69 Investigating the Land Surface–Atmosphere Response in Coupled MONC-JULES and Unified Model Mesoscale Simulations during the U.K. Spring–Summer 2018 Soil Moisture Dry-Down

Monday, 13 January 2020
Hall B (Boston Convention and Exhibition Center)
Jennifer K. Brooke, UKMO, Exeter, United Kingdom; and M. J. Best, J. M. Edwards, A. Hill, A. Lock, and S. Osborne

Traditionally large-eddy models prescribe the land surface state and do not represent the feedback between the evolving land surface and boundary layer development. The Met Office-NERC (MONC) large-eddy model has recently been coupled to the community land surface model, Joint UK Land Environment Simulator (JULES). This coupled system enables more realistic turbulence-resolving simulations of the boundary layer development to be performed because it captures the local-scale feedbacks between land state and surface turbulent fluxes.
The impact of land surface-atmosphere interactions on boundary-layer development during the UK spring-summer 2018 soil moisture dry down is examined. The soil moisture dry-down in the UK during 2018 was historically very unusual with conditions ranging from very wet in April (0.48 kg m-2) to very dry in July (0.11 kg m-2). Our study contrasts different land states (wet, intermediate and dry surfaces) for case studies using observations from the relatively homogenous Met Office Cardington site in Bedfordshire, UK. Observations include eddy-covariance flux measurements at four heights (up to 50 m) which include temperature, friction velocity, and turbulent heat and moisture fluxes. Surface and sub-surface measurements include soil temperatures, ground heat flux and volumetric soil moisture content.
We perform LES for a full diurnal cycle and the results show differences in the vertical transport rate of heat and moisture from the surface into and through the boundary layer between the uncoupled MONC and coupled MONC-JULES simulations. The coupled MONC-JULES simulations have a deeper mixed layer associated with a larger vertical momentum flux, particularly for the dry soil case. Comparisons between the coupled MONC-JULES simulations and Halo Doppler Lidar observations show that the simulation represents the boundary layer top well although overestimates the vertical velocities. The results from coupled MONC-JULES simulations have been contrasted with 300 m and 1.5 km mesoscale simulations of the Unified Model. The partitioning between sensible and latent heat fluxes in the mesoscale models is initially well simulated for the wet soils, however the latent heat flux remains large during the drying leading to an incorrect Bowen ratio compared with observations.
Land surface-atmosphere interactions play a critical role in the development of the boundary layer and help to determine the near surface temperature and humidity throughout the diurnal cycle. We will examine the near surface atmospheric states of temperature and humidity for all model simulations throughout the diurnal cycle using the Local Land-Atmosphere Coupling (LoCo) mixing diagram approach.
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