Ashok K. Luhar

Diagnosing errors in a land surface model (CABLE) in the time and frequency domains

[1]xa0We present an approach for diagnosing errors in land surface models in the time and frequency domains. The approach is applied to the Community Atmosphere-Biosphere-Land Exchange (CABLE). We compare modeled and observed fluxes of net ecosystem carbon exchange (NEE), latent heat (LE) and sensible heat (H) at two different forested flux station sites. Using wavelet analysis, we identify the frequencies where model errors are relatively large, and then analyze the sensitivities of model errors at those frequencies to selected model parameters, yielding significant improvements in several measures of model performance. At Harvard Forest, the predictions by CABLE using the modified parameter values reduce model errors at daily to yearly timescales for all three fluxes, but not at interannual timescales for NEE. At the Tumbarumba site, predictions by CABLE with modified parameter values reduce the variance of model errors of all three fluxes from daily to interannual timescales, but do not improve the agreement with the observed mean diurnal or daily time series for H. We conclude that analyzing both mean and variance of model errors at a range of time/frequency scales is useful for identifying and reducing errors of a land surface model.

The Kwinana Coastal Fumigation Study: II – Growth of the Thermal Internal Boundary Layer

Aircraft measurements of potential temperature and turbulent kinetic energy are used to examine the growth of the thermal internal boundary layer (TIBL) in sea-breeze flows on four selected days of a coastal fumigation study performed in 1995 at Kwinana in Western Australia. The aircraft data, together with radiosonde measurements taken on the same days, show a multi-layered low-level onshore flow in the vertical with a superadiabatic layer extending to about 50 m above the water surface on all four days. On the first three days the layer above the superadiabatic layer was neutral, typically 200 m deep, capped by a stably stratified region, whereas on the remaining day it was fully stable. The occurrence of the neutral layer on most experimental days contrasts with the more usual situation involving an entirely stable onshore flow. A composite approach based on both temperature and turbulence data is used to provide a pragmatic but self-consistent definition of the TIBL height. The data for the first three days indicate that the TIBL grows rapidly into the neutrally stratified region to the top of the region within about 2 km from the coast, with a very slow subsequent growth into the stable stratification aloft. On the other hand, the TIBL grows only to about 200 m within a distance of 7 km from the coast on the fourth day due to a strong stable stratification.An existing numerical TIBL model based on the slab approach, capable of describing the TIBL growth in both neutral and stable environments, and a recent analytical model, more efficient for operational use, are used to simulate the aircraft TIBL observations. The predictions by both models agree reasonably well with the data.