JP1.21 Partitioning of turbulent energy flux over Lake Chungsing as determined by eddy-correlation method and lake surface parameterization

Tuesday, 29 April 2008
Floral Ballroom Magnolia (Wyndham Orlando Resort)
Arumugam Alagesan, National Chung Hsing University, Taichung, Taiwan; and B. J. Tsuang and J. L. Tsai

The study site, Lake Chungsing is located at 24º 12' N latitude and 12º 67' E longitude in the eastern part of Central Taiwan. This is an urban lake system belong to the National Chung Hsing University, Taichung, Taiwan. Partitioning of the available energy into turbulent fluxes of sensible and latent heat fluxes are estimated through the eddy covariance (EC) system consists of Li-cor 7500 open path infrared CO2/H2O analyzer and CSAT 3 sonic amemo-thermometer. The entire signal from the EC system are measured at 20 Hz and averaged over 30 minutes period. The signals were logged through CR 5000 data logger. A Solar Infrared Radiation Station (SIRS) system was used to measure each component of the radiation balance. The SIRS consisted of four individual sensors, including both upward and downward pyranometers (PSPs), and both upward and downward pyrgeometers (PIRs), manufactured by the Epply Laboratory. Lake water profile temperature and soil temperature were measured through temperature probe and soil heat flux plate respectively. The analyzed data reveals that, the turbulent fluxes of heat are always positive throughout the periods. During day time, the available energy was partitioned more in the form of latent heat flux than sensible heat flux. Though both H and LE are of lesser value during night hours, there is an appreciable difference in the behavior of energy portioning. LE being the dominant energy flux term over the entire time scale. During night hours, the LE value appears to be as low as 26.02 W m-2, there after there is a minor fluctuation over the value in the time course up to early morning, afterwards the LE become an increasing phenomena and shows the sign of upward trend over the time course up to 1400 LT where the LE reached its maximum value of 48.98 W m-2, then there was a steady decrease during afternoon. In contrast to LE, H showed a significantly opposite behavior during day time. After 0800 LT, the H started declining. Though during night hours H showed a stable value, after sunrise and around 0800 LT in particular, the value of H started declining and it reached its day time lowest value around 1200 LT, there after, the value started to ascend. The combined value of H+LE followed the same pattern as that of LE. The combined value of H+LE ranged from as low as 26.02 W m-2 during night hours to high a highest value of 48.08 W m-2 during day time at 1400 LT in particular. With respect to energy storage in the lake water, he observed diurnal hourly composite value shows that , as the depth of water increases, the quantity of energy storage tends to decrease., thus showing a pattern as , the top layer of the water body tends to store more energy than the underlying layers and up to ultimate bottom wet soil. It was observed that lake bottom soil heat flux and storage is very minor when compare to rest of the energy components. During night time all the storage values, irrespective of the water profile depth registered negative values of energy storage and the values started positive after sunrise and reached its maximum at noon, thereafter the values started declining and at sunset, the values become negative. The highest value of total energy storage was about 400 W m-2 at 1200 LT. It is well observed that after imposing co-ordinate rotation and WPL term correction as a post processing measure, the value of turbulent energy fluxes has increased significantly than the raw flux. Though the increase in the value of the turbulent fluxes after these corrections was smaller during night hours, the day time value shows a marked improvement after imposing the coordinate rotation correction. The ordinary least square relationship between the uncorrected and corrected flux values shows a R2 value of 0.933. The energy closure for the lake is evaluated through energy balance ratio (EBR) method after taking into consideration of energy stored in water and soil heat flux. The evaluated EBR for the lake Chungsing is found to be 0.73 with an energy gap of 22.0 W m-2. Albedo was computed as a ratio of reflected solar radiation to the incident solar radiation. The day time albedo was calculated from 0900 LT to 1600 LT. The observed albedo during the measurement periods ranges from 0.04 to 0.08. The highest albedo was observed during morning and started declining gradually as time advances up to noon and thereafter the value started ascending with U shaped pattern. Surface emissivity was parameterized for lake water surface as the ratio of the radiation emitted by the standing lake water surface. For the Lake Chungsing the parameterized surface emissivity value is said to be 0.96.
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