CHANGES IN RADIATION PROPERTIES AND HEAT BALANCE WITH SEA ICE GROWTH IN SAROMA LAGOON AND THE GULF OF FINLAND

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 22 International Association of Hydraulic Engineering and Research CHANGES IN RADIATION PROPERTIES AND HEAT BALANCE WITH SEA ICE GROWTH IN SAROMA LAGOON AND THE GULF OF FINLAND Nobuyoshi Ishikawa 1, Atsushi Takizawa 1, Toshiyuki Kawamura 1, Kunio Shirasawa 1 and Matti Lepparanta 2 ABSTRACT The change in radiation properties with sea ice growth was examined at Lake Saroma, Japan and Tvarminne, the Gulf of Finland. The and transmissivity changed greatly with ice growth at Saroma but displayed a small change in the and a large change in the transmissivity at Tvarminne. These differences were explained by the ice structure differences, such as bubble volume, grain size and brine volume of sea ice. The change of heat balance with ice growth was measured on thin ice surface and compared with those of thick ice and snow surfaces. The surface temperature of the thin ice was higher than the air temperature. This resulted in the heat loss from the surface by the negative radiation balance and negative sensible heat flux. INTRODUCTION The heat exchange between atmosphere and underlying surface in the polar ocean depends on the surface conditions, owing to the different thermal properties of water, ice or snow. A number of radiation and heat balance studies have been carried out on sea ice (e.g. Weller, 1968; Allison et al., 1982; Perovich et al., 1998). These observations were usually done on thick and fast sea ice. In polar sea ice zone, there are some areas (called leads or polynyas) where open water remains throughout the winter. These open water areas occupy about 1 % of the sea ice zone. The total heat loss from the open water exceeds the heat loss from the entire area covered by sea ice (Ono et al., 198). Radiation properties of the surface change drastically after sea ice formation, which might influence the local climate conditions. So the heat exchange processes over polynya or thin sea ice must be quantified in spite of logistic difficulties. Recently, the optical properties of thin ice were examined through field observations as well as in cold laboratories (Perovich, 1996; Pegau et al., 1996; Ishikawa et al., 1998). The purpose of this study is to describe the dependence of radiation properties on thickness and the structure of sea ice, and to characterize the changing heat balance with sea ice growth. 1 Institute of Low Temperature Science, Hokkaido University, N-19, W-8, Sapporo, Japan 2 Department of Geophysics, University of Helsinki, P.O. Box 64, Helsinki, Finland

2 OBSERVATION SITES AND INSTRUMENTATION In order to investigate the change in radiation properties with sea ice growth, micro-meteorological observations were conducted in an artificially opened pool made on Saroma Lagoon, Japan in February, 1999 and the Baltic Sea near Tvarminne, the Gulf of Finland in March, The Saroma Lagoon is connected to Okhotsk Sea by two narrow channels and the salinity is 3.1 %. The Baltic Sea is semi-enclosed brackish water, so the salinity is only about.6 %. The ice thickness was.4 m to.5 m and the surface conditions were almost flat and smooth with thin snow cover at both observation sites. Pools were established by removing m ice, and the snow of the surrounding area (5 1 m) was cleared to expose the thick ice surface. Observations at both sites were the same: incoming and outgoing solar radiation, outgoing longwave radiation, transmitted solar radiation through ice and water, temperatures of air, surface and sea water, wind velocity, and relative humidity, which were continuously recorded on a digital data logger every 1 min. The spectral albedo was measured manually on the pool, thick ice and snow surfaces, respectively. Radiation intensity was measured in the 4 to 15 nm wavelength ranges with 25 nm resolution using a photometer. The ice thickness was measured several times a day. OBSERVATIONAL RESULTS Change of radiation properties with ice thickness Reflectivity (spectral albedo) of thin ice was measured at solar noon on clear days. Figure 1 shows the relationship of with ice thickness at two sites, which are averaged for two wide ranges of wavelengths; visible (4 75 nm) and near infrared (75 15 nm). On the water, the in both wavelength ranges takes the same low value (.3). Reflectivity of both wavelengths ranges increases with ice growth, and the of visible range increasing more than that of the near infrared. The change of with ice growth at Saroma is more dramatic than at Tvarminne Saroma(visible) Saroma(infrared) Tvarminne(visible) Tvarminne(infrared) Figure 1: Changes of with sea ice thickness at Saroma and Tvarminne The extinction coefficient can be evaluated from the incoming and transmitted solar radiation. TR z = SR exp( µz), (1)

3 where TR z is the intensity of the transmitted radiation at depth Z, SR is the incoming solar radiation and µ is the extinction coefficient. The transmitted solar radiation was measured by two small underwater solarimeters fixed at.5 m and.15 m below the surface of ice. When the total ice thickness was less than.15 m, the calculated values of µ include ice and water. However, once the ice thickness and the extinction coefficient of water are obtained, we can evaluate the extinction coefficient of ice. Figure 2 represents the variations of extinction coefficients with ice thickness at Saroma and Tvarminne. The values slightly increase with ice growth up to a thickness of.6m at both places. Beyond this thickness the coefficients are almost constant, but the value at Tvarminne (5 to 8 m 1 ) is much smaller than Saroma (1 13 m 1 ). extinction coefficient (m -1 ) Saroma Tvarminne Figure 2: Extinction coefficients with ice thickness at Saroma and Tvarminne Figure 3 shows the changes of, transmissivity and absorptivity with ice thickness. Those are ratios of reflected, transmitted and absorbed solar radiation to incoming solar radiation, respectively. Reflectivity (α) and transmissivity (β) are obtained by observation and absorptivity (γ) is estimated by the following relationship: α + β + γ = 1 (2) We can see the increase of and decrease of transmissivity with sea ice growth at both places. However, at Saroma the variations of and transmissivity are much larger than at Tvarminne. Surface temperatures (Ts) of the thin ice were measured by a thermistor thermometer and an infrared thermometer. The infrared thermometer obtains the brightness temperature (T *), so the emissivity (ε) of the sea ice can be evaluated by comparison of two temperatures (method II). We also obtained the upward longwave radiation (Lu) directly and evaluated the emissivity (method I). The emissivity by two different methods shows high values in the range of.96 to 1., which seems to be independent of ice thickness (Fig. 4). The fluctuation of emissivity obtained by method II is much larger than method I, because the infrared thermometer is very sensitive and measures the spot temperature, so it may useful to use a pyrgeometer for measuring the surface temperature (method I).

4 coefficient coefficient Tvarminne transmissivity absorptivity Saroma transmissivity absorptivity Figure 3: Reflectivity, transmissivity and absorptivity of thin sea ice. 1. emissivity method I method II Figure 4: Emissivity of thin sea ice obtained by two different methods. Relations between radiation property and ice structure The radiation properties of thin sea ice display different values at Saroma Lagoon and Tvarminne even when the ice thickness is equal. This means that the properties do not

5 depend only on the ice thickness. Therefore, the ice structure, such as ice volume, brine volume, air bubble volume or grain size were examined to determine the radiation properties. Ice samples were brought into a cold laboratory and cut into 1 cm section for measuring ice structures. Brine volume was estimated using salinity and ice temperature, and bubble volume was obtained using density, ice temperature and salinity (Cox and Weeks, 1983). Grain size was directly measured from the thin section of ice. We measured, transmissivity and absorptivity of each ice sample using a multi-spectrometer, which detected the radiation intensity in the 4 17 nm wavelength range with a resolution of 1 nm. Radiation penetration through the ice decreases by two processes; absorption and scattering. When the sea ice contains many air bubbles such as in snow-ice, the scattering becomes dominant, which leads to a large extinction coefficient and high. We observed larger structural differences in the thin sea ice between the two study areas. At Saroma, a mixture of snow and ice formed in the upper part of sea ice, below which columnar ice formed. At Tvarminne, the sea ice was totally columnar ice, no snow-ice was observed. Therefore, the total bubble volume of Saroma ice is much larger than that of Tvarminne. Saroma ice contains higher brine volume because the salinity of the source water is higher Saroma(h=.9m) Tvarminne(h=.11m) bubble volume (cm 3 ) Figure 5: Relationship between and total bubble volume. Figure 5 represents the relationships between and total bubble volume of sea ice sample of Saroma and Tvarminne. When ice thickness of Saroma reaches.9 m, the takes the value of.55. At that time the total bubble volume is.68 cm 3. On the other hand, at Tvarminne when ice thickness is.11 m, the is.15 and the bubble volume is.36 cm 3. So we can see that the difference in bubble volume of sea ice is a primary cause for the difference in for the same ice thickness. Figure 6 represents the relationships between transmissivity and total brine volume. At Tvarminne, the transmissivity takes the value of.42 and the brine volume is.6 cm 3 at the thickness of.11 m. At Saroma, the larger change in transmissivity with brine volume is seen. The transmissivity decreases to.2 and the brine volume increases to.42 cm 3 when the ice thickness reaches to.9 m. So we can say that the difference in ice structure and ice thickness both significantly affects the radiation properties of ice.

6 transmissivity Tvarminne(h=.11m) Saroma(h=.9m) brine volume (cm 3 ) Figure 6: Relationship between transmissivity and total brine volume. HEAT BALANCE CHARACTERISTICS OF SEA ICE The change of heat balance with sea ice growth was studied on the thin ice and compared with the heat balance of the thick ice and snow surface. The heat balance equation of the thin ice expressed by Q N + Q H + Q E + Q M =, (3) where Q N is net radiation, Q H is sensible heat, Q E is latent heat of vaporization and Q M is the term of the remainder, which is composed by the change of heat storage and ice growth. The positive sign of Q M means the heat loss or freezing of seawater. Methods of calculation and variables of each heat balance component were described previously (Ishikawa and Kobayashi, 1984). Figure 7 shows the comparison of heat balance components on three different surfaces at Saroma. The value was expressed by the mean amount for 12 hours in nighttime and daytime, and averaged for the whole observation period. On the thin ice, the heat exchange is largest, compared with other surfaces. 1 8 nighttime daytime heat flux (MJ/m 2 ) thin ice thick ice snow thin ice thick ice snow -6-8 QH QE QN Figure 7: Heat balance of various surfaces. In nighttime, the three components are all negative, which means heat is lost from the surface and the heat loss is compensated by ice growth. The major heat loss is due to

7 radiation and sensible heat flux. Conversely, in daytime, the net radiation on the thin ice and thick ice becomes a heat source, and sensible heat and latent heat become heat sink. Sensible heat flux markedly decreases with ice growth. The surface temperature of thin ice is higher than the air temperature. This results in the negative sensible heat flux all day and warms the overlying atmosphere. However, it decreases rapidly with sea ice growth. CONCLUSIONS The changes of radiation properties with sea ice thickness were examined in two seasonal sea ice zone where the salinity of sea was different. The surface albedo was very low, less than.5 (water) and increased to.15 at Tvarminne and.4 at Saroma when the ice was.1 m. Reflectivity and transmissivity changed substantially with ice growth at Saroma but a small change in and a large change of transmissivity were observed at Tvarminne. These differences were explained by the differences in ice structure, such as bubble volume and brine volume contained in the sea ice. The heat balance with ice growth was measured on the thin sea ice and compared with those of thick ice and snow surfaces. The surface temperature of thin ice was higher than air temperature. This resulted in the heat loss from the surface by the negative radiation balance and negative sensible heat flux. ACKNOWLEDGEMENT The technical members of the Monbetsu Sea Ice Research Laboratory, Hokkaido University, and graduate students of Department of Geophysics, University of Helsinki helped the authors to collect data in Saroma Lagoon and the Gulf of Finland. The Shinjo Branch of Snow and Ice Studies, National Research Institute for Earth Science and Disaster Prevention provided the Cold Laboratory. Prof. L.D. Hinzman, University of Alaska read the manuscript. Part of this research was supported by the scientific research fund from the Ministry of Education, Culture and Science of Japan. REFERENCES Allison, I., Tivendale, C.M., Akerman, G.J., Tann, J.M. and Wills, R.H. Seasonal variations in the surface energy exchanges over Antarctic sea ice and coastal waters. Annals of Glaciology 3: (1982). Cox, G.F.N. and Weeks, W.F. Equations for determining the gas and brine volumes in seaice samples. Journal of Glaciology 29(12): (1983). Ishikawa, N, Kodama, Y., Ikeda, T., Takatsuka, T. and Ishikawa, M. Changes of the heat and radiation properties with sea ice growth. In Proceedings of the 13 th International Symposium on Okhotsk sea and sea ice, Monbetsu, Japan (1998) Ishikawa, N. and Kobayashi, S. Experimental studies of heat budget of very thin sea ice. Journal of the Japanese Society of Snow and Ice 46(3): (1984). Ono, N., Wakatsuchi, M. and Kawamura, T. Freezing phenomena at sea water surface opening in Polar Winter I. Low Temperature Science A 39: (198). Pegau, W.S., Paulson, C.A., Zaneveld, J.R.V. Optical measurements of frazil concentration. Cold Region Science and Technology 24: (1996). Perovich, D.K., Roesler, C.S. and Pegau, W.S. Variability in Arctic sea ice optical properties. Journal of Geophysical. Research 13(C1): (1998). Perovich, D.K. The Optical Properties of Sea Ice. CRREL Monogram, U.S. Army Cold Region Research and Engineering Laboratory, Hanover, N.H (1996) 25p. Weller, G. Heat budget and Heat Transfer Processes in Antarctic Plateau Ice and Sea Ice. ANARE Science Report A(IV) Glaciology 12 (1968) 155p.