David A. Krueger

Eight-year climatology of nocturnal temperature and sodium density in the mesopause region (80 to 105 km) over Fort Collins, Co (41°N, 105°W)

Based on 417 nights of lidar observation between 1990 and 1999 at Fort Collins, CO (41°N, 105°W), we tabulate an eight-year composite of monthly mean temperatures and Na densities in the mesopause region, and plot the associated altitude/month contours. The maximum and minimum mean mesopause temperatures were 192 K at 101 km and 178 K at 85.5 km on the 294th and 164th days of the year, respectively. The maximum and minimum monthly mean layer temperatures between 83 and 105 km were 205.8 K and 188.8 K, respectively, in November and July. When the episodic warming that peaked in 1993 is removed, the mean temperatures are typically lowered by ∼4 K. Climatology supports a two-level thermal structure with mesopause occurring at higher altitudes in winter and lower altitudes in summer, and sharp transitions between them taking place in May and August. The maximum and minimum mean peak sodium densities were 4.88 × 109 m−3 at 91 km and 1.49 × 109 m−3 at 90 km on the 310th and 163rd days of the year, respectively.

Climatology of mesopause region temperature, zonal wind, and meridional wind over Fort Collins,Colorado (41°N, 105°W), and comparison with model simulations

[1] Between May 2002 and April 2006, many continuous observations of mesopause region temperature and horizontal wind, each lasting longer than 24 h (termed full-diurnal-cycle observations), were completed at the Colorado State University Na Lidar Facility in Fort Collins, Colorado (41°N, 105°W). The combined data set consists of 120 full-diurnal-cycle observations binned on a monthly basis, with a minimum of 7 cycles in April and a maximum of 18 cycles in August. Each monthly data set was analyzed to deduce mean values and tidal period perturbations. After removal of tidal signals, monthly mean values are used for the study of seasonal variations in mesopause region temperature, zonal and meridional winds. The results are in qualitative agreement with our current understanding of mean temperature and wind structures in the midlatitude mesopause region with an observed summer mesopause of 167 K at 84 km, summer peak eastward zonal wind of 48 m/s at 94 km, winter zonal wind reversal at ∼95 km, and peak summer (pole) to winter (pole) meridional flow of 17 m/s at 86 km. The observed mean state in temperature, zonal and meridional winds are compared with the predictions of three current general circulation models, i.e., the Whole Atmosphere Community Climate Model version 3 (WACCM3) with two different simulations of gravity wavefields, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), and the 2003 simulation of the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM). While general agreement is found between observation and model predictions, there exist discrepancies between model prediction and observation, as well as among predictions from different models. Specifically, the predicted summer mesopause altitude is lower by 3 km, 8 km, 3 km, and 1 km for WACCM3 the two WACCM runs, HAMMONIA, and TIME-GCM, respectively, and the corresponding temperatures are 169 K, 170 K, 158 K, and 161 K. The model predicted summer eastward zonal wind peaks to 71 m/s at 102 km, to 48 m/s at 84 km, to 75 m/s at 93 km, and to 29 m/s at 94 km, in the same order. The altitude of the winter zonal wind reversal and seasonal asymmetry of the pole-to-pole meridional flow are also compared, and the importance of full-diurnal-cycle observations for the determination of mean states is discussed.

High-spectral-resolution lidar with iodine-vapor filters: measurement of atmospheric-state and aerosol profiles.

A high-spectral-resolution lidar can measure vertical profiles of atmospheric temperature, pressure, the aerosol backscatter ratio, and the aerosol extinction coefficient simultaneously. We describe a system with these characteristics. The transmitter is a narrow-band (FWHM of the order of 74 MHz), injection-seeded, pulsed, double YAG laser at 532 nm. Iodine-vapor filters in the detection system spectrally separate the molecular and aerosol scattering and greatly reduce the latter (-41 dB). Operating at a selected frequency to take advantage of two neighboring lines in vapor filters, one can obtain a sensitivity of the measured signal-to-air temperature ratio equal to 0.42%/K. Using a relatively modest size transmitter and receiver system (laser power times telescope aperture equals 0.19 Wm(2)), our measured temperature profiles (0.5-15 km) over 11 nights are in agreement with balloon soundings to within 2.0 K over an altitude range of 2-5 km. There is good agreement in the lapse rates, tropopause altitudes, and inversions. In principle, to invert the signal requires a known density at one altitude, but in practice it is convenient to also use a known temperature at that altitude. This is a scalable system for high spatial resolution of vertical temperature profiles in the troposphere and lower stratosphere, even in the presence of aerosols.

Temperature climatology of the middle atmosphere from long-term lidar measurements at middle and low latitudes

Long-term measurements from several lidar instruments (Rayleigh and sodium) located at 44.0°N, 40.6°N, 34.4°N, and 19.5°N have been used to develop a new climatology of the middle atmosphere temperature. For each instrument, the measurements on every individual day of the year over the entire long-term record were averaged to build a composite year of temperature profiles. These profiles were then interpolated to provide temperature values at 1-km altitude intervals so that the climatology comprises daily temperature values at integer altitudes between 15 and 110 km, depending on the instrument. The climatologies for each lidar were then compared to the CIRA-86 model and to each other. Large differences between the lidar temperatures and the CIRA-86 temperatures are identified and explained. When compared to all instruments, CIRA-86 appears systematically much too cold between 90 and 95 km, by 20 K or more, and possibly 6–8 K too warm around 80 km, making its use as a reference atmosphere model questionable at these altitudes. The annual and semiannual components of the seasonal variability and the 2- to 33-day period variability were investigated. An annual cycle with 6–7 K amplitude in the upper stratosphere, increasing to 15–20 K at 80 km, is observed at midlatitudes. This cycle is in phase with the solar flux in the stratosphere and in opposite phase in the mesosphere with a very cold summer mesopause at 85 km, in good agreement with previous climatologies. At lower latitudes, a semiannual oscillation propagates downward from 85 to 30 km and is characterized by a stronger first cycle than the second (4 and 2 K amplitude respectively). The 2- to 33-day variability at midlatitudes shows a maximum during winter around 40 km and in the mesosphere. The first peak is associated with planetary wave activity and stratospheric warmings and the second to the occurrence of mesospheric temperature inversions. Finally, sudden seasonal transitions, highly consistent between all instruments, have been observed. In particular, in the early winter midlatitudes a two-step warming of the winter mesosphere between 65 and 85 km as well as a cooling of the lower mesosphere appear to be real climatological events rather than some short-term geophysical or instrumental random variability.

Reversible, field induced agglomeration in magnetic colloids

Abstract Reversible agglomeration of magnetic particles (20–300-A diameter) in a colloidal suspension has been induced by applying various magnetic fields: nonuniform static fields and uniform ac or dc fields of magnitudes from 2.5 to 230 Oe. A sensitive Colpitts oscillator circuit monitored the spatial and temporal variations in magnetization in a vertical 11-cm column of magnetic fluid. The results are consistent with the hypothesis that spherical agglomerates of 10 7 –10 9 particles are formed and settle gravitationally. The agglomeration was most pronounced in a commercially available water-base, 200-G ferrofluid; 20% of the particles formed agglomerates of ≥ 10 7 particles and settled out in 1 hr in a 230-Oe uniform ac or dc field. This effect was present to a lesser extent in other fluids (200-G ester base, 3% of the particles agglomerate in 1 hr; 200-G hydrocarbon base, ∼ 0.05% in 2 hr). Electron micrograph studies indicate that all sizes of particles participate in the agglomeration. Centrifuge experiments show that the agglomerates are not limited by close packing upon settling to the bottom of the tube. The Colpitts oscillator should be useful in determining the stability of magnetic fluids used in applications such as magnetic ink printers, metal separation, etc. It may also be useful in experimental investigation of agglomeration hypotheses because the extent of the agglomeration can be conveniently controlled via the applicatioa of magnetic fields.

High-spectral-resolution Rayleigh-Mie lidar measurement of aerosol and atmospheric profiles

We report what is to our knowledge the first demonstration of simultaneous measurement of tropospheric temperature and aerosol extinction coefficient profiles using a high-spectral-resolution Rayleigh-Mie lidar. With the pressure at a single reference height independently provided, our lidar inversion is capable of deducing the vertical atmospheric profiles, including temperature, pressure, and density, as well as aerosol profiles, including backscatter ratio, extinction coefficient, and backscatter phase function.

Daytime mesopause temperature measurements with a sodium-vapor dispersive Faraday filter in a lidar receiver.

Using a dispersive Faraday bandpass filter, we have upgraded our Na temperature lidar to be capable of 24-h operation. Along with a transmitting telescope to reduce the laser beam divergence to 0.2 mrad, the initial use of this unique narrow-band filter in a lidar receiver allowed us to reduce the detected daytime sky background to a level previously encountered at night, making routine daytime temperature measurements in the mesopause region a reality. The implementation, characterization, and results of what we believe are the first daytime mesopause temperature measurements are reported.

Vertical structure of the midlatitude temperature from stratosphere to mesopause (30–105 km)

Data sets from Rayleigh lidar (1979–93) at the Observatory of Haute-Provence (44°N, 6°E) and at Biscarrosse (44°N, 1°W) of southern France, and from narrowband Na lidar (1991–4) at Fort Collins, Colorado (41°N, 105°W) are used to deduce the vertical temperature structure of the midlatitude middle atmosphere from 30–105 km. Nightly averaged temperatures measured across the Atlantic at comparable latitudes between 81 and 90 km showed seasonal variations tracking one another. Harmonic analyses of the measured profiles at these sites have been compared to the reference atmosphere, CIRA 1986. Considerable differences are noted in the annual mean temperatures in the mesopause region. Two temperature minima at 86 and 99 km are observed, suggesting heating and cooling mechanisms not previously incorporated into the reference atmosphere. Using the new 3D TIME-GCM of Roble and Ridley which includes both dynamical and chemical processes, we account for this anomalous behavior by model simulation for the first time. Except the existence of strong semi-annual oscillations, the observed annual and semi-annual temperature variations are compatible with CIRA 1986. Observed annual temperature and solar flux forcing are out-of-phase between 65 and 99 km, reflecting a dynamics dominated atmosphere at these altitudes.

Lidar observations of seasonal variation of diurnal mean temperature in the mesopause region over Fort Collins, Colorado (41°N, 105°W)

Two campaigns from February 1997 to January 1999 have resulted in 18 sets of continuous temperature measurements in the mesopause region over Fort Collins, Colorado (41°N, 105°W), each covering a complete diurnal cycle, reasonably well distributed throughout the year. It is shown that the winter-summer transitions, based on diurnal means, are abrupt and clear, further strengthening the concept of two-level mesopause, and demonstrate the robustness of this picture. Similar to data taken at Urbana, Illinois, our data at Fort Collins also demonstrate the need to observe over a complete diurnal cycle to deduce the uncontaminated mean temperature structure and seasonal variations in the mesopause region. The difference between annual diurnal, nighttime, and daytime means may be ascribed to a temperature wave propagating with a downward phase speed of ∼0.8 km/h. Our data show that the nighttime annual mean is colder than the diurnal annual mean by no more than 2 K below 88 km and warmer by no more than 3 K above 88 km where the three means are nearly the same. As in the Urbana data, we also observed the altitude of a secondary temperature minimum in the summer to be ∼96 km, which without strong dynamical cooling in the summer would have been the summer mesopause altitude.

Observed episodic warming at 86 and 100 km between 1990 and 1997: Effects of Mount Pinatubo Eruption

Along with initial observations, our regular lidar temperature measurements over Fort Collins, CO (41°N, 105°W) in the rarely probed mesopause region has resulted in a unique seven-year data set between 1990 and 1997. After the seasonal variations are removed, a clear episodic warming is observable in the time series of residual temperatures. Least squares fitting the residual temperatures to an episodic impact function plus a linear trend shows maximum temperature increases occurring in 1993 with magnitudes of 9.0±1.7 K and 12.9±1.8 K, at 86 km and 100 km, respectively. By association, we attribute the Mount Pinatubo eruption in June, 1991, as the most probable primary cause for the observed warming, supporting a connection between tropical stratospheric aerosol and temperatures in a midlatitude mesopause region. It is hoped that our lidar observation from a single location will stimulate similar investigations at other latitudes and longitudes.