Cheng, Fang-Yi, Chin-Fang Lin, National Central
University, Taiwan and Jeng-Lin Tsi, National Chung-Hsing University, Taiwan
The aerodynamic
roughness length (Zo) is essential for simulating wind profile and surface
fluxes. The present setup of WRF utilizes a look-up table to provide Z0 that is
only dependent on the dominant land use (LU) types. The LU type in Taiwan is
composed of forestland (~65%), dry/irrigated cropland (~20%), grassland (~5%),
urban (5%) and others (<5%). Specification of Zo from a look-up table would
underestimate surface heterogeneity. In this study, Zo was determined based on
observed surface fluxes and wind, temperature and humidity profiles for urban,
dry and irrigated cropland. For all other LU types, a suggested Z0 from
literature was used. An effective Z0 is estimated using the arithmetic mean of
the consisted LU type. Two gridded Z0 datasets are derived, Zm and Zt
(calculation for momentum and heat flux respectively). The WRF-Z0 (default)
overestimated the surface wind speed due to majority of LU type is specified by
shorter Z0 elements. WRF-Zm is able to reduce wind speed and agrees better to
the observed data due to the enhancement of Zm which increase surface flux
exchanging processes. Inclusion of Zt further reduces the diurnal temperature
variation. Overall, usage of the updated gridded Z0 improves the model
performance.