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Yoshio Kurihara, Christopher L. Kerr, and Morris A. Bender

Abstract

A numerical scheme proposed by Kurihara and Bender is modified so as to improve the behavior of open lateral boundaries of a regional model. In the new scheme, both the local values and the gradients of fields from a larger model are used to define the time-dependent reference values toward which the boundary gridpoint values of the regional model prediction are relaxed at each step of the model integration. Use of the gradients in the boundary forcing imposes constraints on the vorticity, divergence and baroclinicity fields for the regional model. The relaxation time of forcing is set to be short for the normal component of wind. For other variables, the relaxation time at a given boundary gridpoint depends on the wind direction at that gridpoint, with a minimum at a point of normal inflow and a maximum at a point of normal outflow. The forcing strength is reduced in the planetary boundary layer so that the boundary layer structure is determined mainly by the surface condition of the regional model. Also, a simple method to control the total mass in the regional model is described. Numerical results from 96-hour integrations with the improved scheme are compared with those from the previous scheme for the cases of the propagations of a wave and a vortex. The behavior of the model at the lateral boundary was noticeably improved with the use of the new scheme, while the solution in the interior domain was little affected by the scheme modification.

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Filipe Aires, Pierre Gentine, Kirsten L. Findell, Benjamin R. Lintner, and Christopher Kerr

Abstract

Although land–atmosphere coupling is thought to play a role in shaping the mean climate and its variability, it remains difficult to quantify precisely. The present study aims to isolate relationships between early morning surface turbulent fluxes partitioning [i.e., evaporative fraction (EF)] and subsequent afternoon convective precipitation frequency and intensity. A general approach involving statistical relationships among input and output variables, known as sensitivity analysis (SA), is used to develop a reduced complexity metamodel of the linkage between EF and convective precipitation. Two additional quantities characterizing the early morning convective environment, convective triggering potential (CTP) and low-level humidity (HIlow) deficit, are included. The SA approach is applied to the North American Regional Reanalysis (NARR) for June–August (JJA) conditions over the entire continental United States, Mexico, and Central America domain. Five land–atmosphere coupling regimes are objectively characterized based on CTP, HIlow, and EF. Two western regimes are largely atmospherically controlled, with a positive link to CTP and a negative link to HIlow. The other three regimes occupy Mexico and the eastern half of the domain and show positive links to EF and negative links to HIlow, suggesting that both surface fluxes and atmospheric humidity play a role in the triggering of rainfall in these regions. The regimes associated with high mean EF also tend to have high sensitivity of rainfall frequency to variations in EF. While these results may be sensitive to the choice of dataset, the approach can be applied across observational, reanalysis, and model datasets and thus represents a potentially powerful tool for intercomparison and validation as well as for characterizing land–atmosphere interaction regimes.

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Adam J. Clark, Israel L. Jirak, Burkely T. Gallo, Brett Roberts, Andrew R. Dean, Kent H. Knopfmeier, Louis J. Wicker, Makenzie Krocak, Patrick S. Skinner, Pamela L. Heinselman, Katie A. Wilson, Jake Vancil, Kimberly A. Hoogewind, Nathan A. Dahl, Gerald J. Creager, Thomas A. Jones, Jidong Gao, Yunheng Wang, Eric D. Loken, Montgomery Flora, Christopher A. Kerr, Nusrat Yussouf, Scott R. Dembek, William Miller, Joshua Martin, Jorge Guerra, Brian Matilla, David Jahn, David Harrison, David Imy, and Michael C. Coniglio
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