Effects of land-atmosphere coupling strength on coupled WRF/Noah model 0-24 h forecasts of warm-season precipitation in the central United States

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Thursday, 21 January 2010: 2:45 PM
B216 (GWCC)
S. B. Trier, NCAR, Boulder, CO; and F. Chen, K. W. Manning, and M. A. LeMone

The diurnal evolution of the planetary boundary layer and its association with warm-season (convective) precipitation is strongly influenced by the surface heat exchange in both climate and numerical weather prediction models. The strength of this land-atmospheric coupling is related to the roughness lengths for heat and momentum, which can, in turn, be related to each other through an unknown empirical constant, Czil, known as the Zilitinkevich (1995) coefficient. Empirical studies (e.g., Chen and Zhang 2009) have suggested a range of Czil from 0.01 to 1.0 is required to match values of observed heat flux. Simulations using a single intermediate value for Czil will often underestimate (overestimate) in regions of tall (short) vegetation.

In the current study we examine 0-24 h coupled WRF/Noah convection-resolving precipitation forecasts using different specifications of Czil for a six-day retrospective period within the International H20 (IHOP_2002) field program. Results indicate strong sensitivity in the timing of simulated afternoon convection initiation and subsequent precipitation amounts to the strength of the Czil-related variations in the strength of land-atmosphere coupling. Over the western high plains (105-100 W longitude), where deep convection is often locally generated, simulations using empirically derived values for Czil that varied with type (i.e., height) of vegetation produced a diurnal cycle of forecasted precipitation amounts in better agreement with Stage-4 precipitation observations than simulations that used single values of Czil. In this talk we present statistics of the relationship between Czil and forecasted precipitation for this six-day IHOP retrospective period and provide illustrative case studies of the mechanisms by which land-atmosphere coupling strength influences convection initiation and evolution.