Abstract
Abstract. Land surface models are essential parts of climate and weather models. The widely used Noah-MP land surface model requires information on the leaf area index (LAI) and green vegetation fraction (GVF) as key inputs of its evapotranspiration scheme. The model aggregates all agricultural areas into a land use class termed “cropland and pasture”. In a previous study we showed that, on a regional scale, the GVF has a bimodal distribution formed by two crop groups differing in phenology and growth dynamics: early-covering crops (ECC; e.g., winter wheat, winter rapeseed, winter barley) and late-covering crops (LCC; e.g., corn, silage maize, sugar beet). That result can be generalized for central Europe. The present study quantifies the effect of splitting the land use class cropland and pasture of Noah-MP into ECC and LCC on surface energy fluxes and temperature. We further studied the influence of increasing the LCC share, which in the study area (the Kraichgau region, southwest Germany) is mainly the result of heavily subsidized biomass production, on energy partitioning at the land surface. We used the GVF dynamics derived from high-resolution (5 m × 5 m) RapidEye satellite data and measured LAI data for the simulations. Our results confirm that the GVF and LAI strongly influence the partitioning of surface energy fluxes, resulting in pronounced differences between simulations of ECC and LCC. Splitting up the generic crop into ECC and LCC had the strongest effect on land surface exchange processes in July–August. During this period, ECC are at the senescence growth stage or already harvested, while LCC have a well-developed ground-covering canopy. The generic crop resulted in humid bias, i.e., an increase in evapotranspiration by +0.5 mm d−1 (latent heat flux is 1.3 MJ m−2 d−1), decrease in sensible heat flux (H) by 1.2 MJ m−2 d−1 and decrease in surface temperature by −1 ∘C. The bias increased as the shares of ECC and LCC became similar. The observed differences will impact the simulations of processes in the planetary boundary layer. Increasing the LCC share from 28 % to 38 % in the Kraichgau region led to a decrease in latent heat flux (LE) and a heating up of the land surface in the early growing season. Over the second part of the season, LE increased and the land surface cooled down by up to 1 ∘C.
Highlights
Within weather and climate models, land surface exchange processes are simulated by so-called land surface models (LSMs)
As for latent heat flux (LE), the smallest (−1.2 MJ m−2 d−1) and largest (5.3 MJ m−2 d−1) mean differences in H between early-covering crops (ECC) and late-covering crops (LCC) were observed in June and August, respectively (Table 4)
Compared with LCC, the higher latent heat fluxes of ECC in May and June resulted in a cooler land surface, on average by −2.6 and −1.0 ◦C, respectively (Table 4)
Summary
Within weather and climate models, land surface exchange processes are simulated by so-called land surface models (LSMs). The main role of an LSM is to partition net radiation at the land surface into sensible heat flux (H ), latent heat flux (LE) and ground heat flux (G) and determine the land surface temperature. Surface energy partitioning has a significant influence on the evolution of the atmospheric boundary layer (ABL). ABL evolution strongly influences the initiation of convection, cloud formation, and the location and strength of precipitation (Crawford et al, 2001; Koster et al, 2006; Santanello et al, 2013; van Heerwaarden et al, 2009; Milovac et al, 2016). K. Bohm et al.: Distinguishing between early- and late-covering crops
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