Turbulent kinetic energy dissipation rates and eddy diffusivities in the tropical mesosphere using Jicamarca radar data

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Abstract

The 50-MHz MST radar at Jicamarca Radar Observatory (JRO) can detect atmospheric turbulence on the Bragg scale of 3 m in the daytime mesosphere (∼60–85 km). Since 2002, the radar was operated for a certain number of days each year collecting 1-min Doppler spectra in four off-vertical (2.5°) beam directions and 150 m resolution. The spectral widths have been used to compute the kinetic energy dissipation rate ε due to atmospheric turbulence. A small beam broadening effect has been removed from the observed spectral widths.

The daily median energy dissipation rates ε increase from 5 to 30 mW/kg between 67 and 80 km, and the eddy diffusivities increase from 3 to 20 m2/s, consistent with similar studies conducted by two other large 50-MHz radars in Japan and India. The energy dissipation rates are about the same magnitude as the ε estimates for low-latitudes from a global model and are larger than the averages from rocket observations at high-latitudes, confirming previous comparisons.

Introduction

Knowledge of the turbulent kinetic energy dissipation rate ε in mesosphere is important in understanding the thermal and dynamical structures of that region. Various techniques have been used to observe mesospheric turbulence, including rockets (e.g., Royrvik and Smith, 1984, Lübken et al., 1987, Blix et al., 1990, Lübken, 1997), MF radars (Manson and Meek, 1987, Fritts et al., 1998, Hall et al., 1998, Vincent et al., 1998), and VHF radars (Woodman and Guillen, 1974, Fukao et al., 1994, Hoppe and Fritts, 1995, Narayana Rao et al., 2001). VHF echoes in the daytime mesosphere are caused by coherent scattering from irregularities in the refractive index at the Bragg scale (determined by the electron density). In the low-latitude mesosphere, most irregularities and the radar cross-section are consistent with the view as being generated by neutral turbulence (Woodman and Guillen, 1974). Compared with rockets, radar observations have the advantage of more continuous measurements, but the disadvantage of observing turbulence only at one scale size.

The resolution of VHF radar observations at Jicamarca Radio Observatory (JRO) (11.95°S, 76.87°W) has been greatly improved since the early days of mesospheric Doppler measurements (Woodman and Guillen, 1974) and has helped us learn more about the fine structure of the echo layers. We are now able to obtain data with nominally 150 m resolution by using 64-baud coded pulses. A second paper by Lehmacher (this issue) presents cases of Kelvin–Helmholtz instabilities observed in the echoes.

The method of deriving ε from Mesosphere–Stratosphere–Troposphere (MST) radar spectral width measurements has been described and reviewed elsewhere (e.g., Hocking, 1983, Hocking, 1985, Hocking, 1986, Hocking, 1996, Sato and Woodman, 1982). Seasonal studies of turbulence using the spectral width method to estimate ε and eddy diffusivity K have been conducted with several radars (Fukao et al., 1994, Hall et al., 1998, Narayana Rao et al., 2001). Similar studies have not been performed at Jicamarca. In this paper, we present daily variations of ε in the equatorial mesosphere as well as some monthly comparisons.

The Jicamarca radar is the most sensitive 50-MHz VHF radar in the world with the largest power–aperture product. For comparison, Table 1 lists the parameters for Jicamarca, the MU radar in Japan, and the Gadanki radar in India, when they performed similar studies.

The next section explains the method we employed to derive turbulent parameters. Section 3 describes the results, including daily median values for ε. Section 4 compares the results with other studies and Section 5 summarizes our findings.

Section snippets

Data analysis

Jicamarca MST radar receives echoes from the daytime D region caused by coherent backscatter from small-scale fluctuations of the refractive index. Data from 37 days from three years of high-resolution observations have been analyzed to derive energy dissipation rates ε; the dates of observations are listed in Table 2.

We used four independent antenna beams pointed 2.5° off-zenith in geomagnetic north, south, east, and west directions. The four beam set-up makes the estimation of vertical and

Results

Data from three years of observations in the altitudes of 67–80 km daytime D region were analyzed to derive ε (see Table 2). First, we present an example for one day of our observations, 27 May 2003 between 0700 and 1700 LT (Local Time). We show altitude–time maps of SNR and spectral width for the South beam.

The SNR image is displayed in Fig. 2. A wide and strong echo layer is present during most of the day; the bottom side descends from 74 to 72 km. A weaker layer is observed below the main

Discussion

Results from other VHF radars are presented as eddy diffusivities K which are also derived from spectral widths (e.g., Fukao et al., 1994, Narayana Rao et al., 2001). The equationK=βεωB2relates eddy diffusion coefficients with energy dissipation rates. The parameter β is related to Richardson number, and different authors have used different constant values. Lübken (1997) used 0.81 to convert rocket measurements of ε, and Fukao et al. (1994) chose 0.3 in their analysis of VHF radar data. For

Summary

We have analyzed 37 days of Jicamarca mesospheric data from 2003, 2005 and 2006. From the spectral widths of the radar signals, turbulent energy dissipation rates ε were derived using supplementary temperature data from the MSIS model. We have shown daily median profiles for June and September, since they represented consecutive days with consistent, long-lasting layers. On other days, turbulent layers are sparser, and not all altitudes provide reliable median values. Nevertheless, all days show

Acknowledgements

The authors thank Karim Kuyeng for operations and data processing at the Jicamarca radar. The cooperation of the entire staff of the Jicamarca Observatory is gratefully acknowledged. The authors are also grateful to Dr. Jorge L. Chau for his useful comments. This project has been funded by the National Science Foundation under Grants ATM-0422837 and ATM- 0422661. The Jicamarca Radio Observatory is a facility of the Instituto Geophysico del Peru and is operated with support from the National

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    Present address: Lincoln Laboratories, Lexington, MA, USA.

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