John R. McAfee

Long-Term Mean Vertical Motion over the Tropical Pacific: Wind-Profiling Doppler Radar Measurements

Measurement from Christmas Island (2�N, 157�W) of long-term mean vertical motions in the tropical atmosphere using very-high-frequency wind-profiling Doppler radar show that there is a transition from downward motion in the free troposphere to upward motion in the upper troposphere and lower stratosphere. The observations in the free troposphere are consistent with a balance between adiabatic and diabatic heating and cooling rates in a clear atmosphere. Comparison of the results at Christmas Island during El Ni�o and non-El Ni�o conditions with earlier results obtained for stratiform rain conditions over Pohnpei, Federated States of Micronesia, show that cirrus clouds in the vicinity of the tropopause likely play an important role in determining the sense and magnitude of vertical motions in this region. These results have implications for the exchange of mass between the troposphere and stratosphere over the tropics.

Wind profiler‐related research in the tropical Pacific

This paper is concerned with the application of wind-profiling Doppler radar technology to tropical atmospheric research. Examples of the use of wind profilers in the tropics are drawn from the Aeronomy Laboratorys wind profilers located on Pohnpei, Micronesia (7°N, 158°E), and Christmas Island (2°N, 157°W). The Pohnpei wind profiler was constructed in 1984 and has been used exclusively to observe vertical motions. The Christmas Island wind profiler has observed horizontal and vertical velocities routinely since 1986. These two wind profilers form part of a planned trans-Pacific network of wind-profiling radars that will eventually span the tropical Pacific.

On the annual variation of tropospheric zonal winds observed above Christmas Island in the central equatorial Pacific

Observations of tropospheric winds have been made nearly continuously since early 1986 in the altitude range 2–18 km using a 50 MHz VHF wind profiler at Christmas Island (2°N, 157°W). Tropospheric zonal winds observed at Christmas Island show a pronounced annual variation in the middle to upper troposphere with westerly winds typically observed during northern winter. The annual variation is strongest during La Nina and weakest during El Nino. The amplitude of the cycle has a broad peak in the upper troposphere. Empirical orthogonal function (EOF) analysis of the vertical structure of the annual variation shows that the principal mode explains nearly 90% of the variance in the 90- to 365-day frequency band. We attribute the annual variation in zonal winds to the annual variation in deep tropical heating associated with deep convection. The annual variation in tropical convection gives rise to annual variations in the surface pressure and pressure difference between Darwin and Tahiti. The pressure difference is greatest during northern hemisphere winter months. Other measures of the annual variation in tropical convection are provided by the tropical tropopause height and by highly reflective clouds (HRC). The tropical tropopause is highest during the northern hemisphere winter months when convection is most active over the maritime continent/western Pacific warm pool region and lowest in northern summer when convection is most active over the land masses of the northern hemisphere. The HRC show an annual migration in the centers of active convection. The annual latitudinal migration of the active centers of convection over the western Pacific with respect to the equatorial waveguide appears to be an important factor in determining the annual variation evident in the remote response of the zonal winds observed over Christmas Island.

A Comparison of Winds Observed at Christmas Island using a Wind-Profiling Doppler Radar with NMC and ECMWF Analyses

Abstract Wind profilers can provide useful wind data from remote regions of the globe, and incorporation of upper-level wind profiler data into analysis products can significantly improve the quality of analyses in data sparse regions. A wind-profiling Doppler radar was installed by the Acronomy Laboratory on Christmas Island during late 1985 as part of the Tropical Ocean Global Atmosphere (TOGA) Program. The Christmas Island profiler is self-contained and operates essentially unattended. Since April 1986, data from the Christmas Island profiler have been telemetered via GOES Satellite to provide hourly-averaged soundings of the wind four times daily keyed to the standard synoptic observing times and incorporated routinely onto the Global Telecommunication System (GTS) for world-wide distribution. In 1987 both NMC and ECMWF began using Christmas Island wind profiler observations in preparing their global analysis and forecast products. Detailed comparisons of NMC and ECMWF analyses with Christmas Island w...

Comparison of observed diurnal and semidiurnal tropospheric winds at Christmas Island with tidal theory

Four years of hourly averaged atmospheric wind measurements from the Christmas Island VHF wind profiler were processed into typical days to show the vertical phase structure of the diurnal and semidiurnal tropospheric zonal winds. The winds from the lowest resolvable height were compared with both the surface winds calculated from a three year record of surface pressure using the equations of motion on a rotating, frictionless sphere, and the winds from a tidal model. It is concluded that at the lowest height, the observed diurnal and semidiurnal zonal winds have tidal (1,1) and (2,2) mode structures, respectively. Further analysis is necessary to evaluate the tidal mode(s) of the zonal wind throughout the troposphere.

Diurnal variation in vertical motion over the central equatorial Pacific from VHF wind-profiling Doppler radar observations at Christmas Island (2° N, 157° W)

Wind-profiling Doppler radars measure vertical motions along a vertically-directed beam. By averaging over long periods it is possible to measure very small mean vertical motions. Long-term measurements at Christmas Island have been analyzed to determine the diurnal variation in the vertical motion field. The outstanding feature that emerges from the analysis is a pronounced diurnal cycle in the subsiding motion of the main troposphere (4–10 km) and an equally pronounced diurnal cycle in upward motion centered around the altitude of the tropopause. The possibility of radiative forcing of the observed diurnal cycle is considered in the context of the diurnal variation of atmospheric diabatic heating and cooling rates.

Recent changes in tropospheric circulation over the central equatorial Pacific

During the past five years an unusually prolonged warm event has occurred in the equatorial Pacific ocean. As recently discussed by Trenberth and Hoar [1996], this prolonged warm event represents a major climate perturbation in the historical record of the Southern Oscillation. The region most affected by the warm event is the central equatorial Pacific. Upper air observations from this region are sparse. However, since 1986 observations of tropospheric winds have been made nearly continuously using the Christmas Island VHF wind profiler. These profiler observations are examined for evidence of a deep tropospheric change in circulation during the past five years by comparing observations for 1990–1995 with those taken before 1990. It is shown that there has been a substantial decrease in the magnitude of tropospheric zonal winds observed at Christmas Island. At the same time, the magnitude of the pressure difference between Tahiti and Darwin has also been substantially reduced. These observations are consistent with a prolonged warm event in which the centers of active convection over the western Pacific have moved eastward toward the dateline. We conclude that there has been a reduction in the tropospheric zonal winds over the central equatorial Pacific that are normally associated with the gradients of surface pressure and SST across this region.

Wind profiler yields observations of ENSO signal

Nearly continuous observations of tropospheric wind at Christmas Island (2°N, 157°W) have been made since March 1986 using a VHF wind-profiling Doppler radar. The interannual zonal wind variations observed at this central Pacific location show a clear El Nino/Southern Oscillation (ENSO) signal with weaker than normal lower tropospheric easterlies and upper tropospheric westerlies during El Nino and stronger than normal lower tropospheric easterlies and upper tropospheric westerlies during La Nina. These dramatic changes in the tropospheric circulation over the central Pacific are related to variations in the zonal Walker Circulation, which in turn are associated with the Southern Oscillation. Observations recorded from 1986 to 1989 illustrate a complete ENSO cycle with the 1986–1987 El Nino followed by the 1988–1989 La Nina.