Using three and a half years of 50 MHz radar data collected from Pohnpei, Federated States of Micronesia (6.96degreesN, 158.19degreesE geographic, 0.3degrees magnetic dip latitude), we show for the first time that the occurrence frequency of 150 km (or F-1) echoes has a broad but conspicuous maximum during northern summer in this longitude sector with minimal activity in other months. Given a seasonal similarity to sporadic E (E-s), we suggest that an E-s-layer instability [Cosgrove and Tsunoda, 2002] generates a polarization electric field ((E) over right arrow), which maps along geomagnetic field lines to the F-1 region. There, (E) over right arrow forms thin plasma sheets that provide gradients to excite 3-m-scale plasma waves via an interchange process.

Simultaneous measurements of type‐2 echoes in the equatorial electrojet (EEJ) and the 150 km echoes were made with a tri‐beam radar on Pohnpei (Federated States of Micronesia). Using the mean Doppler velocity (〈VD〉) to estimate the line‐of‐sight component of electron drift velocity (), we show that its vertical component (Vez) increases with altitude and exceeds Vez measured at 150 km, between 96 and 103 km. At 103 km, the ratio of vertical 〈VD〉 to Vez at 150 km was five. This enhancement explains the hitherto puzzling east‐west asymmetry in 〈VD〉 of type‐2 echoes. More importantly, we show how the enhanced Vez appears to be associated with the discrepancy in the altitude of peak current density of the EEJ, between measurement and model. We discuss roles played by kilometer‐scale irregularities and close by describing apparent effects by atmospheric gravity waves.

Data collected during the Maritime Continent Thunderstorm Experiment (MCTEX) (10 November-10 December 1995) have been used to analyze boundary layer development and circulations over two almost flat, tropical islands. The two adjacent islands have a combined length of about 170 km from east to west and 70 km from north to south. Intense thunderstorms formed over these islands every day of the field campaign. The boundary layer depth, temperature, and circulation over the island have been measured over the full diurnal cycle using a multiple radar analysis combined with surface and radiosonde measurements. On average, the island boundary layer depth reaches 1.5 km by early to midafternoon coinciding with the development of the deep convection. Thus, the island boundary layer is significantly deeper than the typical tropical oceanic boundary layer. In the midafternoon, thunderstorm outflows and their associated cold pool stabilize the lower boundary layer, suppressing late convection. This is followed by a period of partial boundary layer recovery for 1-2 h. After sunset, cooling leads to a deepening ground-based inversion below a residual mixed layer. Near the island center, the residual mixed layer of island-modified air is replaced by air of oceanic origin by about 2300 LST (local standard time) that then persists until sunrise the next day. The advection of boundary layer air of oceanic origin over the islands every evening resets the boundary layer development cycle. It is shown that much of the variation in the diurnal temperature profile is a result of thunderstorm activity, radiative processes, and the advection of island and oceanic boundary layer air.

Abstract Profilers operating in the UHF range are sensitive to both Bragg scattering from radio refractive index structure and to Rayleigh scattering from small point targets. Identification of the scattering process is critical for proper interpretation of these observations, especially the data collected from the vertical incident beam. This study evaluates the performance of Doppler velocity thresholds as a means to separate air motions from hydrometeor motions in vertical incident profiler observations. This evaluation consists of three different steps. First, using two collocated profilers operating at different frequencies, the observations are unambiguously identified as Bragg or Rayleigh scattering processes. Second, the observations are separated into either air or hydrometeor motion using only the data from one profiler. The third step quantitatively evaluates the performance of the single profiler separation techniques by counting the number of correct classifications and adjusting the count by...

The National Oceanic and Atmospheric Administration's Aeronomy Laboratory has modified a standard 915-MHz profiler for use as a precipitation profiler in support of Tropical Rainfall Measuring Mission ground validation field campaigns. This profiler was modified to look vertically with a fixed dish antenna. It was operated during the Texas and Florida Underflights Experiment (TEFLUN) A in south Texas in April-May 1998 and during TEFLUN B in central Florida in August-September 1998. Collocated with the profiler was a Distromet, Inc., RD-69 Joss-Waldvogel disdrometer in Texas and Florida and a two-dimensional video disdrometer in Florida. The disdrometers are used to calibrate the profiler at the lowest range gates. At higher altitudes, the calibrated profiler reflectivities are compared with observations made by scanning radars such as the Weather Surveillance Radar-1988 Doppler in Dickinson, Texas, and Melbourne, Florida, and the S-band Doppler dual-polarization radar in Florida. The authors conclude that it is possible to use profilers as transfer standards to calibrate and to validate the reflectivities measured by the scanning radars.

We used the ratio of simultaneous observations of radar reflectivity by S- and UHF-band radars together with Hill's model of refractivity fluctuations due to turbulence to infer epsilon, the rate of viscous dissipation of turbulent kinetic energy per unit mass. Observations were made for 25 days from November 13 to December 7, 1995, at 11.4 degrees S, 130.4 degrees E (about 100 km northwest of Darwin, Australia) during the Maritime Continent Thunderstorm Experiment (MCTEX). The 500 m pulse length data covered the height range 872 to 3032 m MSL. The observed distribution of epsilon has a strong diurnal variation, with mean daytime and nighttime values of epsilon of the order of 10(-3) and 10(-5) m(2)s(-3), respectively. With the dual-wavelength technique most non-turbulent echoes (including particulate echoes) are identified and filtered out, since the ensemble of turbulent observations is identified by its conformity to Hill's model. The technique is self-calibrating, requiring only the relative calibration of the two radars using observations during rain, and does not require precise absolute calibration of either radar.

We present new experimental findings that provide insight into the nature of 150‐km echoes. A finding, using data obtained with a 49.92‐MHz radar on Pohnpei (6.96°N, 158.19°E, 0.5° magnetic dip angle), is an unusual east‐west asymmetry in which the altitude profile of echo occurrence depends on viewing direction. The other is the narrowness of Doppler spectral widths associated with these echoes. When considered with other known properties of 150‐km echoes, a scenario for 150‐km echoes emerges in which 3‐m‐scale field‐aligned irregularities (FAI) are imbedded in tilted, sheetlike structures in plasma density. These meter‐scale FAI are envisioned to have narrow Doppler spectral widths if excited directly by a linear plasma instability, and they would consist of a narrow angular spectrum of plane plasma waves if the instability is weak. The latter could produce the observed east‐west viewing asymmetry.

A 3-GHz profiler has been developed by the National Oceanic and Atmospheric Administration's Aeronomy Laboratory to observe the evolution and vertical structure of precipitating cloud systems. The profiler is very portable, robust, and relatively inexpensive, so that continuous, unattended observations of overhead precipitation can be obtained, even at remote locations. The new profiler is a vertically looking Doppler radar that operates at S band, a commonly used band for scanning weather radars (e.g., WSR-88D). The profiler has many features in common with the 915-MHz profiler developed at the Aeronomy Laboratory during the past decade primarily for measurement of lower-tropospheric winds in the Tropics. This paper presents a description of the new profiler and evaluates it in the field in Illinois and Australia in comparison with UHF lower-tropospheric profilers. In Illinois, the new profiler was evaluated alongside a collocated 915-MHz profiler at the Flatland Atmospheric Observatory. In Australia it Was evaluated alongside a 920-MHz profiler during the Maritime Continent Thunderstorm Experiment. The results from these campaigns confirm the approximate 20-dB improvement in sensitivity, as expected for Rayleigh scatter. The results show that the new profiler provides a substantial improvement in the ability to observe deep cloud systems in comparison with the 915-MHz profilers.

A multi-frequency radar that combines millimeter-wave (MMW) and microwave signals provides additional hydrometeor microphysical information that is not available from a single frequency system. The additional information is contained in the Mie scattering and signal extinction experienced by the MMW signal. These effects can be quantified by comparing MMW data, with measurements simultaneously collected at a second, usually lower, frequency. This technique has been used previously to estimate particle size distributions in rain and in deep ice clouds. Unfortunately, entire precipitating cloud columns cannot yet be characterized using these methods because scattering and extinction from melting particles is not well understood at millimeter-wave frequencies.

The motivation for this research is to move in the direction of improved algorithms for the remote sensing of rainfall, which are crucial for meso- and large-scale circulation studies and climate applications through better determinations of precipitation type and latent heating profiles. Toward this end a comparison between two independent techniques, designed to classify precipitation type from 1) a disdrometer and 2) a 915-MHz wind profiler. is presented, based on simultaneous measurements collected at the same site during the Intensive Observing Period of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. Disdrometer-derived quantities such as differences in drop size distribution parameters, particularly the intercept parameter N-0 and rainfall rate, were used to classify rainfall as stratiform or convective. At the same lime. profiler-derived quantities, namely, Doppler velocity, equivalent reflectivity, and spectral width, from Doppler spectra were used to classify precipitation type in four categories: shallow convective, deep convective, mixed convective-stratiform. and stratiform,Overall agreement between the two algorithms is Found to be reasonable, Given the disdrometer stratiform classification, the mean profile of reflectivity shows a distinct bright band and associated large vertical gradient in Doppler velocity, both indicators of stratiform rain. I;or the disdrometer convective classification the mean profile of reflectivity lacks a bright band, while the vertical gradient in Doppler velocity below the melting level is opposite to the stratiform case. Given the profiler classifications, in the order shallow-deep-mixed-stratiform, the composite raindrop spectra for a rainfall rate of 5 mm h(-1) show an increase in D-o, the median volume diameter, consistent with the dominant microphysical processes responsible for drop formation. Nevertheless, the intercomparison does reveal some limitations in the classification methodology utilizing the disdrometer or profiler algorithms in isolation. In particular, 1) the disdrometer stratiform classification includes individual cases in which the vertical profiles appear convective, but these usually occur at times when the disdrometer classification is highly variable; 2) the profiler classification scheme also appears to classify precipitation too frequently as stratiform by including cases that have small vertical Doppler velocity gradients at the melting level but no bright band; and 3) the profiler classification scheme includes a category of mixed (stratiform-convective) precipitation that has some features in common with deep convection (e.g., enhanced spectral width above the melting level) but other features in common with stratiform precipitation (e.g., well-developed melting layer signature). Comparison of the profiler-derived vertical structure with disdrometer-determined rain rater reveals that almost all cases of rain rates greater than 10 mm h(-1) are convective. For rain rates less than 5 mm h(-1) all four profiler-determined precipitation classes are well represented.

In this paper we review the development and application of Doppler radar profilers to precipitation measurement with emphasis on ground validation of satellite precipitation measurements such as are obtained with the TRMM satellite. The principal objective of this research is to develop the capability to obtain height-resolved drop-size spectra from the profiler-observed Doppler velocity and equivalent reflectivity measurements. Profiler measurements can also be used in conjunction with disdrometers to calibrate ground-based measurements of equivalent reflectivity. Two profilers operating near 1 GHz and 3 GHz respectively, were used in conjunction with scanning radars and disdrometers during 1998 in Texas and Florida. In this paper we present preliminary comparisons of profiler, scanning radar and disdrometer observations obtained in Houston, Texas in April 1998.

A 2835-MHz (10.6-cm wavelength) profiler and a 920-MHz (32.6-cm wavelength) profiler were collocated by the NOAA Aeronomy Laboratory at Garden Point, Australia, in the Tiwi Islands during the Maritime Continent Thunderstorm Experiment (MCTEX) field campaign in November and December 1995. The two profilers were directed vertically and observed vertical velocities in the clear atmosphere and hydrometeor fall velocities in deep precipitating cloud systems. In the absence of Rayleigh scatterers, the profilers obtain backscattering,a from the refractive index irregularities created from atmospheric turbulence acting upon refractive index gradients. This kind of scattering is commonly referred to as Bragg scattering and is only weakly dependent on the radar wavelength provided the radar half-wavelength lies within the inertial subrange of homogeneous, isotropic turbulence. In the presence of hydrometeors the profilers observe Rayleigh backscattering from hydrometeors much as weather radars do and this backscatter is very dependent upon radar wavelength, strongly favoring the shorter wavelength profiler resulting in a 20-dB enhancement, of the ability of the 2835-MHz profiler to observe hydrometeors. This paper presents observations of equivalent reflectivity, Doppler velocity, and spectral width made by the collocated profilers during MCTEX. Differential reflectivity is used to diagnose the type of echo observed by the profilers in the spectral moment data. When precipitation or other particulate backscatter is dominant, the equivalent reflectivities are essentially the same for both profilers. When Bragg scattering is the dominant process, equivalent reflectivity observed by the 1-GHz profiler exceeds the equivalent reflectivity observed by the 3-GHz profiler by approximately 18 dBZe. However, when the 3-GHz profiler half-wavelength is smaller than the inner scale of turbulence, the equivalent reflectivity difference exceeds 18 dBZe, and when both Rayleigh scattering and Bragg scattering are observed simultaneously, the equivalent reflectivity difference is less than 18 dBZe. The results obtained confirm the capability of two collocated profilers to unambiguously identify the type of echo being observed and hence enable the segregation of "clear air" and precipitation echoes for studies of atmospheric dynamics and precipitating cloud systems.

Two different frequency radar wind profilers (920 and 50 MHz) were used to retrieve rain rates from a long-lasting rainfall event observed near Darwin, Northern Territory. Australia, during the 1993-94 wet season. In this technique, 50-MHz data are used to derive the vertical air motion parameters (vertical velocity and spectral width): the 920-MHz data are then used to obtain the precipitation characteristics with the vertical air motion corrections. A comparison of the retrieved rain rates with rain gauge measurements shows excellent agreement, A detailed examination of the mean vertical Velocity and spectral width corrections in the rain retrieval shows that the error due to an uncorrected mean vertical velocity can be as large as 100%, and the error for an uncorrected spectral width was about 10% for thr range of mean vertical velocity and spectral width considered. There was a strong functional dependence between the retrieved mean vertical velocity and percentage difference between observed and retrieved rain rates with and without vertical air motion corrections. The corresponding functional dependence with and without the spectral width corrections was small but significant. An uncorrected upward mean vertical velocity overestimates rain rates, whereas an uncorrected downward mean vertical velocity underestimates rain rates. Uncorrected spectral width estimates have a tendency to overestimate rain rates. There are additional errors in the width correction because of antenna beam mismatching. A mc thud is discussed to quantitatively evaluate this effect. and it is shown to be relatively small compared to the first-order mean vertical velocity correction.

Measurements and simulations of multiple-wavelength radar scattering have demonstrated the feasibility of using multiple-wavelength systems to estimate effective hydrometeor size in ice-phase clouds, and in various forms of precipitation. Radar reflectivity differences occur when the higher frequency experiences non-Rayleigh scattering. For cloud particle sizing, the higher frequency must exhibit non-Rayleigh scattering from small hydrometeors. This occurs at mm-wavelengths where radar systems typically operate in one of several atmospheric transmission windows centered near 35 GHz, 95 GHz, 140 GHz and 215 GHz. The authors present realistic electromagnetic simulations of scattering from ice crystals and spheres that show the importance of ice crystal shape and orientation effects for particle sizing and for calibration. While simulation results show that shape and orientation effects are significant for larger particles they can be neglected for distributions of small, Rayleigh-scattering particles. This implies that calibrations are insensitive to particle shape and orientation.