Weather Report. PCAS Camp 2009/10. By Chris Mckenzie

Transcription

1 Weather Report PCAS Camp 2009/10 By Chris Mckenzie

2 Abstract An AWS (Automatic Weather Station) was set up at PCAS camp, located on the Ross Ice Shelf in the Windless Bight (figure 1). Data was measured in 10 minute intervals from the 24 th of December 2009 to the 31 st of December Data from this AWS was used to correlated wind direction with temperature. Results showed cooler air from westerly quadrant and warmer from the East. Stakes measuring the accumulation and ablation of the snow surface were also set up at camp. Results showed large accumulation events during snow and high ablation rates during windy sunny conditions. Four Balloon launches were also undertaken at camp, which measured temperature inversions during calm weather. Comparing these observations with Scott Base and Arrive Heights data showed local weather conditions. Introduction Located along the Transantarctic Mountains in Antarctica is Ross Island, which is subjected to a strong southerly wind regime. Anomalous calm occurs in Windless Bight on the island's southern coast. This calm is caused by the steep topography of Ross Island that causes a stagnation zone. The deflection of highly stable, low-level air around Hut Point Peninsula causes persistent north-easterly winds at Scott Base. The warm maritime air from cyclonic systems to the east is associated with the less frequent, strong southerly winds which override the peninsula. This report focuses on weather observations made in Windless Bight by K220 (Location of PCAS camp: S , E ). The differences in weather conditions at Scott base over the same time period are also assessed.

3 Arrival Heights Scott Base K220 Camp Figure 1, Location of K220 camp, Arrival Heights and Scott Base. Methods Equipment used: Two Kestrel 4500 Pocket Weather Tracker Bamboo stakes Measuring tape Pencil & recording sheet HOBO Weather Station Skyhook Helikite (helium balloon system), with radiosonde & PC connected base station Two observation periods a day, one at approximately 7:30am and 6pm, were observed using the two Kestrel 4500 Pocket Weather Trackers. The measurements made were: Temperature, wind-chill, relative humidity, dew point, wet bulb, pressure (0m & 15m), wind speed, gust & direction (average over 30s). Observations made by the observer were: Cloud cover (eighths), cloud direction, visibility and comments of weather conditions. On the first day, four stakes were placed into the snow leaving 40cm of bamboo exposed. The length of the exposed bamboo also was measured twice daily.

4 The HOBO Weather Station automatically took readings every 10 minutes. The same observations were made at the Kestrel 4500 (as above), except for the additional observations of incoming and outgoing solar radiation. The temperature and humidity was taken at 1.5m and at near surface. Four Skyhook Helikite balloon launches were undertaken, two on the 27th December, one 28th and another one on the 29th. Readings were taken approximately every 10 seconds, measuring pressure, temperature, relative humidity, wind speed and direction. Observations from AWS at Arrival Heights and Windless bight are also used in this report. Results Most of the observations have not been analysed due to the large amount of them and time constraints. The graphs below have been chosen for their importance for understanding the weather during the K220 camp period. Note: this data is the average of all the stakes. Snow is accumulating during snow events. This fresh unconsolidated snow either compacts very quickly or is heavily influenced by wind and sun. Approximately 3cm was lost on the 22nd, which had winds averaging 3.6m/s. The 26th was the sunniest day which coincided with an approximate 5cm snow loss from the previous day. Over the whole nine days at camp there was a net accumulation of approximately 3cm.

5 This graph measures the amount of reflectivity of suns radiation on the snow. Any albedo values over 1 mean more sunlight is being reflected than received, which is very unlikely. Overall the albedo values are high, as is expected for a fresh snow surface. Results show humidity gradually decreasing with height.

6 With temperature plotted against height an inversion layer up to approximately 25m can be seen. No inversion present on the balloon launce on the 28th. An increase in temperature is also noted for the decent.

7 Temperatures ranged from approximately 6C on a clear sunny day to approximately -12C during light snow on Christmas night. The lowest temperatures each day are early in the morning and the warmest during the middle of the day. Wind direction appears to have a moderate correlation with temperature. It is clear that westerly winds on the 25th coincided with cool air. The wind direction changed to an easterly quarter giving way to warmer air. Relationships for the following days are less clear but the relationship can still be seen.

8 This is the temperature and wind direction measured at arrival heights and Scott Base over the K220 camp period. The air is warmer during north westerly than North easterlies, but the relationship is less defined with these observations. Discussion The measurements from the Kestrel 4500 were not used due to inconsistencies in the data. The stakes and general weather observations were used. The observations of snowfall are very useful as the only way of measuring precipitation was from the stakes, which are subjected to ablation. The graph of snow accumulation and ablation clearly showed the snowfall events. The amount of ablation appears to be greatly influenced by the amount of incoming solar radiation and wind speed. From the amount of snow we received during the period, It was surprising to measure the small net gain in snow accumulation. The snow on the 28th was wetter and more consolidated. One would predict this layer to be less prone to wind ablation. For fresh dry snow albedo readings would be expected to be ~0.9, as seen for the first four days. From the 28th onwards albedo is greater than 0.9 and on the 29th it was above 1 for a significant part of the day. Reasons for this could be the level of the ground snow with respect to the solar receiver. Reflections off the HOBO weather station could also increase the reflected radiation. An incorrect calibration of the solar receiver could also affect the reading. The snow from the 28th onwards was not as dry as the previous days, which has increased reflectivity, which contradicts expected albedo for wet snow. Radiosonde taken 27th December in the morning showed a temperature inversion up to ~25 meters. Humidity gradually decreases with height with no clear effects from the inversion layer. The 28th was windy and during the time the balloon was in the air the wind

9 changed direction. This would explain the increase in temperature for the descent. The wind strength must have been great enough to prevent an inversion from developing. Temperature observations during the days that had low wind and cloud cover were highly dependent on the amount of solar radiation reaching the ice. The direction of the wind had the greatest influence on temperature during windy cloudy days. Temperatures were lower during days of westerly quarter winds. Temperatures were higher during easterlies, as the air was probably coming off the warm moist Ross Sea. Winds directions measured at Scott Base and arrival heights were more from a northerly quadrant than measured at K220 camp. The high topography of Erubus would most likely contribute to these dominant wind directions. Conclusion The amount of data collected is sufficient for understanding the weather in Windless Bight. Observations would need to be taken over a longer period of time and correlated with synoptic charts for greatest results.