The energy
imbalance problem' in micrometeorology arises because at most flux measurement
sites the sum of eddy fluxes of sensible latent heat (*H* + *λE*)
is 50 200 W m^{-2} less than the available energy (*A*) at
half-hourly time scales. *A* is net radiation minus changes in energy
stored in soil, air column and biomass. Thus either eddy fluxes of * H*
and *λE* are
underestimated or *A* is overestimated. Lack of energy closure is caused
by: 1) a failure to satisfy the fundamental assumption of one-dimensional
transport needed for measurements on a single tower to represent
spatially-averaged fluxes to/from the underlying surface; 2) measurement errors
in: i) eddy fluxes, ii) net radiation, iii) changes in energy storage in soils,
air, and biomass below the measurement height.

Review of
the literature shows that radiometric errors are unlikely to cause significant overestimates
of *A* at *all* flux measurement sites, but phase lags between *H*
+ *λE* and *A* caused by incorrect estimates of the energy
storage terms that contribute to *A* can explain why *H* + *λE
systematically* underestimates *A* at half-hourly time scales.

Energy
closure is observed at only 8% of flux sites in the La Thuile dataset (http://www.fluxdata.org/DataInfo/default.aspx),
with a median slope of 0.75 for *H* + *λE* versus *A* for half-hourly
averages. Using 24h averages leads to energy closure at 45% of the La Thuile
sites, and the median slope of *H* + *λE* versus *A* increases
to 0.90. Increasing the averaging time reduces the bias in *A* because
energy entering the soil, air and biomass in the morning is returned in the
afternoon and evening.

Unrealistically
large positive horizontal temperature gradients of 4 10 ºK km^{-1}
are needed for horizontal advective flux divergences to explain the 50 200 W
m^{-2} underestimate of *H* + *λE* compared to *A*
often observed at half-hourly time scales. Similarly, unrealistically large
mean vertical velocities and temperature gradients are needed for vertical
advective flux divergences to account for the missing' energy. Imbalances
between *H* + *λE* and *A* still occur in daily averages
but the small residual energy imbalances are explicable by positive and
negative horizontal and vertical advective flux divergences. Systematic
underestimates of the vertical heat flux also occur if horizontal *u'T' *covariances
contaminate the vertical *w'T'* signal due to incorrect coordinate
rotations. An incorrect tilt angle of 2º will cause a 5% underestimate of *H*.

Closure of the energy balance is possible at half-hourly time scales by careful selection of horizontally homogeneous sites to satisfy the assumptions of one-dimensional transport underpinning eddy covariance measurements on single towers , attention to all sources of measurement and data processing errors in the eddy covariance system, and by accurate measurement of net radiation and every energy storage term needed to calculate available energy.