Abstract
Abstract Fair-weather data from the May–June 2002 International H2O Project (IHOP_2002) 46-km eastern flight track in southeast Kansas are compared to simulations using the advanced research version of the Weather Research and Forecasting model coupled to the Noah land surface model (LSM), to gain insight into how the surface influences convective boundary layer (CBL) fluxes and structure, and to evaluate the success of the modeling system in representing CBL structure and evolution. This offers a unique look at the capability of the model on scales the length of the flight track (46 km) and smaller under relatively uncomplicated meteorological conditions. It is found that the modeled sensible heat flux H is significantly larger than observed, while the latent heat flux (LE) is much closer to observations. The slope of the best-fit line ΔLE/ΔH to a plot of LE as a function of H, an indicator of horizontal variation in available energy H + LE, for the data along the flight track, was shallower than observed. In a previous study of the IHOP_2002 western track, similar results were explained by too small a value of the parameter C in the Zilitinkevich equation used in the Noah LSM to compute the roughness length for heat and moisture flux from the roughness length for momentum, which is supplied in an input table; evidence is presented that this is true for the eastern track as well. The horizontal variability in modeled fluxes follows the soil moisture pattern rather than vegetation type, as is observed; because the input land use map does not capture the observed variation in vegetation. The observed westward rise in CBL depth is successfully modeled for 3 of the 4 days, but the actual depths are too high, largely because modeled H is too high. The model reproduces the timing of observed cumulus cloudiness for 3 of the 4 days. Modeled clouds lead to departures from the typical clear-sky straight line relating surface H to LE for a given model time, making them easy to detect. With spatial filtering, a straight slope line can be recovered. Similarly, larger filter lengths are needed to produce a stable slope for observed fluxes when there are clouds than for clear skies.
Highlights
We focus on the impact of surface heterogeneity on horizontal variability in the convective boundary layer (CBL) along a 46-km flight track in southeastern Kansas
The numerical simulations are done with the coupled Advanced Research Weather Research and Forecasting modeling system (ARWWRF; Skamarock et al 2005), initialized using the HighResolution Land Data Assimilation System (HRLDAS; Chen et al 2007), and coupled to the Noah land surface model (LSM; Ek et al 2003)
The horizontal variability in H and latent heat flux (LE), which is highly correlated with vegetation patterns through normalized differential vegetation index (NDVI) and Ts (LeMone et al 2007b), bears little resemblance to that produced by the Noah LSM, and volumetric soil moisture is underestimated
Summary
This paper the first part of a two-part series that uses a combination of numerical simulations and observations of the fair-weather convective boundary layer (CBL) to explore the relationship of surface heterogeneity and associated fluxes (W m22) of sensible heat. The data and results discussed and in Part II are being used to evaluate and improve the Noah LSM and the performance of the ARW-WRF model in representing fair-weather CBL structure and evolution. Ci; isolated small Cu; haze at BL top Scattered Cu humulis in streets Scattered Cu humulis in streets; Ci Scattered Cu humulis in streets In this part, we assess the performance of HRLDAS (an offline version of the Noah LSM, hereafter referred to as ‘‘offline’’ or ‘‘offline Noah LSM’’ in comparisons) and the Noah LSM coupled to ARW-WRF, in simulating the variability in surface fluxes, as well as ARWWRF in simulating the spatial and temporal variability of the CBL along the eastern track. No of legs high (avg height range, m AGL) 6 (523–688) 6 (574–743) 5 (545–765) 6 (752–897)
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