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Research Article
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Evaluating Past Surface Air Temperature Change in Tabriz, Iran Using Hourly-Based Analyzing
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M. Gholipoor
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ABSTRACT
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Past temperature (T) change has been
extensively studied as evaluating increase/decrease in maximum (Tmax)
and minimum (Tmin) Ts. This study aimed to evaluate trends
of T, using hourly T data and comparing them with trends of Tmax
and Tmin. Data set was years 1966-2004, for Tabriz, Iran and
contained daily values for Tmax and Tmin. Firstly,
hourly Ts were estimated and averaged over month, over growing period
of 5 crops and over yearly period. Also, like hourly T, averaged values
of Tmax, Tmin and mean T were calculated. Then,
simple linear regression model (Y = a+bX) was used to determine rate of
change (b) in temperature. Results indicated that among months for which
warming was significant, Jan and July tended to have warming only for
20 to 8 h am and 23 to 5 h am, respectively; Feb and Oct with faster warming
rate at daytime h had highest warming rate for 13-15 h and 12-16 h, respectively;
the difference for lowest warming rate was also sensible; despite of these
two months, June and September with more prominent warming at daytime
hours showed no difference for time of occurrence of highest and for that
of lowest warming rate; in August, the negligibly faster warming was related
to 20 to 8 h am. Warming rate was relatively more prominent across nighttime
h for growing period of winter wheat, maize and for yearly period, but
across daytime hours for chickpea and spring barley; for lentil it was
negligible. Comparing these results with trends of Tmax and
Tmin it is revealed that when trends of Tmax and
Tmin are asymmetric, as it is most evident across the world,
hourly-based analyzing provides more detailed information and increased
insights; such information could be more useful for selecting appropriate
climate-change-related managing/breeding programs for plants.
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INTRODUCTION
In recent years, the climate change phenomena, especially
change of surface air temperature, have been extensively studied. Generally,
in the last century, the global temperature has increased 0.7 °C and
1990s were the warmest decade on record (Rosenzweig et al., 2000).
Although there is more evidence for warming in different regions (Tao
et al., 2003), some reports suggesting that surface air temperature
has had decreasing trend, which is related to quite inhomogeneous in various
respects. For instance, Skinner and Gullett (1993) indicated that in Atlantic,
Canada, autumn and winter temperature (Tmax and Tmin)
has diminished.
Besides the spatial and seasonal variability of the
temperature trend, reports suggesting that globally averaged minimum temperatures
(Tmin) continue to increase at a faster rate than the maximum
temperatures (Tmax), resulting in a narrowing of the diurnal
temperature range (Karl et al., 1993; Razuvayev et al.,
1997; Vose et al., 2005). In Beiging, China it has been shown that
the linear rate of increase in Tmin is 4.08 °C/100 year;
whereas the Tmax decreases with a linear rate of 2.45 °C/100
year (Karl et al., 1993; Xie and Cao, 1996). Other reports for
China indicating that in north regions, increasing trend of Tmin
for period 1951-1990 has been more sensible, when compared to Tmax
(Tao et al., 2003). Analyzing daily and monthly maximum and minimum
surface air temperatures at 66 weather stations over the eastern and central
Tibetan Plateau for temporal trends also confirmed the asymmetric pattern
of greater warming trends in Tmin or nighttime temperatures
as compared to the daytime temperatures (Liu et al., 2006). In
central Europe, despite of low-lying stations for which the above named
behavior has been found, data from mountain top stations show a similar
increase for both Tmin and Tmax of daily temperatures
(Weber et al., 1994). In Iran, results regarding Kermanshah showed
that except for January, February and March, the increase in Tmin
is considerable for all other months, especially June (Gholipoor et
al., 2006); whereas Tmax appears to show increasing trend
only for April, May, June and August; in other regions, like Gorgan, Tmin
appears to be changed only in May; on the other hand, Tmax
shows no statistically change in all months (Ghorbani and Soltani, 2002).
Asymmetric change has been also reported for many other regions/countries,
including southeastern Europe (Brázdil et al., 1996), United
States and Canada (Karl et al., 1984) and Italy (Brunetti et
al., 2000).
As mentioned, the change of surface air temperature has
been extensively studied as Tmax and Tmin based
evaluation. It seems that such evaluations provide no detailed information
about temperature change, when pattern of diurnal temperature change is
asymmetric. For example, when temperature change is significant only for
Tmin, Tmax Tmin based results provide
no information about the number of nighttime h with temperature change.
Such detailed information are needed for decision making and managing,
especially plants in which growth and development processes are highly
dependent on temperature of day and night time moments. This study was
aimed to evaluate climate-change-resulted temperature increase/decrease
in Tabriz, Iran, using hourly-based-analyzing and comparing them with
trends of Tmax and Tmin.
MATERIALS AND METHODS
The historical weather data for Tabriz (38 ° 50`
N, 46 °7` E and 1361 m asl), Iran, was used for this evaluation. Data
set was years 1966-2004 and contained daily values for sunshine hours,
Tmax, Tmin and rainfall. The values of Tmax
and Tmin were used for calculating hourly temperature, using
the model of Cesaraccio et al. (2001). Such estimations have been
extensively used by many researchers for different goals (Fernandez, 1992;
Floyd and Braddock, 1984; Goudriaan and Waggoner, 1972; Johnson and Fitzpatrick,
1977; Jones et al., 2003; Keating et al., 2003; Kline et
al., 1982; Lemon et al., 1971; Parton and Logan, 1981; Snyder
et al., 1999; Wilkerson et al., 1983; Worner, 1988). This
is due to this fact that hourly data records are currently available for
only a limited number of stations because of instrumentation constraints
in rural agriculture areas and memory limitations of computers sorting
hourly (or even more frequent) temperature observations.
The planting date for spring (chickpea, lentil, barley
and maize) and winter (wheat) crops were first determined; the 7 day with
no rainfall and with mean temperature above the base temperature were
considered as planting date for spring crops, using long-term weather
data; planting date for winter wheat calculated based on required Growing
Degree Days (GDD) for rosette growth before occurrence of growth cession
which is imposed by winter freezing; growing period for each crop was
calculated on the basis of required GDD from planting to maturity. A simple
Qbasic program was used for averaging hourly temperature over month, over
growing period of crops and over yearly period; like hourly temperature,
the averaged values of Tmax, Tmin and mean temperature
were also calculated.
If the changing trend of temperature, for example, to
be increasing across first 20 years, but decreasing across last 19 years,
it may be described by quadratic model and should be considered as periodic
fluctuation in temperature, rather than consistent change, which is described
by linear model. Therefore, it was used only a simple linear regression
model Y = a+bX to determine rate of increase/decrease (b) in temperature;
in this equation, Y is dependent variable (temperature), X year, a intercept
and b slope of regression line; the value of parameters (a and b) was
calculated, using the procedure REG in SAS (SAS, 1989).
RESULTS AND DISCUSSION
The rate of change (value of b) for temperature of 1
to 5 h am was presented in Table 1. Warming was statistically
considerable for January, February, June, July, August, September and
October, but negligible for other months. The rate of warming was the
same across these h for August (0.046 °C year-1), September
(0.051-0.052) and October (0.058-0.059), but ranged from 0.092 to 0.096
for January, 0.082 to 0.085 for February, 0.058 to 0.060 for June and
0.039 to 0.042 for July. Based on these values, the highest and 2nd highest
warming rate has been happened for January and February, respectively;
the lowest warming has been done for July. It was equal to 0.029-0.030
for barley, 0.036 for chickpea, 0.037 for lentil, 0.040-0.041 for maize,
0.036-0.037 for wheat and 0.049-0.050 for yearly period.
The values of b for 6-10 h presented (Table
2). Based on values of Table 2 and 1,
it can be said that the warming tended to be statistically negligible
for 1-10 h in March, April, May, November and December, for 6-10 h in
July and for 9-10 h in January. Results regarding 6-10 h indicate that
with hour incrementing, the rate of warming showed decreasing trend for
January (only across 6-8 h for which the warming was significant), June,
September, but increasing trend for October and February; in August, it
was nearly constant (0.045-0.046 °C year-1). Averaged over
growing periods and over yearly period, these h had statistically significant
warming; the rate of warming showed directed-change across named h for
barley, but inversed-change for maize, wheat, yearly period and nearly
no change for chickpea and lentil; like 1-5 h, the highest warming rate
was found for yearly period.
Warming for 11-15 h was significant only in February,
June, August, September and October (Table 3). In contrast
to 1-10 h , warming rate was higher for 11-15 h in February [0.097 °C
year-1 (for 11 h) to 0.099 (13-15), versus, 0.083 (1) to 0.095
(10)] and October [0.064 (11) to 0.065 (12-15),
Table 1: |
The rate of change (value of b in equation Y = a+bX)
and its probability level (P) for averaged hourly temperature (1 to
5 am) over month, over growing period of 5 crops and over year ( °C
year-1) during past 39 years for Tabriz, Iran |
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The bold values are statistically significant |
Table 2: |
The rate of change (value of b in equation Y = a+bX)
and its probability level (P) for averaged hourly temperature (6 to
10 am) over month, over growing period of 5 crops and over year (
°C year-1) during past 39 years for Tabriz, Iran |
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The bold values are statistically significant |
Table 3: |
The rate of change (value of b in equation Y = a+bX)
and its probability level (P) for averaged hourly temperature (11
to 15 ) over month, over growing period of 5 crops and over year (
°C year-1) during past 39 years for Tabriz, Iran |
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The bold values are statistically significant |
versus, 0.058 (1-3) to 0.064 (10)], but lower in June
[0.050 (11) to 0.048 (13-15), versus, 0.060 (1-3) to 0.051 (10)] and September
[0.042 (11-12) to 0.041 (13-15), versus, 0.052 (1-3) to 0.044 (10)]; on
the other hand, it was nearly constant (0.045-0.046) across these h for
August. Considering these values and those of other months, it becomes
clear that in first h of morning, as mentioned previously, the highest
and 2nd highest warming rates have happened for January and February,
respectively, but due to difference in trends of warming rate, February
possessed the highest values after 6 am and that of January statistically
equaled to zero after 8 am; across 1-10, warming rate appeared to show
relatively rapid directed-change for February and October, but inversed-change
for June and September; with incrementing from 11 to 16, it had relatively
slow upwardly change for February, but downwardly change for June; the
change of warming rate across these h was nearly negligible for October
and September. Compared to 1-10 h, warming rate was considerably higher
for 11-15 h in growing period of barley and sensibly in that of chickpea,
but lower in that of maize, wheat and in yearly period; it was nearly
the same in growing period of lentil.
The values of b for 16-20 h, which were positive, appeared
to be statistically considerable only in February, June, August, September
and October (Table 4); in January, the warming rate
was significant only for 20 h; with incrementing h from 16 to 20, warming
rate showed downwardly trend for February and October, but upwardly trend
for June and September; it was nearly constant across these hours for
August. Despite of chickpea and lentil, for which the warming rate indicated
no change across 16-20, it displayed nearly directed-change for maize,
wheat and for yearly period, but inversed-change for barley.
The warming was significant for January (it ranged from
0.085 °C year-1, for 21 h, to 0.094 for 24 h), February
(0.089 to 0.083), June (0.056 to 0.059), August (0.046 to 0.046), September
(0.048 to 0.051), October (0.061 to 0.059) and July (0.039, for 23, to
h 0.041 for 24 h) (Table 5). It was also significant
for barley (0.033 to 0.030), chickpea (0.037 to 0.036), lentil (0.037
to 0.037), maize (0.039 to 0.041), wheat (0.035 to 0.037) and yearly period
(0.048 to 0.049).
In February, June, August, September and October, the
value of b was significant for all cases, including Tmax, Tmin
and mean temperature (Table 6); among these months,
August appeared to have nearly the same value of b for Tmax
and Tmin; compared to Tmax, value of b was higher
for Tmin in September and June, but lower in February and October.
In July, that of b was statistically considerable only for Tmin.
In January, it was true for both Tmin and mean temperature.
Rate of warming for Tmin was about two times higher in January,
as compared to July. For growing period of crops and yearly period, value
of b was significant for Tmin, Tmax and mean temperature;
warming found to be faster for Tmin in growing period of wheat,
maize, yearly period, but slower in that of chickpea and barley; warming
rate nearly appeared to be the same for lentil. Considering these results
it is cleared that there is no values of b<0, reflecting no climate-change-resulted
decrease in temperature for period from 1966 to 2004 in Tabriz, Iran;
this is in agreement with reports which are most evident, i.e., warming
(Tao et al., 2003), but not with findings of Skinner and Gullett
(1993); additionally, as it has
Table 4: |
The rate of change (value of b in equation Y = a+bX)
and its probability level (P) for averaged hourly temperature (16
to 20 ) over month, over growing period of 5 crops and over year (
°C year-1) during past 39 years for Tabriz, Iran |
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The bold values are statistically significant |
Table 5: |
The rate of change (value of b in equation Y = a+bX)
and its probability level (P) for averaged hourly temperature (21
to 24 ) over month, over growing period of 5 crops and over year (oC
year-1) during past 39 years for Tabriz, Iran |
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The bold values are statistically significant |
Table 6: |
The rate of change (value of b in equation Y = a+bX)
and its probability level (P) for averaged maximum temperature (Tmax),
minimum temperature (Tmin) and mean temperature over month,
over growing period of 5 crops and over year ( °C year-1)
during past 39 years for Tabriz, Iran |
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The bold values are statistically significant |
been found for many other regions (Vose et al.,
2005; Gholipoor et al., 2006; Ghorbani and Soltani, 2002), there
is considerable difference between months for temperature change; more
over, except for August and growing period of lentil, rate of change is
asymmetric for Tmax and Tmin which is similar to
published reports for other regions/countries (Gholipoor et al.,
2006; Xie and Cao, 1996). There are a number of possible factors, such
as an increase in cloud cover, contributing to decreases in the diurnal
temperature range (Easterling et al., 2000); urban heat island
is another factor influencing climate, which often tends to manifest itself
strongest during the nighttime h; increase in urbanization may differentially
increase the Tmin relative to the Tmax (Karl et
al., 1991).
Overall, hourly-based results indicating that in January
and July, warming found to be significant only for nighttime (20 h to
8 am) and for nearly half of nighttime (23 h to 5 am), respectively; across
these hours, rate of warming was higher in January compared to July; the
highest warming rate was obtained for 2 h in January and for 1 to 3 h
in July, then showed decreasing trend with hour decrementing/incrementing;
in contrast to these results, the results obtained from Tmax-Tmin-based
analyzing only showing that warming has been happened for Tmin;
additionally, rate of warming is higher for January than for July. In
August, rate of warming was equal to 0.045 and 0.046 °C year-1
for Tmax and Tmin, respectively; based on hourly-results
it was appeared that negligibly prominent warming rate is related to (night
time) 20 to 8 am (h). For February and October, in which the warming was
faster for Tmax than for Tmin, warming rate tended
to be the highest for 13 to 15 h and for 12 to 16 h, respectively; whereas
it appeared to be lowest in 2 am h and 1-3 am h for named months, respectively;
across day and night hours, rate of warming was higher in February than
October. Despite of February and October, in June and September, rate
of warming was higher for Tmin, as compared to Tmax;
more detailed results, which are only obtainable from hourly-based analyzing,
indicate that there is no difference between June and September for time
of occurrence of highest warming rate (1 to 3 am h) and that of lowest
warming rate (13-15 h); warming rate was relatively higher for June, compared
to September. It showed relatively prominent state at nighttime h for
growing period of wheat (the highest rate obtained for 24 to 4 am h; the
difference between Tmax and Tmin for warming rate
was equal to 0.004 °C year-1), maize (24 to 4 am; 0.007),
for yearly period (1 to 3 am; 0.004), but at daytime h for that of chickpea
(9 am to 19; 0.002) and barley (13 to 15; 0.011); negligibly faster warming
was related to night time h for lentil (21 to 7 am; 0.001).
CONCLUSION
Generally results indicate that in March, April, May,
November and December, warming was not significant for all day time and
night time hours. February and October with faster warming rate at daytime
h appeared to have highest warming rate for h 13-15 and 12-16, respectively;
the difference for lowest warming rate was also sensible (hour 2 am versus
1-3 am). Despite of these two months, June and September with more prominent
warming at daytime h showed no difference for time of occurrence of highest
1-3 am (h) and for that of lowest (13-15 h) warming rate. In August, the
negligibly faster warming (0.046 versus 0.045 °C year-1)
was related to 20 to 8 am (h). In January and July, warming found to be
significant only for nighttime 20 to 8 am (h) and for nearly half of nighttime
23 to 5 am (h), respectively; based on this difference and on existence
of relatively higher warming rate for January, it is concluded that when
there is significant change only for Tmin, it will be expected
a proportional relation between rate of change for Tmin and
number of nighttime h with temperature change; this conclusion may be
true for Tmax, which should be confirmed in future studies.
Based on huge difference between months for temperature change and on
this fact that with changing scale from month to growing period of crops
and finally to yearly period, variations are hidden, it seems that more
variations may be exist between days of each month, which should be studied
in future investigations.
Overall, when rate of change is asymmetric for Tmin
and Tmax [as it is evident in most reports (Karl et al.,
1993; Gholipoor et al., 2006; Ghorbani and Soltani, 2002), hourly-based-analyzing
provides more-detailed results and therefore increased insights, compared
to Tmax-Tmin-based-analyzing; for better understanding,
suppose that in 9 April 1971, the temperature has equaled to 8.20, 7.40,
6.80, 6.10, 6.80, 7.40 and 8.20 °C for 23, 24, 1, 2, 3, 4 and 5 am
(h), respectively; if climate-change-resulted increase in temperature
of theses h has been 0.94, 0.96, 0.98, 1.00, 0.98, 0.96 and 0.94 °C/30
year, it will be cleared that the h with temperature-constraint has been
diminished from 5 h in 1971 to 1 h in 2000, for, e.g., a plant with base
temperature 7.5 °C; such relatively comprehensive results may be more
useful for selecting appropriate climate-change-related managing/breeding
programs for plants, due to this fact that the plant growth and development
processes, including respiration, are continuous at different day and
night moments.
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