Download The role of air temperature on the skin temperature of the sea

STATE OF ISRAEL
THE MINISTRY OF NATIONAL INFRASTRUCTURES
EARTH SCIENCE RESEARCH ADMINISTRATION
‫מדינת ישראל‬
‫משרד התשתיות הלאומיות‬
‫מינהל המחקר למדעי האדמה‬
The role of air temperature on the skin
temperature of the sea surface –
The case of the Dead Sea
Nehorai R, Lensky NG, Lensky IM
December 2010
ES-35-2010
GSI/30/2010
STATE OF ISRAEL
THE MINISTRY OF NATIONAL INFRASTRUCTURES
EARTH SCIENCE RESEARCH ADMINISTRATION
‫מדינת ישראל‬
‫משרד התשתיות הלאומיות‬
‫מינהל המחקר למדעי האדמה‬
The role of air temperature on the skin
temperature of the sea surface The case of the Dead Sea
Nehorai R1,2, Lensky NG2, Lensky IM1*
1
Department of Geography and Environment, Bar-Ilan University,
Ramat-Gan 52900, Israel.
2
Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel.
*Corresponding author: Itamar Lensky, fax: +972-3-7384033, email: [email protected]
December 2010
ES-35-2010
GSI/30/2010
Table of contents:
Abstract ................................................................................................................. 1
Introduction ........................................................................................................... 2
Data and Methods .................................................................................................. 4
The diurnal and seasonal cycles of the skin effect .................................................... 5
Correlation of Ts to Tb and Ta ............................................................................ 5
Correlation of Ts to Ra and Ws ........................................................................... 6
Summary and Conclusions ..................................................................................... 7
Acknowledgments.................................................................................................. 8
References ............................................................................................................. 9
Figures ................................................................................................................ 11
Abstract
We explored the governing factors controlling the skin layer of the Dead Sea by means
of in situ measurements of water temperature, short wave and long wave radiation,
wind speed and air temperature. Continuous measurements were conducted in different
seasons reflecting different states of the Dead Sea. The skin temperature was found to
be mostly correlated to the air temperature (0.93-0.98) with no time shift. The skin
temperature is much less correlated to the bulk water temperature of the surface layer
with a significant time lag of 0.3-2 hours. An even lower correlation was found
between the skin temperature and the solar radiation and wind speed with time shifts of
2-5 hours. These findings call for reassessment of two basic concepts: Does the
satellite based SST represent bulk water temperature or air temperature? Should the
evaporation rate be calculated using the skin or the bulk temperature?
1
Introduction
The Sea Surface Temperature (SST) is a critically important parameter in the study
of ocean-atmosphere interactions. SST has a major role in atmospheric models,
weather forecasting, climate change, and energy balances. SST measured from
satellites represents a very thin boundary layer (~10 μm skin layer) between the
turbulent ocean and atmospheric layers. At this boundary layer, exchanges of sensible
and latent heat occur and long-wave radiation is emitted and absorbed [Emery et al.,
2001]. Different processes act on the skin layer and on the water body beneath it (bulk
layer), resulting in a difference between the skin and bulk temperatures.
The effects of wind, waves, and the upper layer mixing on the boundary layer have
been investigated [Barton, 2001; Donlon et al., 2002; Emery et al., 2001; Merchant et
al., 2008; Oesch et al., 2005]. These studies have shown that wind mixes the upper
layer cooling the skin layer, and that breaking waves momentarily destroy the skin
layer, which reestablishes itself within less than one second [Jessup et al., 1997]. The
difference between bulk temperature and skin temperature (ΔT or skin effect) varies
between day and night and depends on the wind speed and the heat flux between the
sea (upper 1 m) and the air [Wick et al., 1996]. The gradient between skin and bulk
temperatures is estimated using measured long wave radiation from which the skin
temperature is calculated, and measured in-situ bulk temperature [Robinson et al.,
2003; Donlon et al., 2002]. The lack of in situ measurements of skin temperature had
led to the common approach of calibration of satellite SST against measured bulk
water temperature. This forces the satellite skin SST to estimate buoy bulk SST and
ignores the physics that connect the skin and bulk SST [Emery et al., 2001]. The use of
bulk temperature as representing the skin temperature in SST algorithms is therefore
questionable [Donlon et al., 1998; Minnett, 2003].
Physical processes that control the skin effect vary along the seasonal and diurnal
cycles. Emery et al. [2001] described three mixing regimes in the water body affecting
the skin effect: Free convection, forced convection driven by wind stress, and forced
convection driven by micro-scale wave breaking. They used four different models to
represent the physics of the skin layer, and applied the models to five data sets. These
models reproduced the overall tendency of the skin effect, nevertheless most of the
variance was not explained (R2=0.28 was the highest of all models).
2
Little is known about the dynamics of the skin layer of the Dead Sea and on the
skin effect there. The Dead Sea is a hypersaline terminal lake with a reduced
evaporation rate due to the low water activity [Lensky et al., 2005], and is the warmest
large water body on Earth. Nehorai et al. [2009] characterized the Dead Sea surface
temperature using sequences of 15-minute interval satellite images and in-situ
measurements of wind speed, solar radiation, and air temperature. They found that at
night the SST over the Dead Sea is relatively uniform, whereas during daytime the
spatial variability is much larger. This observation is more pronounced during summer.
They also found that the uniformity of SST at nighttime is correlated with high wind
speed, while the high spatial variability of SST during daytime is correlated with solar
radiation and low wind speed. They concluded that horizontal uniformity of the Dead
Sea surface temperature during the nighttime is due to the strong night winds that
cause vertical mixing of the upper few meters. During the day, the skin temperature
rises due to intense solar radiation and the low intensity of winds. Calm winds locally
destroy the fragile skin layer, causing the non-uniformity in SST.
In this paper we use in situ measurements of skin, bulk and air temperatures, solar
radiation, and wind speed measured from a buoy in the Dead Sea to explore the diurnal
and seasonal cycles and the major forcing of the skin effect.
3
Data and Methods
Meteorological data including air temperature (Ta), wind speed (Ws), and solar
radiation (Ra) were collected every 20 min at a hydro-meteorological buoy located 5
km offshore of Ein-Gedi [Gertman and Hecht, 2002; Hecht and Gertman, 2003]. Bulk
temperature (Tb) was measured using temperature sensor (Solnist Levelogger) placed
at depth of 5 cm with a data recording frequency of 1 min and temperature sensitivity
of ± 0.1°C. The temperature sensors were tied to a small buoy 2 m away from the
hydro-meteorological buoy to avoid the influence of the hydro-meteorological buoy on
the thermal structure of the top 5 cm (Fig. 1). The skin temperature (Ts) was measured
using two long-wave radiometers (Kipp & Zonen, CGR4) installed on the buoy. These
radiometers are sensitive to long-wave radiation in the range of 4.2-42 μm. The
downward directed radiometer was placed at the edge of an extension arm 2 m away
from the buoy and one meter above the water surface. The second radiometer was
placed besides the meteorological instruments approximately 3 m above the water
surface, directed upward. It received the long-wave radiation emitted downward from
the atmosphere ( L ↓). The downward directed radiometer received the total long-wave
radiation flux ( L ↑ ) consisting of the radiation emitted from the sea surface and the
radiation reflected upwards from the sea surface. To calculate Ts we use the following
equation:
(1)
L ↑ − L ↓ (1 − ε ) = ε ⋅ σ ⋅ Ts 4
where ε is the water emissivity (and absorption); (1-ε) is the water reflectance, and σ is
the Stefan–Boltzmann constant.
We use cross-correlation to analyze the correlation and time lag between Ts and the
other measured quantities (Ta, Tb, Ws and Ra). Since the skin layer is very thin with a
very short thermal response time (seconds), we expect that the measured quantity that
will show the minimum time lag and the highest correlation to be the major forcing of
the skin layer.
4
The diurnal and seasonal cycles of the skin effect
The principle finding of this research is that the diurnal cycle of the skin
temperature mostly correlated to Ta (0.93-0.98) with minimal time lag (0-0.3 hr) in all
seasons. Ts is much less correlated to Tb and has a larger time lag, and Ts even less
correlated to Ws and Ra with en even higher time lag. Figure 2 presents the time series
of all measured quantities of four representative days in winter, summer and autumn.
Figures 3 presents scatter diagrams of Ts vs. Ta, Tb, Ws and Ra in the three seasons,
and Figures 4 and 5 presents the cross correlation of these pairs.
Correlation of Ts to Tb and Ta
During the winter the Dead Sea is fully mixed and the water temperature is almost
uniform throughout the entire water column (300 m) with Tb diurnal amplitude of
approximately ±0.2°C (Figs. 2a and 3a, 3b). Tb is higher than Ta by ~3°C, leading to
continuous cooling of the whole sea through the skin at a rate of ~0.02°C/day. The
amplitude of the diurnal cycle of Ts and Ta is 2-4°C which is much larger than that of
Tb. The skin layer is cooler and saltier and thus is unstable which drives the winter
convection and cooling. Accordingly, Ts and Tb indicate no correlation in winter (0.17), whereas the correlation between Ta and Ts is much higher (0.97) practically no
time lag (presented in Figures 4a and 5).
In the summer the Dead Sea is stratified with an upper mixed layer above a
thermocline at depth of 20-30 m [Gertman and Hecht, 2002]. As in the winter, Ts is
mostly correlated to Ta (0.93) with no significant time lag (Figs. 4b, 5). The
correlation between Tb and Ts is highest in the summer (0.79) with a time lag of an
hour, and higher than the correlation of Ts-Ra and Ts-Ws (Figs. 4 and 5). The diurnal
amplitude of Tb is smaller than Ta and Ts (±1°C and ±2-3°C, respectively, Figs. 2b),
but still significantly higher than in the winter, representing stronger coupling between
the sea and the atmosphere in the stratified period.
In the autumn the Dead Sea is still stratified, but the stability of the upper layer
decreases together with the decrease of water temperature and its reduced diurnal
amplitude. Ts is highly correlated to Ta (0.98) with no time lag, and less correlated to
Tb (0.68, Figs. 4c, 5). The correlation of Tb to Ts is higher in the summer than in
autumn (Figure 4b, 4c), probably due to the weaker incoming solar radiation (Fig. 2e,
5
2f) which results in the reduced diurnal amplitude of Tb in autumn. The bulk surface
water (Tb) cools in this season at a rate of 0.18°C/day, 10 times faster than in winter.
The reason for the faster cooling of the bulk water in the upper one meter in autumn
relative to winter, is that in the winter the cooling from the skin removes heat from the
entire water column (300 m) while in the autumn cooling is limited to the upper mixed
water layer (20-30 m).
Correlation of Ts to Ra and Ws
Low correlation and significant time lags were found between the Ts and the
atmospheric forcing (Ws and Ra) in all seasons (Figs. 3, 4, 5). Ra and Ts both show a
diurnal cycle with high values at daytime and low values at nighttime (Figs. 2d, 2e,
2f). However there is a lag of 4-5 hours in which the Ra precedes Ts. In the scatter
diagram it is seen as a counterclockwise cycle, as is best demonstrated in Figures 3h
and 2e. From sunrise (no solar radiation and Ts = 33oC), the solar radiation increases
to ~1100 Wm-2 with very minor change in Ts, then at noon Ts increases rapidly to
about 37oC, and gradually decreases to 36oC at sunset, falling back to 33oC at
nighttime.
Ws show low correlation to Ts (0 to -0.71) with even less ordered diurnal cycle
than Ra. The correlation of Ts-Ws is negative with time lag of 2-3.3 hours. Negative
shifts in Ts-Ws occur while Ts-Ta is positive due to strong dry winds in the night time,
cooling the skin layer by increased evaporation.
6
Summary and Conclusions
The diurnal cycle forces the skin layer system via the solar radiation and winds
[Barton, 2001; Donlon et al., 2002; Wick et al., 1996]. They also drive the spatial
variance of the Dead Sea SST measured from satellites [Nehorai et al., 2009]. Here we
showed that the skin temperature is mostly correlated to the air temperature in all
seasons with practically no time lag. High air and skin temperatures in summer
evenings decrease from the peak temperature few hours after sunset and after the bulk
temperature has decreased (Figs. 2b and 2e). The warm air that does allow the evening
chilling a few hours after sunset derives from the Mediterranean breeze (Fig. 2e), that
adiabatically heats while descending from the Judean mountains to the Dead Sea. The
high temperature of the skin layer at nighttime has no other explanation other than
being affected by air temperature, suggesting that the skin temperature is affected by
air temperature. In addition the skin layer (~10μm) is not expected to affect the air
temperature measured by thermometers 3 m above the sea surface.
In the winter when the whole water column of the Dead Sea is mixed the bulk
water temperature is almost fixed whereas the air temperature has a significant diurnal
cycle, and the skin and air temperature are very similar (~4°C), thus the skin and bulk
layers show no correlation. In the summer, the diurnal cycle of the bulk surface water
temperature is similar to the diurnal cycle of the air temperature, only with smaller
bulk temperature diurnal amplitude relative to the air temperature (2°C vs. 4°C,
respectively). Accordingly, the correlation between skin temperature and bulk
temperature is much stronger than in the winter. Bussières and Granger [2007] also
found strong signal–coupling between bulk and skin temperatures in the summer in
lakes in cold climate regions.
The skin layer of the Dead Sea can be classified into two mixing regimes: (i)
Unstable conditions in winter when the skin temperature is controlled by free
convection (where Ts<Tb). As was shown previously, the spatial variations of SST are
low in such conditions [Nehorai et al., 2009]. In free convection conditions the skin
layer temperature is less affected by wind since it is unstable and it continuously sinks
and rebuilds. The strong correlation between air temperature and skin temperature
suggests that the skin temperature is directly affected by the air temperature. To some
extent this is also the case in summer nights, when the night cooling takes place. (ii)
Stable conditions in summer daytime when the skin is affected by solar radiation. The
7
stable structure of the upper water layer is very sensitive to wind gusts that cause
significant spatial variations [Nehorai et al., 2009].
SST measured from satellites is an important geo-physical parameter in the
research of ocean - atmosphere interactions and ocean circulation. Nevertheless, the
understanding of the factors controlling SST is still limited [Emery et al., 2001]. The
strong correlation between skin and air temperature as well as uncorrelated bulk-skin
temperatures calls for reassessment of the interpretation of satellite based SST. Does
SST represent water temperature or air temperature? Another question these findings
raise is how to calculate the evaporation rate using either the skin temperature or the
bulk temperature? The bulk formulas for calculating evaporation take into account the
bulk temperature, despite the fact that evaporation occurs at the skin layer.
Acknowledgments
We thank Isaac Gertman for supplying the in situ measurements from the buoy, which
enabled this research, and for his critical reading. We thank Raanan Bodzin for critical
reading and helpful discussions and Ittai Gavrieli, Vladimir Lyakhovsky and Gerald
Stanhill for fruitful discussions. We also thank Uri Malik and Shabtai Cohen for
helping with the installation and calibration of the instruments, and the late Moti
Gonen, Silvy Gonen and the team "Taglit" for cruise services. We thank Tal Ozer,
Boris Katsanelson for assistance in the field. The research was supported by the Earth
Science Research Administration, the Ministry of National Infrastructures (Israel).
8
References
Barton, I. J. (2001), Interpretation of satellite-derived sea surface temperatures, Adv.
Space Res., 28(1), 165-170, doi: 10.1016/S0273-1177(01)00337-4.
Bussières, N., and R. J. Granger (2007), Estimation of Water Temperature of Large
Lakes in Cold Climate Regions during the Period of Strong Coupling between
Water and Air Temperature Fluctuations, J. Atmos. Ocean. Tech., 24(2), 285296, doi: 10.1175/JTECH1973.1.
Donlon, C. J., S. J. Keogh, D. J. Baldwin, I. S. Robinson, I. Ridley, T. Sheasby, I. J.
Barton, E. F. Bradley, T. J. Nightingale, and W. Emery (1998), Solid-State
Radiometer Measurements of Sea Surface Skin Temperature, J. Atmos. Ocean.
Tech., 15(3), 775-787. doi: 10.1175/15200426(1998)015<0775:SSRMOS>2.0.CO;2
Donlon, C. J., P. J. Minnett, C. Gentemann, T. J. Nightingale, I. J. Barton, B. Ward,
and M. J. Murray (2002), Toward Improved Validation of Satellite Sea Surface
Skin Temperature Measurements for Climate Research, J. Climate, 15(4), 353369.
Emery, W. J., S. Castro, G. A. Wick, P. Schluessel, and C. Donlon (2001), Estimating
Sea Surface Temperature from Infrared Satellite and In Situ Temperature Data,
B. Am. Meteorol. Soc., 82(12), 2773-2785.
Gertman, I., and A. Hecht (2002), The Dead Sea hydrography from 1992 to 2000, J.
Marine. Syst., 35(3-4), 169-181, doi: 10.1016/S0924-7963(02)00079-9.
Hecht, A., and I. Gertman (2003), Dead Sea meteorological climate, in Fungal Life in
the Dead Sea, edited by E. Nevo, A. Oren, and S. P. Wasser, pp. 68– 114,
A.R.G. Ganter, Ruggell, Lichtenstein.
Jessup, A. T., C. J. Zappa, M. R. Loewen, and V. Hesany (1997), Infrared remote
sensing of breaking waves, Nature, 385(6611), 52-55, doi: 10.1038/385052a0.
Lensky, N. G., Y. Dvorkin, V. Lyakhovsky, I. Gertman, and I. Gavrieli (2005), Water,
salt, and energy balances of the Dead Sea, Water Resour. Res., 41(12),
W12418. doi:10.1029/2005WR004084.
Merchant, C. J., M. J. Filipiak, P. Le Borgne, H. Roquet, E. Autret, J. Pioll , and S.
Lavender (2008), Diurnal warm-layer events in the western Mediterranean and
European shelf seas, Geophys. Res. Lett., 35(4), L04601,
doi:10.1029/2007GL033071.
Minnett, P. J. (2003), Radiometric measurements of the sea-surface skin temperature:
the competing roles of the diurnal thermocline and the cool skin, Int. J. Remote
Sens., 24(24), 5033-5047, doi: 10.1080/0143116031000095880.
Nehorai, R., I. M. Lensky, N. G. Lensky, and S. Shiff (2009), Remote sensing of the
9
Dead Sea surface temperature, J. Geophys. Res., 114(C5), C05021, doi:
10.1029/2008JC005196.
Oesch, D. C., J. Jaquet, A. Hauser, and S. Wunderle (2005), Lake surface water
temperature retrieval using advanced very high resolution radiometer and
Moderate Resolution Imaging Spectroradiometer data: Validation and
feasibility study, J. Geophys. Res., 110(C12), C12014,
doi:10.1029/2004JC002857.
Robinson, I. S., and C. J. Donlon (2003), Global Measurement of Sea Surface
Temperature from Space: Some New Perspectives, J. Global Atmos. Ocean
Syst., 9(1), 19-37, doi: 10.1080/1023673031000080385.
Wick, G. A., W. J. Emery, L. H. Kantha, and P. Schlüssel (1996), The Behavior of the
Bulk – Skin Sea Surface Temperature Difference under Varying Wind Speed
and Heat Flux, J. Phys. Oceanogr., 26(10), 1969-1988.
10
Figure 1: Schematic diagram of the hydro-meteorological buoy with the relevant
instruments. The measurements used here include: Ws– wind speed; Ra– short wave
radiation; Ta– air temperature; Tb– bulk temperature; L– long wave radiation.
11
28
26
24
Autumn
30
28
Tb
Ta
Ts
12
10
8
WS
SR
6
4
2
0
29-Jun-09
(f)
2-Nov-09
(e)
28-Jun-09
30
10
1-Nov-09
Summer
27-Jun-09
Figure 2: Time series of bulk (Tb), skin (Ts), air temperatures (Ta), wind speed (Ws)
and solar radiation (Ra) during four representative days in: (a), (d) winter, when the
water column is homogeneous; and (b), (e) and (c), (f) summer and autumn, when the
water column is stratified.
13-Jan-09
12-Jan-09
11-Jan-09
16
10-Jan-09
18
8
31-Oct-09
32
10
9-Jan-09
8-Jan-09
20
26-Jun-09
25-Jun-09
Tb
Ta
Ts
Wind speed (m/s)
Solar radiation (W/m2 )/100
Winter
30-Oct-09
(c)
29-Oct-09
34
Wind speed (m/s)
Solar radiation (W/m2 )/100
36
Wind speed (m/s)
Solar radiation (W/m2)/100
(b)
13-Jan-09
12-Jan-09
11-Jan-09
10-Jan-09
22
29-Jun-09
28-Jun-09
27-Jun-09
9-Jan-09
8-Jan-09
T (°C)
(a)
2-Nov-09
1-Nov-09
31-Oct-09
32
26-Jun-09
25-Jun-09
T (°C)
38
30-Oct-09
29-Oct-09
T (°C)
24
(d)
WS
SR
6
4
2
0
8
6
4
2
0
Figure 3: Correlations between surface, bulk water temperature, air temperature, wind
speed and solar radiation in: (a, b, c, d) – winter (22/12/2008 – 14/1/2009); (e, f, g, h) –
summer (25-29/6/2009); and (i, j, k, l) – autumn (27/10-10/11/2009). Note the high
correlation of skin and air temperatures and the much lower correlation between skin
and bulk temperatures.
13
Ts-Ta
Correlation
1
(a) Winter
Ts-Tb
0.8
Ts-Ws
0.6
Ts-Ra
0.4
0.2
0
-12
-10
-8
-6
-4
-2
-0.2
0
2
4
6
8
10
12
Shift (hours)
-0.4
-0.6
-0.8
-1
Ts-Ta
Correlation
1
(b) Summer
Ts-Tb
0.8
Ts-Ws
0.6
Ts-Ra
0.4
0.2
-12
-10
-8
-6
-4
0
-2 -0.2 0
2
4
6
8
10
12
Shift (hours)
-0.4
-0.6
-0.8
-1
(c) Autumn
Correlation
Ts-Ta
1
Ts-Tb
Ts-Ws
0.8
Ts-Ra
0.6
0.4
0.2
-12
-10
-8
-6
-4
0
-2 -0.2 0
-0.4
2
4
6
8
10
Shift (hours)
-0.6
-0.8
-1
Figure 4: Cross correlation between the skin temperature (Ts) and the following
measured quantities in winter, summer and autumn: air temperature (Ta), bulk water
temperature (Tb), wind speed (Ws) and solar radiation (Ra).
14
12
1
S
A W
S
0.8
S
Ta
Tb
Ws
Ra
A
0.6
W
Correlation
0.4
A
0.2
0
-1
0
1
2
3
4
5
Shift (hrs)
W
-0.2
W
A
-0.4
-0.6
S
Figure 5: The maximum correlations from Figure 4 with the corresponding time lags.
The three seasons are denoted by: S-summer, W-winter, A-autumn.
15
PUBLICATION DOCUMENTATION PAGE
1. Publication No.
2.
3. Recipient Accession No.
ES-35-2010
4.Title and Subtitle
5. Publication Date
Nov. 2010
The role of air temperature on the skin
temperature of the sea surface –
the case of the Dead Sea
6. Performing Organiz. Code
7. Author (s)
Nehorai R, Lensky NG, Lensky IM
8. Performing Organiz. Rep. No.
9. Performing Organization Name and
Address
10. Project/ Task / Work Unit No.
GSI/30/2010
Geological Survey of Israel
30 Malkhei Israel St., Jerusalem 95501
11. Contract No.
29-17-008
12.Sponsoring Organization (s) Name and
Address
13. Type of report and period covered
The Ministry of National Infrastructures
P.O.B. Box 13106, 91130 Jerusalem
14. Sponsoring Organiz. Code
GSI/30/2010
15. Supplementary Notes
16. Abstract (Limit 200 Words)
We investigated the skin temperature of the Dead Sea by means of thermal IR remote sensing,
in situ measurements of water temperature, short wave and long wave radiation, wind speed
and air temperature. Strong correlation were found between air and skin temperatures in all
seasons. The skin temperature is much less correlated to the upper 1m bulk water temperature,
and even less correlated to the solar radiation and wind speed. In winter no correlation was
found between the skin and bulk temperatures whereas in the summer the correlation is
significantly higher, this is due to the seasonal layering. These findings call for reassessment
of two basic concepts: Does the satellite based SST represent bulk water temperature or air
temperature? Should the evaporation rate be calculated using the skin or the bulk temperature?
17. Keywords
Dead Sea, SST, air temperature, long wave radiation, skin effect
18. Aviability Statement
19. Security Class
Geological Survey of Israel
20. Security Class
21. No. of Pages
22. Price