Automatic weather stationobservations of strong katabatic

winds near Terra Nova Bay,Antarctica

THOMAS R. PARISH

Department of Atmospheric ScienceUniversity of Wyoniiiig

Laramie, Wijoning 82071

DAVID H. BROMWICH

Byrd Polar Research CenterOhio State UniversityColumbus, Ohio 43210

For 9 months of the year, net radiative cooling characterizesthe heat budget of the ice slopes of the antarctic continent. Asa result, the surface wind regime over Antarctica is stronglyinfluenced by the underlying terrain. Katabatic winds are apredominant boundary-layer phenomenon; some of the strong-est surface winds on Earth are found along the coastal sectionsof Antarctica. Currently underway is an observational study ofthe intense katabatic winds found near Terra Nova Bay (75°S163°E). Six automatic weather stations have been installed alongReeves Glacier and upslope from the glacier on the plateau. Allstations were deployed by Charles Stearns and colleagues atthe University of Wisconsin, Madison. In addition, an array offour automatic weather stations near Terra Nova Bay has beendeployed by the Italian Antarctic Expedition. Positions and re-sultant wind data for the period from February 1988 throughJanuary 1989 can be seen in Bromwich, Parish, and Pellegrini(1990). Here, we will focus on data collected by automaticweather station units 8905 and 8909, which are situated beyondthe foot and near the head of Reeves Glacier, respectively.

Composite depictions of the annual course of state parame-ters for the entire automatic weather station array have beenprepared for the calendar year 1988. The trends were computedby first determining daily averages (vector averages for wind)and then passing the time series through a nine-point low-passfilter to remove much of the high-frequency variance. Figure 1shows the evolution of potential temperature and wind speedduring the course of 1988 for units 8905 (Inexpressible Island)and 8909 (head of Reeves Glacier). Pronounced is the decreaseof potential temperature during February (days 32-60). Therapid change is, in all likelihood, a result of the rapidly chang-ing solar geometry and resulting decrease in solar insolation.This sudden onset of winter has been alluded to in Bromwich(1989). Accompanying the abrupt temperature decrease is apronounced increase in the intensity of the katabatic wind. Ina period of approximately 1 month, resultant wind speeds in-crease in excess of 100 percent. By mid-April (day 110) winterconditions are fully established with daily resultant windspeeds near 15 meters per second at 8905 and mean potentialtemperatures near 250 kelvin. Note that the potential temper-ature trends at 8909 are nearly identical to those found at 8905,indicating similar synoptic influences and/or local radiative con-ditions. As noted in the aircraft studies of katabatic winds near

Terra Nova Bay by Parish and Bromwich (1989), values of po-tential temperature are nearly identical at the head and foot ofthe Reeves Glacier. This feature has also been noted in mod-eling studies (Parish and Waight 1987) and is characteristic ofkatabatic-prone areas. The wind speeds seen at 8909 are con-sistently some 3-5 meters per second less than correspondingwind speeds at 8905, implying that the katabatic flow acceler-ates down the approximately 40-kilometer stretch of ReevesGlacier. Bromwich (1989) has noted that the katabatic windexhibits little deceleration from the terminus of the ice edge ofReeves Glacier and 8905 despite the 34-kilometer trek acrossthe flat Nansen Ice Sheet. Only minor enhancements to theautumn changes are observed during the following 5 1/2 months(days 110-274). The potential temperature trend observed dur-ing the winter months suggests a coreless winter. Both poten-tial temperature and wind-speed trends display a similar, al-though possibly weaker, return to summertime conditionsduring the austral springtime months of October and Novem-ber (days 275-335). Short-term variations, presumably due tothe effects of transient synoptic systems, are present in bothpotential temperature and wind speed throughout the entireyear.

To depict interrelationships in weather between the two au-tomatic weather station sites, records were subjected to spec-trum and cross-spectrum analysis. Results here will featurewind speeds from the transitional period from January throughMarch 1988 only. Power spectra for wind speeds at 8905 and8909 for this period are shown in figure 2. As is expected duringsummer months, a diurnal cycle is present (peak at 1 day). Inaddition, there is a suggestion of synoptic variability at 8905;secondary peaks in the percentage of explained variance overthe 2-10 day time scale. Wintertime (May through July, days121-213) power spectra at 8905 for wind and pressure (notshown) display appreciable variance over time scales of 2-10days, presumably associated with transient synoptic activity.

Cross-spectrum analysis shows that the wind speeds meas-ured by the automatic weather station units are highly coherentwith each other. Wind speeds for the period from Januarythrough March 1988 at sites 8905 and 8909 show a correlationcoefficient of 0.68. Similarly high coherence in wind speed (val-ues in excess of 0.65) can be seen over nearly all frequencies atsurrounding automatic weather stations near Reeves Glacierand situated on the sloping antarctic ice fields. Pressure differ-ences between the head of the Reeves Glacier and InexpressibleIsland and the wind speed at Inexpressible Island were alsoshown to be significantly correlated (correlation coefficient of0.61). This suggests that katabatic winds are associated with ahorizontal pressure gradient directed down Reeves Glacier. Fig-ure 3 shows the cross spectra. Note that a high degree of co-herence is present over time scales of 1 to 7 days; no phase lagis seen between the katabatic wind at Inexpressible Island andthe pressure difference along Reeves Glacier. This feature isevident when comparing horizontal pressure differences be-tween 8909 and 8905 during periods of high winds at Inex-pressible Island. Analyses show that higher wind speed eventsat 8905 are associated with correspondingly larger pressuredifferences between 8909 and 8905 with higher sea level pres-sures at the head of Reeves Glacier. For wind speeds in excessof 30 meters per second at 8905, horizontal pressure differencesat these two sites are 7.5 hectopascals greater than what isfound during weak wind periods (8905 winds less than 10 me-ters per second). This suggests that extreme katabatic winds atInexpressible Island undergo significant acceleration down theglacier valley and perhaps along the flat Nansen Ice Sheet.

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Figure 1. Annual course of potential temperature (in degrees kelvin) and wind speed (in meters per second) at 8905 (Inexpressible Island)and 8909 (head of Reeves Glacier) during 1988.

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Figure 2. Power spectra of wind speed at 8905 and 8909 during the period from January through March 1988.

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Figure 3. Cross spectrum of pressure difference between 8905 and 8909 and wind speed at 8905. Power spectra of pressure difference attop, coherency and phase spectra at bottom. (COH denotes coherency.)

This research has been supported by National Science Foun-dation grants DPP 89-16998 (to Thomas R. Parish) and DPP 89-16134 (to David H. Bromwell).

References

Bromwich, D.H. 1989. An extraordinary katabatic wind regime at TerraNova Bay, Antarctica. Monthly Weather Review, 117, 688-695.

Bromwich, D.H., T.R. Parish, and A. Pellegrini. 1990. The katabaticwind regime near Terra Nova Bay, Antarctica. Antarctic Journal of theU.S., 25(5), 267-269.

Parish, T.R., and D.H. Bromwich. 1989. Instrumented aircraft obser-vations of the katabatic wind regime near Terra Nova Bay. MonthlyWeather Review, 117, 1570-1585.

Parish, T.R., and K.T. Waight. 1987 The forcing of Antarctic katabaticwinds. Monthly Weather Review, 115, 2214-2226.

1991 REVIEW 267