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M. Baldi, G. A. Dalu, and R. A. Pielke Sr.

the vegetation enhances the gradients of the surface fluxes providing the solenoidal forcing to local frontogenetic flows ( Shaw et al. 1997 ). Moreover, observations show that in dryline regions the probability of deep convection initiation is high in the late afternoon when the updraft forced by a mesoscale secondary circulation can locally break the boundary layer capping, and that the wind shear can modulate the convective initiation in the mesoscale updrafts at the dryline ( Ziegler and

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Young-Kwon Lim, Ming Cai, Eugenia Kalnay, and Liming Zhou

separating the surface temperature change signals due to global and regional forcings in the observed data. The basis of this study is the fact that the surface temperature change response to land vegetation types is not present in the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (NNR) surface data, and is only partially present in the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40), while the station

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K. W. Oleson, G. B. Bonan, J. Feddema, M. Vertenstein, and C. S. B. Grimmond

the top of the urban canopy layer (UCL) down to the depth of zero vertical heat flux in the ground ( Oke 1987 ). The current state of the atmosphere (wind, temperature, and humidity) and downwelling fluxes (longwave and shortwave radiation and precipitation) at a given time step is used to force the urban model. The urban model then provides fluxes that are area averaged with other land cover (e.g., forests or cropland) if present within the model grid cell. The area-averaged fluxes are used as

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K. W. Oleson, G. B. Bonan, J. Feddema, and M. Vertenstein

relative to observations. One probable reason for this discrepancy is the nature of the offline experimental setup. The rural and urban surfaces are forced with identical atmospheric variables. In particular, the atmospheric temperature at the forcing height is prescribed identically over the two surfaces. Because the atmospheric temperature is linked to the UCL and rural air temperatures through the aerodynamic resistance for heat, the difference in air temperature between the two surfaces is

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Vinodkumar, A. Chandrasekar, K. Alapaty, and Dev Niyogi

the analysis fields. Much scientific literature on the uses of four-dimensional data assimilation (FDDA) already exists ( Anthes 1974 ; Stauffer and Seaman 1990 , 1994 ; Stauffer et al. 1991 ). The present study utilizes the continuous (every model time step) dynamical FDDA, using the analysis nudging approach, where forcing functions are added to the model’s governing equations in order to “nudge” the model state toward the observations. In recent years, several studies have attempted to

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