Characteristics of summer-time energy exchange in a high Arctic tundra heath 2000–2010

Figures & data

Fig. 1 Map of the study area indicating the location of eddy covariance (EC) mast (Universal Transverse Mercator (UTM) zone 27, World Geodetic System (WGS) 84). White contour lines indicate 100 m elevation intervals. Inserted map shows the location of Zackenberg in Northeast Greenland (74.47°N, 20.55°W). The average footprint of the EC system was ca. 200 m long, directed towards southeast, and dominated by Cassiope heath plant community type (Lund et al., 2012).

Fig. 2 Mean July air temperature (T _a ) 1996–2010 (a) and maximum active layer depth (Max. ALD) 1997–2010 (b) in Zackenberg, NE Greenland.

Table 1. Mid-summer period surface energy balance and environmental characteristics during the study period 2000–2010

	Year	2000	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010	Mean	St. dev.	Change^a
Mid-summer	Start	167	175	171	166	167	161	179	161	174	154	166	167	7	−0.7
	End	237	250	247	253	250	246	236	241	254	240	236	245	7	−0.6
	Length	71	76	77	88	84	86	58	81	81	87	71	78	9	0.1
Diurnal	T a	4.8	5.1	4.9	6.0	5.8	5.1	6.3	5.7	7.1	4.9	5.4	5.6	0.7	0.08
	T 0	8.1	7.4	7.7	8.3	8.3	9.2	9.7	8.9	7.2	7.4	7.9	8.2	0.8	0.01
	T 2.5	6.1	5.9	6.2	6.7	7.0	6.9	7.0	6.9	6.3	6.1	6.3	6.5	0.4	0.02
	SW in	215	206	184	191	195	198	215	225	184	210	233	205	16	1.8
	θ v	0.36	0.36	0.34	0.37	0.36	0.36	0.33	0.30	0.30	0.29	0.27	0.33	0.04	−0.010**
	Precip	13.7	26.8	26.1	9.9	21.3	33.1	13.9	21.9	60.0	59.4	13.3	27.2	17.5	2.2
	Max. ALD	63.4	63.3	70.5	72.5	76.3	79.4	76.0	74.8	79.4	79.4	78.2	73.9	6.0	1.54**
	Max. SD	0.48	0.68	1.33	0.60	0.69	0.73	1.09	0.57	1.30	0.16	0.73	0.76	0.35	−0.005
Midday^b	H	151	139	130	125	135	140	142	159	126	129	151	139	11	0.2
	LE	52	42	44	48	47	52	58	54	50	46	56	50	5	0.7
	G	83	84	66	58	61	41	58	49	29	27	24	53	21	−5.9**
	R n	318	307	250	235	242	n/a	268	277	225	231	266	262	32	−4.8
	H/R n	0.49	0.45	0.49	0.56	0.55	n/a	0.52	0.57	0.52	0.56	0.58	0.53	0.04	0.009*
	LE/R n	0.16	0.16	0.19	0.23	0.22	n/a	0.22	0.20	0.21	0.23	0.23	0.20	0.03	0.006*
	G/R n	0.27	0.29	0.29	0.28	0.25	n/a	0.23	0.20	0.14	0.13	0.10	0.22	0.07	−0.019**
	B	3.7	3.6	3.0	2.8	3.2	2.9	2.6	3.1	2.7	2.9	2.8	3.0	0.4	−0.071*
	ra	90	101	89	85	84	87	96	89	95	94	94	91	5	0.3
	rs	414	449	276	343	350	306	313	309	373	323	309	342	52	−7.6
	Ω	0.37	0.34	0.41	0.37	0.35	0.39	0.40	0.41	0.37	0.44	0.41	0.39	0.03	0.005*
	A	0.50	0.50	0.62	0.55	0.51	0.52	0.57	0.57	0.56	0.58	0.56	0.55	0.04	0.005
	Da	334	317	227	351	373	331	394	370	391	283	333	337	49	3.7
	Ds	864	771	694	753	826	803	826	1012	643	569	637	763	124	−14.3

Mid-summer period is defined as the period with daily average T_a and T₀ above 0°C, positive SW_in and R_n, and albedo between 10 and 20%.

*Linear change (F statistic in linear regression) is significant at the α=0.05 level; **α=0.001 level.

^aSlope of linear regression with years as independent variable.

^bMean midday (11:00 – 15:00 local time) values during mid-summer period.

Fig. 3 Environmental characteristics and surface energy dynamics during average year mid-summer period. (a) net radiation (R_n); (b) air temperature (T_a); (c) ratio of sensible heat (H) to R_n; (d) soil volumetric water content (θ_v); (e) ratio of latent heat (LE) to R_n; (f) surface resistance (r_s); (g) ratio of ground heat flux (G) to R_n; (h) Priestley-Taylor α coefficient; (i) Bowen ratio (β); and (j) McNaughton & Jarvis Ω value. Black dots represent means and error bars standard error. Mid-summer period is defined as the period with daily average T_a and T ₀ above 0°C, positive SW_in and R_n, and albedo between 10 and 20%. Mean start and end dates during the study period (2000–2010) were 16 June and 1 September, respectively.

Fig. 4 Relationship between within-year variations in Bowen ratio (β) and environmental characteristics during 2003. (a) net radiation (R_n); (b) atmospheric vapour pressure deficit (D_a); and (c) surface saturation vapour pressure deficit (D_s). Observations of β was averaged within bins of 50 W m⁻² (R_n) and 100 Pa (D_a, D_s), respectively. Black dots represent means and error bars standard deviation.

Fig. 5 Relationship between surface saturation vapour pressure deficit (D_s) and environmental characteristics during 2007. (a) McNaughton & Jarvis Ω value; (b) Priestley-Taylor α coefficient; and (c) surface resistance (r_s). Observations of Ω, α and r_s was averaged within D_s bins of 100 Pa. Black dots represent means and error bars standard deviation.

Table 2. Average midday, mid-summer meteorological conditions and surface energy partitioning characteristics during 2002 (means±standard deviation)

	Offshore winds	Onshore winds	p ^a
Ta	7.6±2.3	6.5±3.2	0.003
Da	311±204	204±190	<0.001
H/Rn	0.33±0.32	0.54±0.15	<0.001
LE/Rn	0.30±0.12	0.17±0.08	<0.001
G/Rn	0.35±0.11	0.27±0.08	<0.001
β	0.72±1.29	3.51±1.42	<0.001
α	1.20±0.58	0.49±0.19	<0.001
ra	71±29	90±31	<0.001
rs	161±114	301±158	<0.001

Offshore and onshore winds are defined as coming from 225–45° and 45–225° (relative to north), respectively. Mid-summer period is defined as the period with daily average T_a and T ₀ above 0°C, positive SW_in and R_n, and albedo between 10 and 20%.

^a p values depict the probability that means are equal (two-sample t-test).

Fig. 6 Changes in mid-summer surface energy balance partitioning between 2000 and 2010. Lines indicate significant change in data. Error bars for G/R_n 2008–2010 indicates uncertainty (St. dev.) in extrapolating ɛ [eq. (4)]. 1-(H/R_n+LE/R_n) is a residual term in the energy balance serving as an independent proxy for G. Mid-summer period is defined as the period with daily average T_a and T ₀ above 0°C, positive SW_in and R_n, and albedo between 10 and 20%.

Table

Name	Unit	Explanation
Ta	°C	Air temperature
T 0	°C	Surface temperature
T 2.5	°C	Soil temperature at 2.5 cm depth
Da	Pa	Atmospheric vapour pressure deficit
Ds	Pa	Surface saturation vapour pressure deficit [eq. (9)]
Rn	W m^−2	Net radiation
SWin	W m^−2	Incoming short-wave radiation
Precip	mm	Precipitation
θv	m^3 m^−3	Volumetric soil water content
ALD	cm	Active layer depth
SD	m	Snow depth
H	W m^−2	Sensible heat flux
LE	W m^−2	Latent heat flux
β	–	Bowen ratio (H/LE)
G meas	W m^−2	Measured soil heat flux
G meas, sur	W m^−2	Measured soil heat flux plus storage term [eq. (1)]
G	W m^−2	Modelled soil heat flux [eq. (4)]
rs	s m^−1	Bulk surface resistance [eq. (5)]
ra	s m^−1	Aerodynamic resistance [eq. (6)]
Ω	–	McNaughton & Jarvis Ω value [eq. (7)]
A	–	Priestley-Taylor coefficient α [eq. (8)]

Lund M. , Falk J. M. , Friborg T. , Mbufong H. N. , Sigsgaard C. , co-authors . Trends in CO2 exchange in a high Arctic tundra heath, 2000–2010. J. Geophys. Res. Biogeosci. 2012; 117: 02001.

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