Go/min et al.: Frazil Ice Formation durin~ the Soring Flood
127
Laptev Sea in the near-delta region of the Lena river from mid-May to mid-June during the
period that directly precedes the spring flood, and during the period of its development. The
array of comprehensive oceanographic stations is presented in Figure 1. Observations of water
temperature and salinity were conducted by means of a fine-structure CTD sonde. We used an
OTS-PROBE Serie 3 sonde (Meerestechnik Electronic CrnBH, Germany). The main technical
characteristics of an OTS-PROBE are the following:
- water temperature: accuracy 0.01 °c ; time costant 160 ms;
- electric conductivity: accuracy 0.02 mS/cm; time constant 100 ms;
- hydrostatic pressure: accuracy 0.1 %, time constant 40 ms;
- information exchange rate 1200 bit/so
Temperature and salinity sensors of the sonde are situated close to each other. They are of the
same size. Multi-channel sonde is able to perform a parallel cross-examination of sensors. The
rate of vertical sounding is about 0.5-1.0 mis, i.e. about 12-24 parameter values per 1 meter of
sea water body (average IS-17). All values of temperature and salinity parameters have been
used for analyzing the principal possibility and the value of supercooling in the extremely sharp
halo-pycnocline during flood period. Data filtration with depth has not been performed.
Observation results
Processing of temperature and salinity observational data has shown the existence of zones of
supercooled water S cm to I.S m thick in the pycnocline (the interface between river and sea
water). Supercooling in some thin layers of these zones ilT f = T- 'ts , where T is the measured
(actual) water temperature and 'ts is the water freezing temperature at the given salinity, was
calculated using standard algorithms (Fofonoff and Millard, 1983). At a first glance, it was
reaching the improbable values of -0.6 to -0.8°C (Figure 2c). Mean super-cooling values varied
from -0.0 I °c up to -0.1 Dc. The persistence of such supercooling under natural conditions has
not been earlier observed. At least the authors are not aware of cases where similar values were
directly determined in such an extensive region.
The depth of the supercooled layer, its thickness characterizing the extent of penetration of
cooling to the pycnocline and the value of supercooling itself depended on the time and place of
measurements. Prior to the spring flood (end of May), supercooling was only observed at
stations located at the traverse of the main discharge branches Trofimovskaya and Bykovskaya.
The upper boundary of supercooled water is confined to the lower boundary of freshened
surface water (Figure 2). Similar position of the location of the upper boundary of the
supercooled layer was observed at all stations where this layer was recorded. The supercooled
layer itself at the weak inflow of river water occupies a small upper part of the pycnocline
(Figure 2a). At the time of maximum discharge from early to mid-June it occupied either much
of the pycnocline or the entire pycnocline (Figures 2b, c). Prior to the flood, the value of
supercooling in the upper part of the pycnocline was small and rarely exceeded O.OSoC (Figure
2a). With the rising flood the intensity of freshwater inflow sharply increased, causing the
lense of fresh water to greatly expand its area below the fast ice. The thickness of the fresh river
water layer gradually increased with entrainment and mixing of the underlying layer of saline
and cold water (Figure 3). This is especially well seen at the stations located along the traverse
crossing the outflow from the main branches (Figure 3). Finally, river water at some
comparatively shallow stations occupied the entire water column between the fast ice and the
seafloor. With increasing thickness of the river water layer (Figure 3), the upper boundary of
the supercooled layer is lowered (Figure 2). The increased rate of river water inflow, associated
127
Laptev Sea in the near-delta region of the Lena river from mid-May to mid-June during the
period that directly precedes the spring flood, and during the period of its development. The
array of comprehensive oceanographic stations is presented in Figure 1. Observations of water
temperature and salinity were conducted by means of a fine-structure CTD sonde. We used an
OTS-PROBE Serie 3 sonde (Meerestechnik Electronic CrnBH, Germany). The main technical
characteristics of an OTS-PROBE are the following:
- water temperature: accuracy 0.01 °c ; time costant 160 ms;
- electric conductivity: accuracy 0.02 mS/cm; time constant 100 ms;
- hydrostatic pressure: accuracy 0.1 %, time constant 40 ms;
- information exchange rate 1200 bit/so
Temperature and salinity sensors of the sonde are situated close to each other. They are of the
same size. Multi-channel sonde is able to perform a parallel cross-examination of sensors. The
rate of vertical sounding is about 0.5-1.0 mis, i.e. about 12-24 parameter values per 1 meter of
sea water body (average IS-17). All values of temperature and salinity parameters have been
used for analyzing the principal possibility and the value of supercooling in the extremely sharp
halo-pycnocline during flood period. Data filtration with depth has not been performed.
Observation results
Processing of temperature and salinity observational data has shown the existence of zones of
supercooled water S cm to I.S m thick in the pycnocline (the interface between river and sea
water). Supercooling in some thin layers of these zones ilT f = T- 'ts , where T is the measured
(actual) water temperature and 'ts is the water freezing temperature at the given salinity, was
calculated using standard algorithms (Fofonoff and Millard, 1983). At a first glance, it was
reaching the improbable values of -0.6 to -0.8°C (Figure 2c). Mean super-cooling values varied
from -0.0 I °c up to -0.1 Dc. The persistence of such supercooling under natural conditions has
not been earlier observed. At least the authors are not aware of cases where similar values were
directly determined in such an extensive region.
The depth of the supercooled layer, its thickness characterizing the extent of penetration of
cooling to the pycnocline and the value of supercooling itself depended on the time and place of
measurements. Prior to the spring flood (end of May), supercooling was only observed at
stations located at the traverse of the main discharge branches Trofimovskaya and Bykovskaya.
The upper boundary of supercooled water is confined to the lower boundary of freshened
surface water (Figure 2). Similar position of the location of the upper boundary of the
supercooled layer was observed at all stations where this layer was recorded. The supercooled
layer itself at the weak inflow of river water occupies a small upper part of the pycnocline
(Figure 2a). At the time of maximum discharge from early to mid-June it occupied either much
of the pycnocline or the entire pycnocline (Figures 2b, c). Prior to the flood, the value of
supercooling in the upper part of the pycnocline was small and rarely exceeded O.OSoC (Figure
2a). With the rising flood the intensity of freshwater inflow sharply increased, causing the
lense of fresh water to greatly expand its area below the fast ice. The thickness of the fresh river
water layer gradually increased with entrainment and mixing of the underlying layer of saline
and cold water (Figure 3). This is especially well seen at the stations located along the traverse
crossing the outflow from the main branches (Figure 3). Finally, river water at some
comparatively shallow stations occupied the entire water column between the fast ice and the
seafloor. With increasing thickness of the river water layer (Figure 3), the upper boundary of
the supercooled layer is lowered (Figure 2). The increased rate of river water inflow, associated