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The Amazon rainforest plays an important role in the global water and carbon cycle, and though it is predicted to continue drying in the future, the effect of drought remains uncertain. Developments in remote sensing missions now facilitate large-scale observations. The RapidScat scatterometer (Ku band) mounted on the International Space Station observes the Earth in a non-Sun-synchronous orbit, which allows for studying changes in the diurnal cycle of radar backscatter over the Amazon. Diurnal cycles in backscatter are significantly affected by the state of the canopy, especially during periods of increased water stress. We use RapidScat backscatter time series and water deficit measurements from dendrometers in 20 trees during a 9 month period to relate variations in backscatter to increased tree water deficit. Morning radar bacskcatter dropped significantly with increased tree water deficit measured with dendrometers. This provides unique observational evidence that demonstrates the sensitivity of radar backscatter to vegetation water stress, highlighting the potential of drought detection and monitoring using radar.

A higher frequency of severe droughts under warmer temperatures is expected to lead to large impacts on global water and carbon fluxes and on vegetation cover—including possible widespread mortality. Monitoring the hydraulic state of vegetation as represented by the canopy water content will allow rapid assessment of vegetation water stress. Here we show the potential of active microwave backscatter observations at Ku band for monitoring the diurnal and seasonal variations of top-of-canopy water content. We focus on the humid tropical forests of Central Africa and examine spatiotemporal variations of radar backscatter from QuikSCAT (2001–2009) and RapidScat (2014–2016). Diurnal variations in RapidScat backscatter demonstrate the occurrence of widespread midday stomatal closure in this region. Increases in backscatter during the dry seasons in humid forests could be explained by both dry season leaf flushing (as supported by canopy structure) and vapor pressure deficit-driven increases in evapotranspiration rates.

In general, given an oceanic thermal front, there is a strong positive correlation between sea surface temperature (SST) gradients and surface winds, and the marine atmospheric boundary layer (MABL) is unstable over the warm side of the oceanic thermal front. The Gulf Stream is a notable example of an oceanic thermal front and its warm side is often detected as enhanced backscatter in Synthetic Aperture Radar (SAR) images. However, in some “anomalous” SAR images, low backscatter is sometimes observed on the warm side of the front, which seems inconsistent. Therefore, we propose a mechanism to interpret the generation of the low backscatter, based on interactions between ocean surface wind waves and intense nonlinear Ekman divergence. This mechanism is verified by showing that patterns in an observed “anomalous” SAR image are in good agreement with those in the simulated radar signature. In addition, this methodology and analysis demonstrate that SAR is potentially important for detecting and diagnosing small scale air-sea interactions and upper ocean dynamics with strong vertical transports induced by submesoscale processes.

The postsunset vortex in the equatorial ionosphere exhibits clockwise plasma motions after sunset in longitude (time) and altitude coordinates when the equatorial ionosphere is viewed looking northward. We describe the typical morphology of the postsunset vortex using incoherent scatter radar observations at Jicamarca in Peru during the previous solar maximum (2000–2002). A pronounced vortical plasma motion appears around 1700 LT along with the onset of the prereversal enhancement (PRE). The center of this vortex is located near an altitude of 270 km. A smaller-scale vortex also appears about 0.5 ~ 1 h later at higher altitudes. However, the morphology and occurrence time of this small vortex depend on the characteristics of the coherent backscatter region. We find that the earlier vortex is the major feature of the postsunset vortices because it is repeatable, associated with the PRE, and independent to the occurrence of the coherent backscatter region.

This paper uses spaceborne lidar data to study how near-cloud aerosol statistics of attenuated backscatter depend on cloud fraction. The results for a large region around the Azores show that: (1) far-from-cloud aerosol statistics are dominated by samples from scenes with lower cloud fractions, while near-cloud aerosol statistics are dominated by samples from scenes with higher cloud fractions; (2) near-cloud enhancements of attenuated backscatter occur for any cloud fraction but are most pronounced for higher cloud fractions; (3) the difference in the enhancements for different cloud fractions is most significant within 5km from clouds; (4) near-cloud enhancements can be well approximated by logarithmic functions of cloud fraction and distance to clouds. These findings demonstrate that if variability in cloud fraction across the scenes used to composite aerosol statistics are not considered, a sampling artifact will affect these statistics calculated as a function of distance to clouds. For the Azores-region dataset examined here, this artifact occurs mostly within 5 km from clouds, and exaggerates the near-cloud enhancements of lidar backscatter and color ratio by about 30. This shows that for accurate characterization of the changes in aerosol properties with distance to clouds, it is important to account for the impact of changes in cloud fraction.

Sporadic E ionization layers have been observed in the daytime subauroral ionospheric E layer by a 30 MHz radar in Alaska. The radar detects coherent backscatter from meter-scale field-aligned plasma density irregularities. The irregularities were generated by ionospheric modification—by the emission of strong HF electromagnetic waves directly beneath the layers—making the layers visible to the radar. Aperture-synthesis methods are used to generate imagery of the layers from the radar data. The layers are patchy, with patches organized along fronts spaced by tens of kilometers and propagating slowly toward the southwest. Similar, naturally occurring layers are commonly observed at middle latitudes at night in the absence of ionospheric modification. That the patchy layers can be found at high magnetic latitudes during the day argues that they are most likely produced through the interaction of the ionospheric layer with neutral atmospheric waves and instabilities. Attenuation of the radar echoes when the HF emission frequency exceeded the third harmonic of the electron gyrofrequency was observed and is discussed.

We present a technique for the automatic detection of Pc5 (150 s to 600 s period) ultralow frequency (ULF) pulsations in ground and ionospheric backscatter from the Super Dual Auroral Radar Network (SuperDARN). This new technique enables rapid identification and visualization of ULF oscillations over the very wide geographical coverage of SuperDARN. The technique detects ULF oscillations using the Lomb-Scargle periodogram method, which provides a natural test for periodic behavior against the null hypothesis of a pure noise distribution. This does not require any interpolation across data gaps and is thus appropriate for SuperDARN data. We demonstrate the application of the technique to SuperDARN data for March 2014 and find that Pc5 pulsations are frequently observed by multiple radars simultaneously. A preliminary investigation using data from all Northern Hemisphere SuperDARN radars indicates that Pc5 pulsation activity occurs most often on the nightside of the magnetosphere, with a mean frequency of about 2 mHz.

[1] Formation of broad plasma depletions (BPDs) at night in the equatorial F region is understood in association with plasma bubbles. However, we report BPDs that do not show a connection with bubbles. The characteristics of BPDs are investigated using the observations of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite on 31 December 2008, 28 July 2010, and 1 February 2011. On those days, BPDs are detected in the longitude regions where C/NOFS did not detect bubbles prior to the detection of BPDs. The coincident C/NOFS and radar observations over Jicamarca in Peru show the occurrence of BPDs at the time when backscatter echoes are absent and at the height below backscatter echoes. These observations indicate that bubbles are not a prerequisite for those BPDs. The detections of those BPDs can be explained by the uplift of the equatorial F region peak height above the satellite orbit.

The first results of a multisensor airborne survey conducted off the western Iberian Coast are presented (including visible, lidar, and infrared imagery) and reveal the presence of internal solitary waves (ISWs) propagating into the nearshore region. For the first time, two-dimensional lidar imagery is shown to detect the presence of ISWs, and the results are interpreted in a more comprehensive framework provided by the remaining instrumentation. Sea surface roughness patterns, resulting from the ISWs, are found to be imaged in the lidar data, where specular reflection causes slicks to appear as areas of significantly reduced backscatter. Moreover, the lidar data reveal an unprecedented view into the ISW surface and subsurface structures. Possible interpretations are discussed based on the accumulation of surfactants and air bubble entrainment at the leading edge of the ISWs (where maximum convergence occurs).

[1] When Langmuir waves are driven by an electron beam to large amplitudes, they can undergo electrostatic (ES) decay to smaller wave numbers via a series of backscatters. Truncated ES decay, where the number of backscatters is reduced due to damping, is modeled here using the three-dimensional ES Zakharov equations. Langmuir beats develop in “snake”-like structures parallel to the electron beam direction and are most evident when decay is truncated to a single backscatter. From these results, an analytic form is derived and shown to be consistent with some of the waveforms and spectra observed by STEREO in the source regions of type III solar radio bursts. The agreement between the model and observations provides strong evidence for ES decay and Langmuir “snakes” parallel to the electron beam and so the ambient magnetic field.

Contrail backscatter and polarization measurements made using a 532-1064 nm LIDAR in Florence (Italy) indicated that, in most of the cases, the contrail depolarisation and color ratio resemble the usual cirrus. In 20% of the cases, a depolarisation below 15% was observed, both in isolated contrails and in cirrus-embedded contrails. In a few cases, a very low depolarisation, down to 3%, was observed at temperatures in the -40° to -65° C range. In two measurements, a low depolarization was coupled with a high color ratio in the 2-4 range. The low-depolarisation, high color ratio contrail observations are discussed in the light of the results of recent T-matrix scattering simulations. The two events lasted at least 10-20 minutes in the LIDAR data, suggesting a very slow growth or no growth at all of the initial micron-sized contrail particles in the large ice crystals typical of cirrus.

The stratospheric aerosol layer above Mauna Loa Observatory (MLO), Hawaii, has been at low background levels for the past 5 years. This is the first time that an extended non-volcanic background aerosol period has been observed since modern measurements began in the early 1960s. Lidar backscatter at 532 nm shows a distinct maximum in winter and minimum in summer. The five annual cycles have included three easterly phases and two westerly phases of the quasibiennial oscillation (QBO). Differences in aerosol backscatter versus altitude profiles are seen for different QBO phases. There is also a switching of about 25% in the magnitude of the aerosol backscatter on a weekly time scale with varying particle size derived from multiwavelength data. Assumption of a tropical particle source at background suggests that the differing particle regimes are tropical and midlatitude.

An analysis of mountain polar stratospheric cloud observations by solar absorption FTIR from Kiruna/Sweden was performed and compared with co-located airborne lidar measurements. The infrared cloud spectral signature and optical depth indicated water ice particles compatible with the high backscatter and depolarization ratio of the lidar signal. Retrieval of mean particle properties from the FTIR spectrum depend on the assumed width of the particle size distribution and range from 2.0 to 1.1 µm radius and from 1.9 to 5.1 cm−3 number density. The volume density is well determined with 63.8–60.2 µm³/cm³ equivalent to about 2.5 ppmv of condensed water at 20 hPa. Simulations of lidar backscatter ratio based on these results are consistent with the lidar observations.

Polar mesospheric clouds (PMCs) were observed by a ground-based Fe Boltzmann temperature lidar for 65 h on 17–19 and 24–26 January 2000 above the geographic South Pole. The mean PMC backscatter ratio, volume backscatter coefficient, total backscatter coefficient, layer centroid altitude, and layer rms width are 53.5, 2.9 × l0−9 m−1sr−1, 4.3 × 10−6 sr−1, 85.37 km, and 0.78 km, respectively. Strong semidiurnal and diurnal oscillations were observed in the PMC backscatter ratio, volume backscatter coefficient, total backscatter coefficient, and centroid altitude. The oscillations are all maximum around 0630 and 1900 UT. The variations appear to be linked to vertical advection of the PMC scattering layers by a persistent oscillation in the vertical wind velocity. Scattering is strongest when the PMCs are highest which suggests that the colder temperatures at higher altitudes near the mesopause facilitate the formation of larger PMC particles.

The Valensole high frequency (HF) radar in the south of France is an ionospheric Doppler sounder which can perform E region coherent backscatter measurements over an azimuthal sector of 86°, from 26° E to 58° W, with ∼2° angular resolution. This large azimuthal coverage is taken advantage of in order to study quasiperiodic (QP) echoes in the zonal direction using azimuth-time-intensity (ATI) analysis. ATI plots show sequential sloping striations of scatter reminiscent of those detected routinely in the range-time-intensity (RTI) plots of midlatitude radars which view the medium at a fixed azimuth about the meridian. It was found that ATI striation periods range from a few minutes to less than 30 min, whereas the striation slopes are systematically negative (motions westward) prior to local midnight, and turn positive (motions eastward) in the post-midnight hours. The zonal rates, dx/dt, computed from the striation slopes take values between ∼30 and 160 m/s. These are due to real motions of unstable plasma structures, most likely sporadic E patches that drift along with the neutral wind, that have zonal scale lengths of several tens of kilometers. The present observations imply that the mechanism responsible for QP echoes is independent of azimuth and can basically operate effectively in any direction in the horizontal plane.

Monthly sonde data for Lauder in Central Otago, New Zealand show profiles of aerosol backscatter from the surface to over 30 km altitude. The tropospheric data vary by season, with greater aerosol backscatter throughout the free troposphere in springtime. Aerosol mixing ratios in layers in the upper troposphere at these times are often much higher than anywhere else above the boundary layer, suggesting that they arise from horizontal transport. Ozone measurements from the sonde show correspondence in vertical structure to the backscatter data and also seasonal enhancement. The latter correlates with aerosol, but competing causes of ozone enhancement make the correspondence indistinct. High concentrations of carbon monoxide are observed by Fourier transform spectroscopy in spring, and altitude profiles derived from line shape suggest that the peak in CO occurs in the same altitude range as the aerosol enhancement.

Resonance Lidar observations of neutral sodium and VHF coherent scatter radar observations of field-aligned 3-meter irregularities were obtained during the Coqui II rocket campaign in Puerto Rico. The Lidar, a facility instrument at the Arecibo Observatory (18.3°N, 66.8°W), and the University of Illinois Radar, located near Salinas on the south of the island, both monitored volumes near where the uplegs of the nominal rocket trajectories intersected the E-region. The Observatory's Incoherent Scatter Radar was also used to characterize the plasma layers. Preliminary investigation of the data sets has shown a potential correspondence between VHF backscatter from plasma layers and a new class of characteristic enhancements in the neutral sodium.

On 25 January 1998 mountain induced gravity waves developed over Scandinavia leading to the formation of mesoscale polar stratospheric clouds (PSCs). Balloon-borne mass spectrometric measurements of particle composition and optical backscatter measurements were performed above Kiruna/Sweden. PSCs were encountered twice, showing a correlated increase in the condensed phase water, nitric acid and the backscatter ratio. Thermodynamic modeling allows the PSC particles to be unambiguously identified as supercooled ternary solution (STS) droplets, but cannot account for the measured scatter in the particulate HNO3∶H2O mole ratio. Simultaneous temperature measurements show that the particles were subject to rapid atmospheric temperature fluctuations of ±1 K and cooling/heating rates exceeding 1 K/min caused by the gravity waves. Micro-physical non-equilibrium modeling of STS droplet distributions reveals that the observed temperature perturbations cause particle compositions in close agreement with the measured HNO3∶H2O variations. Non-equilibrium compositions of liquid PSC particles are thus a principal stratospheric characteristic related to gravity waves affecting particle evolution.

A combined coherent backscatter-incoherent scatter experiment with the Millstone Hill UHF radar provided simultaneous observations of electric field magnitude and coherent backscatter parameters on the same L shell. A carefully-designed geometry used sidelobe coherent contamination from two-stream irregularities at 110 km altitude, appearing at ranges corresponding to F-region altitudes in the main beam, in conjunction with simultaneous uncontaminated F-region observations of the drift velocity in adjacent range gates. Both logarithmic coherent power and the magnitude of the coherent phase velocity Vph vary linearly with . With the assumption that the coherent phase velocity is approximately the perturbed ion sound speed in the heated E region, we find an excellent agreement between the electron temperature inferred from Vph and previous incoherent-scatter results relating E-region Te to E. A maximum value of ∼3100 K has been found for such wave-induced E-region heating.

A series of campaigns has been carried out in the Caribbean over a one-year period to study intense mid-latitude spread-F events using a cluster of diversified instrumentation. These events are relatively rare but a number of them have now been captured and will be discussed in this and several companion papers. This paper focuses on 630 nm airglow images obtained by the Cornell All-Sky Imager for two of the more spectacular cases that began on February 17, 1998 and February 17, 1999. In the latter case, and for the first time, a poleward surge of depletion/enhancement airglow zones was captured by radar as well as an airglow imager. In the former case structures grew in place overhead and produced strong VHF F-region backscatter as observed by the CUPRI and University of Illinois radars; the other event, exactly one year later, did not result in detectable 3-m backscatter. The two data sets show quantitatively that the low airglow region is elevated in height and depleted in plasma density and Pedersen conductivity. We suggest an enhanced eastward electric field inside the low conductivity zone may be responsible for the surge. The data also suggest small scale turbulence can only be observed in developing structures.

Advantages introduced by Raman lidar systems for cloud base determination during precipitating periods are explored using two case studies of light rain and virga conditions. A combination of the Raman lidar derived profiles of water vapor mixing ratio and aerosol scattering ratio, together with the Raman scattered signals from liquid drops, can minimize or even eliminate some of the problems associated with cloud boundary detection using elastic backscatter lidars.

Model calculations suggest that PSC-Ia optical properties measured by lidar at ultraviolet and visible wavelengths are strongly correlated; particle depolarization ratios and backscatter coefficients are 17% smaller and 41% larger at a wavelength of 532 nm than at 355 nm, respectively. The model relations are used to compute 532-nm particle backscatter and depolarization ratios from 355-nm lidar data of a PSC Ia-enhanced, i.e., a PSC of type Ia with unusually high backscatter ratios, measured with a UV Raman lidar over Esrange, Sweden (67.9° N, 21.1° E), on 20 Jan 1997. The calculated optical properties agree well with lidar observations at 532 nm of this PSC type.

High resolution HF radar observations of ULF waves in the ionosphere are possible by artificially generating irregularities using the EISCAT heater at Tromso, Norway. The line-of-sight velocities from the CUTLASS radars have been combined for the first time to form bistatic flow vectors during a ULF wave, generating ionospheric electric field measurements with a higher accuracy than ever previously recorded by any instrument. A high-m wave observed in the ionosphere exhibits a frequency which is a harmonic of that of a low-m field line resonance which is observed simultaneously in the ionosphere and at the ground. The high-m pulsation resembles a class of particle driven wave previously recorded on VHF radars. These results are consistent with recent HF radar observations of small scale waves possessing similar characteristics to field line resonances and with the theory that the low-m wave may be driving the high-m wave through a non-linear Kelvin-Helmholtz instability.

Forward scattering in cirrus clouds can significantly affect backscatter lidar signal. Its contribution can be estimated numerically knowing the size and shape of the crystals. Contemporary airborne and space-borne lidar measurements collected during the European Lidar In-space Technology Experiment (E-LITE) field campaign (in the framework of the NASA/LITE mission) are used to show that forward scattering reduces the optical depth derived from space-borne measurements by at least a factor of 2 with respect to the true value. This reduction factor is discussed in terms of crystal size and shape (i.e. forward scattering, and measurement configuration). It is shown that cirrus backscatter-to-extinction ratios derived from airborne and space-borne lidar measurements can be used to illustrate the sensitivity of multiple scattering effects the ice crystal size and shape.

From June to August 1998 the ALOMAR Rayleigh/Mie/Raman lidar, located at 69°N and 16°E in Northern Norway, repeatedly observed noctilucent clouds (NLCs) overhead the lidar. Due to a recent upgrade in detector technology, the lidar was able to obtain 151 hours of NLC observations, simultaneously at 355, 532, and 1064 nm. For the 11 strongest NLC events, we have calculated size distributions for the NLC particles from the backscatter ratios measured at the 3 wavelengths and using the assumptions of spherical ice particles with a monomodal lognormal size distribution. For all events evaluated at the layer maxima, we obtain well-defined median radii rmed, width parameters σ, and particle number densities NNLC for the NLC particle distributions. Mean values for 10 out of the 11 events are rmed = 51 nm, σ = 1.42, and NNLC = 82 cm−3.

In this paper a large database of midlatitude E region coherent backscatter, obtained with a 50 MHz Doppler system, is used to investigate the long-term variability in echo occurrence. The backscatter is found to be dominated by pronounced quasi-periodic variations with periods in the range from about 2 to 9 days that persist for time intervals from about 10 to maybe more than 20 days and have no relation to geomagnetic activity. The most commonly observed periods appear in two preferential bands, that is, the 2 to 3 day and the 4 to 6 day band. Using concurrent ionosonde data we find the variations in backscatter to be exactly in-phase with similar periodicities in the occurrence of relatively strong sporadic E layers. The present findings support the possibility that planetary waves are responsible for the observed long-term periodicities which indicates also a close relation between planetary waves and the well known, but not well understood, seasonal Es dependence. We suggest, the planetary wave option constitutes a new component into the research of midlatitude sporadic E layer formation and occurrence that needs to be considered and closely investigated.

We present first results from a search for quasi-periodic (QP) radar echoes in the American sector. These echoes, which are produced by underdense backscatter from field-aligned irregularities in nighttime midlatitude sporadic E (Es), have been observed in the Asian sector but never before at other longitudes. Using radar data collected during the summer in 1998 from Stanford, California, we show that QP echoes can indeed be detected in the American sector and that they resemble those found in central and southern Japan. We then show that the invariance of a feature in the pattern of QP echoes, which persisted for five hours on 3 August 1998, raises questions regarding the existing models for QP echoes.

Polar stratospheric clouds (PSC) were observed above Andenes, Norway (69°N, 16°E) with the ALOMAR Rayleigh/Mie/Raman-Lidar during three winters and with an optical particle counter on two occasions. Multiwave-lengths measurements allow the derivation of the parameters of a monomodal particle size distribution and the derivation of volume densitiespaper if the refractive index is known. The particle counter results are used to determine the best refractive index value. The relation of volume density to temperature is compared to thermodynamic models. This confirms that the PSCs observed consist mainly of a ternary solution with low water content. PSC Type I a and I b can be distinguished by backscatter ratio, color ratio and cooling rate.

Observations from the CUTLASS Finland coherent HF radar on 23 February 1996 are employed to demonstrate that changes in propagation mode from 1/2F to 1 1/2F and back again, determined from elevation angle measurements, do not significantly alter the ranges over which ionospheric backscatter is observed. This indicates that the latitudinal extent of backscatter in the dayside auroral oval and cusp region correspond to the boundaries of geophysical processes, as opposed to limits in the illumination of the F region ionosphere by the radar. Hence, the HF radar technique is confirmed as an excellent diagnostic of the cusp and other dayside regions.

We report three new findings from the first observations of quasi-periodic (QP) radar echoes at 24.5-MHz. Most interesting is that QP echoes produced by 6.1-m field-aligned irregularities (FAI) can occur at altitudes as high as 150 km. Because Hall currents are negligible there, the existence of these FAI require interpretation at least in terms of a generalized gradient-drift instability that allows for (1) a transition from a Hall current at low altitudes to a Pedersen current at high altitudes, and (2) ion magnetization effects at altitudes above the lower E region. The second finding is the occurrence of extreme spectral broadening that often accompanies the strongest echoes. The third finding, from data accumulated over a two-month period, is evidence that the longitudinal gradient in conductivity associated with the solar terminator plays a role in the first appearance of QP echoes in the even-ing.

During June/July 1997 we observed with a groundbased lidar noctilucent clouds (NLCs) overhead the ALOMAR observatory in Northern Norway (69°N, 16°E). These daylight observations of NLC were made possible by use of a very small field-of-view for the lidar receiving telescope in combination with a narrowband double-etalon Fabry-Perot interferometer in front of the photon counting detectors. During 148 hours of lidar measurements, NLCs were observed for 69 hours. Thus, the mean occurrence frequency of NLC was 46%. The observed NLCs exhibited marked variations of the layer altitude and backscatter ratio with local time (LT). The temporal behaviour of both the NLC altitude and NLC backscatter ratio are dominated by a semidiurnal variation. The NLC altitude attains maxima near 13 LT and 01 LT and minima near 07 LT and 19 LT. The backscatter ratio clearly maximizes at times of minimum NLC layer altitude. These observations point to a strong control of variations in NLC altitude and backscatter ratio by a stable semidiurnal tide. The mean centroid NLC altitude is 82.7 km.

An angular scanning backscatter lidar was deployed on the NASA DC-8 research aircraft as part of SUCCESS. The lidar viewing direction could be continuously scanned from vertically upward to forward to vertically downward. Real-time pictorial displays generated from lidar signatures were used to locate clouds and contrails above, ahead of, and below the DC-8; to depict their spatial structure; and to help select DC-8 altitudes for achieving optimum sampling by onboard in situ sensors. The lidar data are being analyzed to establish their value in the interpretation and extension of the in situ sensor databases. Data examples are presented that illustrate (1) correlation with particulate, gas, and radiometric measurements made by onboard sensors, (2) discrimination and identification between contrails observed by the onboard sensors, (3) a 13.1 km altitude layer that exhibits greatly enhanced vertical backscatter relative to off-vertical backscatter, and (4) mapping of vertical distributions of individual precipitating ice crystals and their capture by cloud layers.

We show that atmospheric UV/visible backscatter spectra obtained by the Global Ozone Monitoring Experiment (GOME) spectrometer on board the ESA ERS-2 satellite may be used to retrieve column amounts of key trace species associated with smoke cloud combustion from biomass burning events. This paper focuses on the recent rain forest burning in SE Asia (August–October 1997). For ground scenes with low cloudiness, differential absorption fitting applied to backscatter spectra yields column distributions of NO2 and H2CO in and around smoke-polluted regions. A two-fold increase in the vertical NO2 content is apparent over large parts of the smoke cloud; this clearly indicates the ability of GOME to measure tropospheric NO2 content. H2CO is detected only in areas closest to combustion sources. Slant column amounts in the range 2.5 - 4 × 1016 mol cm−2 have been determined; these correspond with previous estimations of vertical columns of H2CO for biomass Savannah burning.

Observations of cirrus clouds were made with a host of instruments on board the NASA ER-2 aircraft on April 16, 1996. These instruments provide radiometric measurements covering the visible, infrared, and millimeter-wave regions of the electromagnetic wave spectrum, as well as profiles of lidar backscatter at 1.064 µm. The cirrus clouds are classified into two groups: the modest cirrus clouds with lidar backscatter from the surface and the intense cirrus clouds without lidar surface returns. For the modest clouds, the magnitudes of brightness depressions are typically less than 5 K and are comparable at 150 GHz and 220 GHz. The depressions for the water vapor channels (e.g. 183.3±7 GHz) are smaller. We interpret the microwave brightness depressions for the modest cirrus cases as due to reduced skin temperature from the cloud shadow, rather than ice scattering. For the intense cirrus cases, the magnitude of brightness depression at 220 GHz reach 35 K and are far greater than those at 150 GHz. After correcting for the skin temperature effect, a regression between the observed brightness depressions at 150 and 220 GHz frequencies results in a slope of 2.75. Comparison with radiative transfer modeling for solid spherical ice particles, indicates a median equivalent mass diameter of about 350 µm, and ice water path ranging up to 300 g/m². The results illustrate the potential utility of millimeter and sub-millimeter wave radiometry for characterizing microphysical properties of thick cirrus.

Dual wavelength aerosol lidar observations of tropical high-altitude cirrus clouds were performed during the ALBATROSS 1996 campaign aboard the research vessel “POLARSTERN” on the Atlantic ocean in October–November 1996. On the basis of 57 hours of night-time observations between 23.5°N and 23.5°S we find in 72% of the altitude profiles indications of the presence of cirrus cloud layers. This percentage drops to 32% at subtropical latitudes (23.5°–30°) based on 15 hours of data. About one-half of the subtropical and tropical cirrus layers are subvisual with an optical depth of less than 0.03 at a wavelength of 532 nm. In general the clouds exhibit high spatial and temporal variability on scales of a few tens of meters vertically and a few hundred meters horizontally. No clouds are observed above the tropopause. An abrupt change in the relation between the color ratios of the parallel and perpendicular backscatter coefficients at about 240 K is interpreted in terms of changes of particle shape and/or size distribution. At temperatures between 195 and 255 K only a small fraction of the observations are consistent with the presence of small particles with dimensions of less than 0.1 µm.

The amplitude variations and Doppler spectra of the 50 MHz radar echoes from meteor trails are investigated. The radar echoes from underdense meteor trails particulary during nighttime do not always fit the conventional specular reflection model. The echo was found to spread, sometimes, over as large as 8–10 km in range. In each range bin, the spread echo from the trail is produced with an altitude dependent time delay after the passage of the meteoroid producing the ionized trail. The Doppler spectra of the spread echo in each range bin is asymmetric with a high frequency wing comprised of narrow spectral peaks and successive spectral peaks amplitude decrease with frequency. These observations are interpreted as due to multiple reflections of the radar pulse between the striations generated parallel to the geomagnetic field along the length of the meteor trail. There is evidence to show that within the dynamo region the EW and NS components of the currents at lower and higher altitudes could be in opposite directions during the early morning hours of Sq electric field reversal.

Hurricane Hortense was observed in the Atlantic Ocean on 12 September 1996 by two high-frequency over-the-horizon radars in Texas and Virginia, looking at backscatter from the ocean surface. Dual radar coverage permits construction of an unambiguous surface-wind-direction field and a vector surface-current field with about 15-km resolution in the storm's vicinity. The known location of a storm band coincides with a region of surface convergence indicated by the radar. The surface-current field lags the wind field by about 3 h and shows the strongest ocean response to the right of the storm track, with near-inertial rotation in the storm's wake. Analysis of surface-wave directionality shows that it depends on the recent wind-direction history at each point.

A SuperDARN HF radar pair (Saskatoon-Kapuskasing) has been used to measure Doppler spectra of backscatter from the dayside F-layer. Normally the spectra are single-peaked, but a small fraction exhibit a double-peaked (D-P) signature. On the 2D convection maps, these D-P spectra occur in range cells located poleward of the convection reversal. A comparison with DMSP SSJ/4 particle data measurements and their mapping to magnetospheric boundaries shows that the D-P spectra are concentrated just equatorward of the magnetopause, in regions of spatially/temporally structured soft electron precipitation (about 300 eV) where the highly variable flux can reach 2–5 ergs/cm²/s. The D-P spectra are most easily explained in terms of scattering from small-scale vortices of size less than the radar resolution of 45 km. The D-P spectral measurements are illustrated by SuperDARN and DMSP data for a dayside event on Feb. 20, 1995, when northward IMF conditions prevailed. We conclude that HF radar D-P observations can be used to map in real time the dayside 2D ionospheric footprint of the outer LLBL.

A favourable conjunction of HF coherent radar backscatter, meridian scanning photometer data and an overflight of the DMSP F13 spacecraft has enabled the study of the ionospheric signature of the cusp with these three important techniques simultaneously. Strong HF backscatter power, poleward-moving red line auroral forms and latitude-dispersed ion precipitation features are all observed to be collocated. The precipitation of ions in the 0.1–2 keV energy range is found to be very closely associated with the production of the F region irregularities detected by the HF radar.

Dual-wavelength aerosol lidar backscatter measurements at Mauna Loa Observatory are used to monitor and characterize the 15–30 km stratospheric aerosol layer. The decay of aerosol loading following the El Chichon, Mexico (17°N) and Pinatubo, Philippine Islands (15°N) volcanic eruptions of 1982 and 1991, respectively, depends on the phase of the quasibiennial oscillation (QBO) in tropical stratospheric winds. Averaged over a 3-year period, these effects are removed and an exponential decay with a characteristic (e−1) decay time of about 1 year is observed for both eruptions. By the end of 1996, about 5 ½ years after the Pinatubo eruption, stratospheric aerosol levels at Mauna Loa had decayed to pre-eruption levels, approximately matching the lowest aerosol levels seen here in the past 17 years (about 6 × 10−5 sr−1 at 694 nm integrated between 15.8 and 33 km). However, this background stratospheric aerosol level at Mauna Loa may depend on the QBO, being slightly lower during the westerly phase. Analyses of aerosol backscatter, backscatter wavelength dependence, and trajectories provide evidence for a minor injection of aerosol from the Rabaul eruption in Papua, New Guinea (4°S) in September of 1994.

During October 1994, a dual-frequency GPS receiver was operated in Alcântara, Brazil, at the same site where the Cornell University Portable Radar Interferometer (CUPRI) was performing spread-F measurements for an ongoing rocket campaign. We present data for two nights during which several spread-F plumes drifted through the field-of-view of CUPRI. Fluctuations in the total electron content (TEC) measured with the GPS receiver were correlated with those plumes. We define a “roughness” measure from a specialized high frequency filter. Plots of “roughness” in a magnetic longitude versus time space show initially low activity followed by regions of dramatic localized onsets and a slower subsequent decline. Features in the roughness image are well-correlated with plumes in the CUPRI backscatter power map.

From early July until mid-August 1995 we observed six noctilucent clouds (NLC) above the ALOMAR observatory in Northem Norway (69°N, 16°E) with ground based lidar. The NLC layers had maximum backscatter ratios between 17 and 190 at 532 nm, their mean altitude was 83.0 km. The latest of these events (August 12/13, 1995) was observed by two lidars, both, the Rayleigh- and the ozone lidar, located in the ALOMAR observatory, on wavelengths of 532 nm and 308 nm, respectively. During the period of common-volume observations from 23:55 UT until 01:30 UT, maximum averaged backscatter ratios of 56 and 28 were measured at 532 and 308 nm, respectively. If we assume spherical particles and a lognormal particle size distribution of width s, standard Mie theory yields median radii r m of the NLC particles between 16 nm and 44 nm with number densities between 2780 cm -3 and 160 cm -3 for assumed values of s between 1.6 and 1.2, respectively.

Maurice A. Jarzembski, I Vandana Srlvastava," Eugene W. McCaul, Jr., 2 Gary J. Jedlovec,1Robert J. Atkinson, 3 Rudolf F. Pueschel, 4 and Dean R. Cutten 5Abstract. Measurements of calibrated backscatter, using twocontinuous wave Doppler lidars operating at wavelengths 9. I and10.6p.m were obtained along with cloud particle size distributionsin Hurricane Juliette on 21 September 1995 at altitude -11.7 km.Agreement between backscatter from the two lidars and with thecloud particle size distribution is excellent. Features inbackscatter and particle number density compare well withconcurrent GOES-7 infrared images•IntroductionLidars can be used to measure backscatter of atmosphericaerosols and clouds [Menzies and Trait, 1994], where thebackscatter magnitude is in part indicative of particle numberdensity. Their high resolution capability provides a uniquemethod of measuring fine-scale atmospheric variations[Srivastava et al., 1995]. Recently, two National Aeronauticsand Space Administration (NASA)/Marshall Space Flight Center(MSFC) continuous wave (CW) focused Doppler lidars obtainedin-situ high resolution calibrated backscatter measurements inHurricane Juliette as part of the 1995 NASA/MulticenterAirborne Coherent Atmospheric Wind Sensor (MACAWS)mission on board NASA's DC8 aircraft. Two traverses ofJuliette's eye were made off the west coast of Mexico at altitude11.7 km on 21 September 1995. The two independent lidar datasets offered an opportunity for intercomparison and validation ofthe calibrated backscatter results. These were also intercomparedwith in-situ cloud particle size distributions obtained fromNASA/Ames Research Center's forward scattering spectrometerprobe (FSSP), the DC8 aircraft infrared (IR) surface temperatureradiometer data, and the Geostationary OperationalEnvironmental Satellites (GOES-7) l 11am IR emission imageswith their corresponding estimates of cloud top temperature andheight. At the time of the DC8 flight through Juliette, GOES-7was in geosynchronous orbit over the equator and 135 ° Wlongitude and provided superior viewing geometry of the easternPacific ocean region• GOES-7 IR data for the 12-h periodbeginning at t600 UTC and spanning the DC8 flight, were usedto monitor the cloud development and structure of Juliette.Meteorology, Flight, and GOES-7 DataJuliette developed off the west coast of Mexico on 16September 1995. It moved slowly west-northwestward andgradually intensified, reaching hurricane strength by 18i NASA/Marshall Space Flight Center, Huntsville, AL2 I.Tniversities Space Research Association, Huntsville, AL3 Lockheed Martin Corp., Huntsville, AL4NASA/Ames Research Center, Moffett Field, CA5 University of Alabama in Huntsville, Huntsville, ALCopyright 1997 by the American Geophysical Union.Paper number 97GL00832.0094-8534/97/97GL-0083250500September. Juliette reached its maximum strength at 2100 UTCon 20 September, with peak winds of 64 m s-_ (125 kt) andminimum central pressure of 925 hPa. At this time, eyewalldecay-replacement cycles were occurring, with the old eyewallcontracting and dissipating while a new, larger annular eyewallformed outside [Willoughby et al., 1982]. Late stages of thiscycle were observed at the time of the DC8 traversal of Juliette'score on 21 September. As the storm moved over cooler water itgradually lost strength, ultimately dissipating on 26 September.During the DC8 flight around and through Juliette, the stormdisplayed a nearly symmetrical, rapidly expanding cirrus outflowshield, according to concurrent GOES-7 imagery. The storm'srainfall pattern, however, exhibited considerable azimuthalasymmetry, with heaviest precipitation located in the storm'score and in strong rainband convection along the storm's southernperiphery, as diagnosed from the 85 GHz channel of the SpecialSounder Microwave/Imager (SSM/I) at 1756 UTC on 21September. Outer rainbands were absent on the storm's northernflanks, perhaps because of the presence of cooler ocean water anddrier midtropospheric air. The small, old eyewall, still evidentwithin the center of the new, larger annular eye, remainedconnected to the new eyewall by a vestigial spiral band structurein the storm's north quadrant, with a veil of thin cirrus coveringthe developing new large eye.Flight through Juliette lasted -2.5-h, during which twotraverses of the eye were made separated by -1.6-h. A GOES-7IR image of Juliette at 2100 UTC (-21 rain. after the first eyepenetration) is shown in Fig.1. Flight track from 2010 to 2250Figure 1. GOES-7 IR image of Hurricane Juliette on 21September 1995 at 2100 UTC. The co-located position of theNASA DC8 flight track is shown from 2010 to 2250 UTC.1063

Zenith-enhanced backscattering (ZEB) of a lidar beam by cirrus clouds is a remarkable phenomenon usually explained in terms of specular reflection from large plane facets of horizontally oriented ice plates. Since the standard geometric optics approximation (GO) may be inapplicable in many cases, especially in analyzing infrared measurements, and ignores physical optics effects, we use the recently improved exact T-matrix method to compute the scattering of light by ice plates at visible and infrared wavelengths. Computations for horizontally and randomly oriented thin disks and oblate spheroids with size parameters up to 50 show that while all particles produce a strong Fraunhofer diffraction peak centered at exactly the forward-scattering direction, a strong and narrow ZEB peak can be produced only by horizontally oriented disks but not by horizontally oriented spheroids or particles in random orientation. This finding demonstrates that ZEB can be produced even by particles which are not in the GO domain of size parameters and supports the traditional interpretation of ZEB. Also, we have found that the angular width of the ZEB peak for horizontally oriented disks is equal to half the width of the Fraunhofer diffraction peak. This result can be used in practice to derive a lower estimate of ice particle sizes from high angular resolution measurements of ZEB. We show that our exact T-matrix computations can explain the peculiar zenith-angle dependence of depolarization observed by Platt et al. [1978] in the visible and can be interpreted qualitatively in terms of the modified Kirchhoff approximation.

Balloon borne frost-point hygrometers and backscatter sondes were launched at Sodankyla, Finland in January and February of 1996. These instruments measure water vapor and the backscatter ratio of light due to polar stratospheric clouds in the Arctic stratospheric vortex. Here we report the results of a hygrometer sonde and a backscatter sonde launched within 3.5 hours of each other on January 22/23. Together these soundings show a strong loss of water vapor due to the formation of ice clouds as a result of record cold temperatures in the Arctic stratosphere. The separation of the upper edge of the layer showing water vapor loss and the upper edge of the PSC layer indicates sedimentation of the ice particle layer, possibly leading to a permanent dehydration in the upper part of the layer exhibiting water vapor loss.

In winter 1995/96, extreme conditions were observed in the lower and middle stratosphere over most of the European Arctic. At the ALOMAR facility in Northern Norway, an ozone DIAL system observed PSCs on every measurement occasion in January and over a 10-days period in February 1996. On January 23, 1996, the PSC layer reached a maximum vertical extension, ranging from 19 to 27 km. In narrow layers, superimposed on the “background” PSC, the backscatter ratio R reached maximum values of 16 at 353 nm and about 9 on 308 nm. Morphology and colour index indicate that these narrow layers were type-II PSCs. Just above the PSC layer, temperatures below 190 K were recorded regularly. The temperatures at the edges of the “background” PSC allow this layer to be composed of STS.

The Solar Backscatter Ultraviolet (SBUV/2) instrument on the NOAA-9 spacecraft made total ozone measurements over Antarctica during the 1994 Austral Spring depletion. These measurements continue those made by the SBUV/2 on NOAA-11. In recent years NOAA-9 drifted from a poor orbit to one where earth observations and calibration capabilities are not restricted. An interim calibration of the NOAA-9 SBUV/2 instrument was established with data from June 1994 and applied to observations during September, October, and November 1994. To validate the NOAA-9 ozone measurements, daily zonal ozone averages from NOAA-9 and NOAA-11 measurements in the Northern Hemisphere have been compared. Comparisons have also been made with ground-based measurements from five Dobson stations dispersed on the Antarctic continent. The results show that, on average, the NOAA-9 data agree to within 1-2 percent with the Dobson stations with standard deviations of the difference ranging from 5.3 to 7.7% and to within several percent with NOAA-11 Northern Hemisphere data when restricted to solar zenith angles less than 80 degrees. This agreement makes possible not only a continuation of the Antarctic measurements without a large instrument-related bias, but also establishes the NOAA-9 data as a suitable transition data set during the replacement of NOAA-11 by NOAA-14.