Annual variation of air-water temperature difference at three Estonian coastal stations.

Keevallik, Sirje

1. INTRODUCTION

Land-sea warming contrast is a well-known physical phenomenon that reflects the different heat capacity of these two objects. The temperature difference between air and water affects fluxes of mass and energy through the air-sea interface and is an important input for calculations of sensible heat [1].

Numerical circulation models of the ocean and atmosphere solve the momentum and heat fluxes at the air-sea interface as the principal coupling agents between the sea and the atmosphere [2]. The contrast between the water and air temperatures together with the radiation regime permits one to evaluate whether or not the lower layers of the atmosphere are stably stratified [3]. As a rule, the properties of stratification are estimated by means of observations of vertical profiles of air masses [4], relying on other types of measurements and modelling efforts [5] or using automatic weather stations on ships [6]. It is natural to presume that the measured water temperature to some extent characterizes temperature over some larger region of the sea surface and that the observed air temperature similarly reflects the properties of lower atmosphere over some region. The sign of the temperature difference usually serves as a rough criterion for the stability of the lower atmosphere in the first approximation. Knowledge on the stability permits one to choose the proper method for handling the variations of the air flow properties at different levels, for example, to choose the coefficient to derive the wind speed at the height of 10 m from the measurements in higher levels [7].

In this research note we analyse the temporal behaviour of the water and air temperature measured simultaneously at three Estonian coastal meteorological stations. Not surprisingly, the difference between these temperatures varies during the year. This variation is to some extent site-specific and considerably depends on the wind direction. At one of the sites (Sorve, at the south-western end of the island of Saaremaa) this temperature difference reveals a counter-intuitive feature.

2. MATERIAL AND METHODS

The measurements used in this research note were carried out according to the WMO (World Meteorological Organisation) prescriptions [8] at the meteorological stations of Pakri, Sorve and Ruhnu (Fig. 1). Air temperature was measured at the altitude of 2 m above the observation field and water temperature at the depth of 30 cm on the coast, at a location where harbour buildings and islets did not shelter the site. Principal information about the meteorological stations and time series is shown in Table 1.

[FIGURE 1 OMITTED]

The three air temperature time series do not have any gaps or substantial inhomogeneities, but the routine of the measurements of water temperature was changed rather often. Therefore the longest possible time series was chosen from the measurements at 06:00 UTC (08:00 East-European Time) and the second longest from the data filed at 18:00 UTC (20:00 East-European Time).

3. LONG-TERM PROPERTIES OF AIR AND WATER TEMPERATURE

Due to differences in the heat capacity of water and air it is natural that the water temperature generally follows the variations in the air temperature with a certain delay throughout the year. The air warms up quickly in the spring when the water is still cold. The temperature of the water and the air become slowly equal in summer and the water is warmer than the air in the autumn. In winter, the coastal zone of Estonia is temporarily covered with ice. In such conditions the water temperature is measured in samples that are taken from under the ice with buckets. The resulting water temperature does not represent the sea surface temperature.

The air temperature has a large annual cycle at all three measurement sites. The highest monthly average (in July) is around 17[degrees]C at all stations. The lowest monthly average is -4[degrees]C at Pakri and Ruhnu and -2[degrees]C at Sorve. The water temperature is around zero in winter and at its maximum in July--approximately 17[degrees]C at Pakri and Ruhnu and 18[degrees]C at Sorve. In climatological conditions of Estonia in summer the air is commonly warmer than water (Fig. 2). The situation at Sorve differs greatly from that at the other stations: during a substantial part of the summer the water is warmer than air (Fig. 2). This feature persists for the time series containing only evening records (18:00 UTC). In May the water is even by 2.5[degrees] warmer than the air at Sorve (Fig. 3).

This unusual and interesting feature could be partially explained by different location of the measurement sites in terms of the openness with respect to the nearshore-open sea water exchange. At Pakri and Ruhnu the water temperature is measured at a location where the water exchange with the open sea is not restricted. Therefore the water temperature filed at these locations characterizes the temperature of the surface layer of the sea rather well. At Sorve the measurements are carried out in a shallow bay that is open only to the South. From July to January the West and South winds dominate in the north-western Baltic Proper. These winds usually carry warm surface water to the bay. Therefore the water temperature at Sorve not necessarily exactly represents the thermal conditions in the open sea.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

4. TEMPERATURE DIFFERENCE AND THE WIND DIRECTION

It is widely known that the air-land temperature differences and atmospheric movements are tightly interrelated. In order to evaluate the impact of the latter driver to the highlighted feature, the above analysis was repeated for different wind directions. The measured wind data were divided into four rhumbs: North 360[degrees] [+ or -] 45[degrees], East 90[degrees] [+ or -] 45[degrees], South 180[degrees] [+ or -] 45[degrees], West 270[degrees] [+ or -] 45[degrees]. Due to the asymmetry of the wind roses and different length of the meteorological time series, the number of recorded wind data varies for different wind directions (Table 2). Calm situations are not taken into account.

Figure 4 shows that the difference between the air and water temperatures depends notably on the wind direction. For North and East winds the annual cycle of the temperature difference is similar to the one depicted in Fig. 2. South winds approach Pakri over the Estonian mainland. This is the likely reason why the annual amplitude of the temperature difference is larger at Pakri than at other stations where South winds approach the measurement site over the sea. West winds are onshore for all stations. At Pakri they blow along the Gulf of Finland, at Ruhnu from the Gulf of Riga and at Sorve from the Baltic Proper. This analysis signals that the above-described unusual behaviour of the temperature difference at Sorve is caused by the location of this site at the end of the narrow peninsula. The water temperature is measured on the western coast of the peninsula. Thus, the particular location for the temperature measurements may be a partial reason for the contradiction between the local and the open sea temperature regime because the surface water is carried from the open sea to the water temperature measurement site by the West (and South) winds.

[FIGURE 4 OMITTED]

5. CONCLUSIONS

The research sheds some light to the problem of adequacy of existing estimates of the stratification conditions of the lower layers of the atmosphere in the coastal zone. It is commonly assumed that stratification of air masses is stable when the water is colder than air. The presented results show that in the conditions of selected stations at the Estonian seashore in most cases the water is warmer than the air whereas details of the temperature difference depend substantially on the season. The annual cycle is different at different sites and additionally depends on the wind direction. Most interestingly, the measurements at Sorve demonstrate the frequent presence of an unusual situation when the air-sea temperature difference is reversed and possibly strongly affected by the wind systems of the Gulf of Riga and Baltic Proper.

doi:10.3176/eng.2013.4.07

Received 6 November 2013, in revised form 22 November 2013

REFERENCES

[1.] Clayson, C. A. and Bogdanoff, A. S. The effect of diurnal sea surface temperature warming on climatological air-sea fluxes. J. Climate, 2013, 26, 2546-2556.

[2.] Kara, A. B., Rochford, P. A. and Hurlburt, H. E. Efficient and accurate bulk parameterizations of air-sea fluxes for use in general circulation models. J. Atmos. Oceanic Technol., 2000, 17, 1421-1438.

[3.] Rouse, W. R., Oswald, C. M., Binyamin, J., Blanken, P. D., Schertzer, W. M. and Spence, C. Interannual and seasonal variability of the surface energy balance and temperature of Central Great Slave Lake. J. Hydrometeor., 2003, 4, 720-730.

[4.] Hsu, S. A., Meindl, E. A. and Gilhousen, D. B. Determining the power-law wind-profile exponent under near-neutral stability conditions at sea. J. Appl. Meteorol., 1994, 33, 757-765.

[5.] Pena, A., Gryning, S.-E. and Hasager, C. B. Measurements and modelling of the wind speed profile in the marine atmospheric boundary layer. Boundary Layer Meteorol., 2008, 129, 479-496.

[6.] Niros, A., Vihma, T. and Launiainen, J. Marine meteorological conditions and air-sea exchange processes over the Northern Baltic Sea in 1990s. Geophysica, 2002, 38, 59-87.

[7.] Launiainen, J. and Laurila, T. Wind characteristics at Finnish automatic marine weather stations in the northern Baltic Sea. Finnish Marine Res., 1984, 250, 52-86.

[8.] WMO Guide to Meteorological Instruments and Methods of Observation. WMO-No. 8. Geneva, 2006.

Ohu-ja veetemperatuuri vahe aastane tsukkel kolmes Eesti rannikujaamas

Sirje Keevallik

On vorreldud ohu-ja veetemperatuuri kolmes Eesti rannikujaamas aastatel 1980-2010 eesmargiga hinnata atmosfaari alumiste kihtide stratifikatsiooni stabiilsust. On naidatud, et ohu- ja veetemperatuuri vahe oleneb aastaajast, mootmiskohast ning tuule suunast. Mootmised Sorves naitavad, et seal on tegemist erilise olukorraga, kus ohu-ja veetemperatuuride vahet mojutavad Riia lahe ning Laanemere avaosa tuulesusteemid.

Sirje Keevallik

Marine Systems Institute at Tallinn University of Technology, Akadeemia tee 15a, 12618 Tallinn, Estonia; sirje.keevallik@gmail.com

Table 1. Characteristics of the meteorological stations
and the data series

Station     Coordinates              Elevation,   Time period
                                     m

Pakri       59[degrees]23'22"N,      23           1980-2008
            24[degrees]02'24"E

Sorve       57[degrees]54'49"N,      3            1980-2009
            22[degrees]03'29"E

Ruhnu       57[degrees]47'00"N,      2            1980-1987,
            23[degrees]15'32"E                    2003-2010

Table 2. Number of recorded winds for different rhumbs at 06:00 UTC

Wind direction    Pakri        Sorve        Ruhnu

North             2011         1973         1012
East              2110         2123         858
South             4213         3101         1855
West              2037         3016         1326