Integrated Applications - Transport in the Atmosphere-Vegetation-Soil Continuum - page 295

different land use types. To understand those differences, the response of the energy
partitioning in the energy balance of grass and forest will be compared for two dif-
ferent conditions: normal summer days are compared to exceptionally hot days as
occurring during heat waves. Heat waves are deined here as a period of at least
5 days in which the maximum temperature exceeds the climatological value for that
date by at least 5 K (Frich et al., 2002 ).
The rationale for this analysis is twofold. First, it is intended to show the mech-
anism behind the difference in response to changing atmospheric conditions
between grassland and forest. Second, the differences in partitioning may inlu-
ence the intensity of the heat wave itself.
The data set as well as part of the analysis in this section is based on Teuling et al.
( 2010 ).

8.4.1 Data
To study the actual response of different land use types, direct lux observations are
needed. Teuling et al. ( 2010 ) provide a synthesis of lux observations from 30 stations
(grass or forest) in western Europe, covering the summer months (June-August) in
the period 1997-2006 (data sets per station varying from 2 to 10 years). The data have
been obtained from the FLUXNET data set (Baldocchi et al., 2001 ; Baldocchi, 2008 ).
The data encompass not only the turbulent surface luxes of heat and water vapour,
but net radiation and soil heat lux as well.
To study the response of the surface energy balance, the climatological energy bal-
ance is constructed for both land use types separately: the median luxes for all grass/
forest stations, for the time period 9-13 UTC from days without heat wave condi-
tions. Subsequently, the median anomaly in the energy balance terms is determined
for the 2003 and 2006 heat waves in western Europe. Here the median observations
will be used as if they were representative observations of two single composite sta-
tions: one grass station and one forest station.
To analyse further the normal and heat wave behaviour of the energy balance, we a
need to derive a number of variables that have not been directly observed: air temper-
ature and canopy resistance. We start from the resistance expressions for the sensible
as used in the derivation of the Penman-Monteith equation in Chapter 7 :

TT
r

−

a s

H =− ρ p c

(8.3)

a

Assuming typical values for the aerodynamic resistance for grass and forest (40 and
10 s m -1 , respectively), and deriving the surface temperature from the upwelling long-
wave radiation, the air temperature can be determined using Eq. ( 8.3 ).