regard, the midlatitude land surface effects described here are similar to those previously found for tropical regions.

A numerical simulation study of the influence of surface albedo on July circulation in semi-arid regions, using the general circulation model (GCM) of the Goddard Laboratory of Atmospheric Sciences (GLAS) is presented. The results are based on two 47-day integrations. In the first integration, called the control run, the surface albedo was normally prescribed, whereas in the second integration, called the anomaly run, the surface albedo was modified in four regions: the Sahel in Africa, the western Great Plains in the United States, the Thar Desert border in the Indian subcontinent, and northeast Brazil in South America. This experiment is similar to that of Charney et al. (1977); however, it was performed with the G U S model with vastly different boundary forcings and several changes in the physical parameterizations.Each run was started from observed initial conditions for 15 June 1979, based on NMC analysis. A n analysis of the two simulations shows that in the Sahel, and the Thar Desert border regions the current results again show reduced precipitation with increased surface albedo in accordance with Charney et al. (1977) and Charney (1975).The semi-arid northeast Brazil region, which had a winter circulation, also conforms with Charney's (1975) hypothesis. However, the Great Plains region was an exception. There the total precipitation in two simulations was unchanged, as was the total cloudiness. In this region, the convective precipitation was reduced somewhat, while the large-scale precipitation compensated this decrease by approximately the same amount. The lack of albedo impact on precipitation in the Great Plains region could be due to the influence of the Rocky Mountains in generating variations in the large-scale flow.The purpose of repeating Charney's albedo experiment was to test the sensitivity of his earlier results with the current GLAS GCM which has substantially modified physical parameterizations, particularly the planetary boundary layer (PBL), crucially important for such impact studies. Taking into account that both the GCM and the initial conditions of the atmosphere were different, it is worth noting that this experiment still provides support for Charney 's hypothesis (1975) regarding the influence of surface albedo on mean-monthly climatology in the subtropical desert margin regions.Another important influence noted was the variability of mean monthly simulation in areas far away from the albedo anomaly regions. Because the differences occur in regions of high observed climatological variability, it is necessary to separate the contribution of albedo anomalies as opposed to the model variability. The influence of surface albedo changes to produce large changes in the mean montly circulation elsewhere is very intriguing and needs further investigation.

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