Observations Of Polar Mesospheric Clouds Research Articles

Polar mesosphere and lower thermosphere dynamics: 1. Mean wind and gravity wave climatologies

Mean wind and gravity wave climatologies are presented for the polar mesosphere and lower thermosphere (MLT). The data were derived using MF radars at Davis (69°S, 78°E) and Syowa (69°S, 40°E) in the Antarctic and Poker Flat (65°N, 147°W) and Andenes (69°N, 16°E) in the Arctic. The dynamics of the Antarctic MLT are found to be significantly different from the Arctic MLT. Summer maxima in both the westward and equatorward winds occur closer to the solstice in the Antarctic than in the Arctic. The greater symmetry around the solstice suggests radiative effects may play a greater role in controlling the state of the Antarctic MLT than in the Arctic, where dynamical effects appear to be more important. Gravity wave observations also suggest that wave drag may be greater in the Arctic than in the Antarctic. The equatorward flow near the mesopause persists later in summer in the Arctic than in the Antarctic, as do observations of polar mesospheric clouds and polar mesospheric summer echoes. All three phenomena begin at about the same time in each hemisphere, but end later in the Arctic than in the Antarctic. It is proposed that the magnitude of the meridional winds can be used as a proxy for gravity wave driving and the consequent adiabatic cooling in the MLT. Seasonal variations in gravity wave activity are predominately combinations of annual and semiannual components. Significant hemispheric differences are observed for both the timing and magnitude of these seasonal variations.

Polar mesospheric clouds observed by an iron Boltzmann lidar at Rothera (67.5°S, 68.0°W), Antarctica from 2002 to 2005: Properties and implications

Lidar observations of polar mesospheric clouds (PMC) were made at Rothera, Antarctica, from December 2002 to March 2005. Overall, 128 hours of PMC were detected among the 459 hours of observations, giving a mean occurrence frequency of 27.9%. The mean PMC centroid altitude is 84.12 ± 0.12 km, the mean PMC total backscatter coefficient is 2.34 ± 0.11 × 10−6 sr−1, and the mean layer RMS width is 0.93 ± 0.03 km. The distribution of PMC centroid altitudes over all observations is symmetric (nearly Gaussian), with the most probable altitude (∼84 km) near the center of the distribution. The distribution of PMC brightness is non‐Gaussian and is dominated by weak PMC. The observed PMC altitudes at Rothera support the earlier lidar findings that Southern Hemispheric PMC are on average 1 km higher than corresponding Northern Hemispheric PMC, and higher PMC occur at higher latitudes. Significant interannual and diurnal variations are observed in PMC centroid altitude and brightness. Mean PMC altitude varies more than 1 km from one year to another. In addition, 24‐hour, 12‐hour, and 8‐hour oscillations are clearly shown in PMC centroid altitude and brightness. The altitude distribution of PMC brightness peaks at a nearly constant altitude of 84 km, with weaker PMC found on either side of this altitude. The mean PMC altitudes averaged in brightness bins are anticorrelated with the PMC brightness, where weaker PMC occur at higher altitude and the PMC altitudes are proportional to the logarithm of the PMC brightness.

A quarter-century of satellite polar mesospheric cloud observations

Satellite observations of polar mesospheric clouds (PMCs) are extremely valuable because they typically have daily coverage to characterize seasonal variations, sufficient detections for each season to give good statistics, quantitative information for physical analysis, and coverage of both hemispheres to evaluate global behavior. Continuous spectral measurements in the ultraviolet provide information about particle size distributions. A typical PMC season begins approximately 20 days before summer solstice at 80° latitude, rises rapidly in occurrence frequency to 80–90%, and remains at that level until 50–60 days after solstice. Both occurrence frequency and brightness are latitude dependent, with higher values observed toward the poles. PMCs are normally observed at altitudes of 82–83 km, with higher altitudes at the start and end of each season. Hemispheric differences in behavior are also observed. Northern Hemisphere PMCs are consistently both more frequent and brighter than Southern Hemisphere clouds. Cloud height is generally anti-correlated with cloud brightness. The availability of extended PMC data sets from satellites provides the opportunity to evaluate long-term PMC variations over the past few decades. Analysis of these lengthy data sets shows a clear anti-correlation between seasonally averaged PMC parameters (occurrence frequency and brightness) and solar UV activity over the past two solar cycles, in agreement with model predictions. A time lag of ∼1 year between the solar cycle and the PMC response is present in several data sets (solar variation leads PMC response). The cause is unknown. Multiple regression analysis also indicates long-term increases in both occurrence frequency and brightness, although there is not yet a consensus on the magnitude of the increase. These results are compared with information about concurrent variations in plausible source mechanisms such as mesospheric water vapor and temperature.

Observations of polar mesospheric clouds by the Student Nitric Oxide Explorer

Polar Mesospheric Clouds (PMCs) were observed by a limb‐scanning ultraviolet spectrometer on the Student Nitric Oxide Explorer (SNOE). Radiance profiles at 215 and 237 nm are analyzed to determine the presence of clouds. Once detected, the altitude and brightness of a cloud relative to the background atmosphere is determined. SNOE observations provide the frequency of occurrence of PMC as a function of location and time for the years 1998 through 2003. The observations show at high latitudes a general rise in frequency of occurrence beginning approximately 3 weeks before summer solstice in both hemispheres and lasting for approximately 1 week. These rises are followed by approximately 60 days of relatively high but variable occurrence frequencies. The declines in frequency of occurrence at the ends of the seasons are generally slower and more structured then the beginning of the seasons. One of the major results from the SNOE observations is that significantly more PMCs are observed in the Northern Hemisphere than in the south, leading us to conclude that the southern polar mesosphere must be on average less saturated than the northern polar mesosphere. The SNOE observations also suggest that the frequency of occurrence of PMCs is strongly modulated by local dynamical influences. The SNOE results are in general agreement with results from the Solar Mesosphere Explorer which observed PMC with similar instrumentation in the years 1981 through 1986.

Antarctic mesospheric clouds formed from space shuttle exhaust

New satellite observations reveal lower thermospheric transport of a space shuttle exhaust plume into the southern hemisphere two days after a January, 2003 launch. A day later, ground‐based lidar observations in Antarctica identify iron ablated from the shuttle's main engines. Additional satellite observations of polar mesospheric clouds (PMCs) show a burst that constitutes 10–20% of the PMC mass between 65–79°S during the 2002–2003 season, comparable to previous results for an Arctic shuttle plume. This shows that shuttle exhaust can be an important global source of both PMC formation and variability.

The polar mesospheric cloud mass in the Arctic summer

We infer the polar mesospheric cloud (PMC) mass throughout the Arctic summer using results from two sets of satellite observations and a microphysical model. Solar backscatter ultraviolet (SBUV) PMC observations in July 1999 indicate a burst of activity persisting for ∼8 days after a space shuttle launch and averaging 262 ± 52 t near 4.7 local time. This mass is consistent with the propellant mass available from the shuttle's main engines and accounts for 22% of the total SBUV PMC mass over the season between 65° and 75°N. This is the first evidence that PMCs formed by space shuttle water exhaust can contribute significantly to both the number of observed PMCs and the total PMC mass in a season. In another approach, 11 years of observations by the Halogen Occultation Experiment (HALOE) indicate that on average 90 ± 12 t of water ice is present near local midnight between 65° and 75°N. Using simultaneous HALOE water vapor observations, we find that a one‐dimensional microphysical model reproduces the start and end of the PMC season but overpredicts the ice mass by about a factor of 1.8 when compared with the observations. This overprediction is within the time‐dependent variability of ice formation and the uncertainties of temperature, water vapor, and vertical winds used to initialize the model.

Lidar observations of polar mesospheric clouds at Rothera, Antarctica (67.5°S, 68.0°W)

Polar mesospheric clouds (PMC) were observed by an Fe Boltzmann temperature lidar at Rothera (67.5°S, 68.0°W), Antarctica in the austral summer of 2002–2003. The Rothera PMC are much weaker, less frequent, and not as high as the PMC observed at the South Pole. The mean PMC altitude is 83.74 ± 0.25 km, which is approximately 1.3 km lower than the South Pole clouds. A comparison of numerous cloud observations indicates that southern hemisphere PMC are about 1 km higher than northern clouds at similar latitudes. Lidar measurements also show that the mesopause region temperatures at Rothera in late January are warmer than at the South Pole, while the Fe layer at Rothera has higher density and a lower peak altitude compared to the summertime Fe layer at the South Pole. These Fe density and temperature observations are qualitatively consistent with the PMC observations.

Solar backscattered ultraviolet (SBUV) observations of polar mesospheric clouds (PMCs) over two solar cycles

Previous satellite measurements have provided nearly complete seasonal and geographic coverage of polar mesospheric clouds (PMCs), but previous data sets have not been able to evaluate changes in PMC behavior on decadal timescales. The Solar Backscattered Ultraviolet (SBUV) series of ozone measuring instruments have been flying continuously since 1978. While the instrument design is not optimized for PMC detection, the radiance data can be analyzed to examine the occurrence frequency and intensity of relatively bright PMCs. In this paper, we present PMC results from five SBUV and SBUV/2 instruments covering more than 23 years (1978–2002), starting just before the maximum of solar cycle 21 and extending through the maximum of solar cycle 23. The overlapping data sets from nearly identical instruments give an accurate picture of long‐term variations. Multiple linear regression fits are used to examine solar and secular correlations. PMC occurrence frequency is anticorrelated with solar Lyman alpha irradiance, with an approximate 0.5‐year phase lag in the Northern Hemisphere (Rsolar = −0.87) and no phase lag in the Southern Hemisphere (Rsolar = −0.65). The distribution of cloud brightness by season appears to be changing over time. When the PMC brightness for each season is characterized using an exponential cumulative distribution function, the exponent decreases in magnitude by a factor of 2 from 1978 to 2002 in the Southern Hemisphere (Rtime = +0.85). This implies an increase in the relative proportion of the brightest PMCs. The secular brightness trend is less significant in the Northern Hemisphere (Rtime = +0.58). We discuss possible origins for these changes.

Observations of the 5‐day planetary wave in PMC measurements from the Student Nitric Oxide Explorer Satellite

The Student Nitric Oxide Explorer (SNOE) satellite has been observing Polar Mesospheric Clouds (PMCs) since 1998 and has successfully measured seven PMC seasons. In the summer seasons, the Ultraviolet Spectrometer (UVS) limb measurements include detections of PMCs between 80 – 90 km. SNOE observations of PMCs have a significant advantage over other PMC measurements in that it can observe them globally each day. Because SNOE orbits the earth 15 times a day, daily global images of PMC brightness may be produced. Variations in the PMC brightness with a 5‐day period are observed from the measurements. The 5‐day wave is observed in both the northern and southern hemisphere polar summers at high latitudes. This is the first direct global scale wave analysis performed on PMC measurements and indicates the effects of dynamics on PMC formation.

Lidar observations of polar mesospheric clouds at South Pole: Diurnal variations

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.

Lidar observations of polar mesospheric clouds at South Pole: Seasonal variations

Polar mesospheric clouds (PMCs) were observed above the geographic South Pole by an Fe Boltzmann temperature lidar from 11 Dec 99 to 24 Feb 00. During this 76‐day period 297 h of observations were made on 33 different days and PMCs were detected 66.5% of the time. The mean PMC peak backscatter ratio, peak volume backscatter coefficient, total backscatter coefficient, layer centroid altitude, and layer rms width are 50.59±2.33, 2.70±0.12 × 10−9 m−1sr−1, 3.61±0.22 × 10−6 sr−1, 85.49±0.09 km, and 0.71±0.03 km, respectively. The PMCs are highest near summer solstice when upwelling over the pole is strongest. The altitudes are 2–4 km higher than that typically observed elsewhere, including the North Pole. After solstice the mean altitudes decreases by about 64 m/day as the upwelling weakens.

An analysis of POAM II solar occultation observations of polar mesospheric clouds in the southern hemisphere

The second Polar Ozone and Aerosol Measurement (POAM II) instrument is a space‐borne visible/near IR photometer which uses the solar occultation technique to measure vertical profiles of ozone, nitrogen dioxide, and water vapor as well as aerosol extinction and atmospheric temperature in the stratosphere and upper troposphere. Here we report on the detection of polar mesospheric clouds (PMCs) in the high‐latitude southern hemisphere by POAM II during the 1993 and 1994 summer seasons. These measurements are noteworthy because they are the first measurements of PMCs in atmospheric extinction. The POAM II PMC data set has been analyzed using a simple geometric cloud model. We find that mean cloud altitudes deduced from these data are 82–83 km, consistent with previous ground‐based and satellite measurements. In addition, the 0.7 km vertical resolution of POAM II allows for accurate determination of cloud thickness. For the PMCs detected by POAM II we find a mean thickness of 2.4 km. The mean peak slant optical depth was determined to be 1.2×10−3 for the 1993 season and 1.8×10−3 for the 1994 season, corresponding to a cloud extinction coefficient of 3.9×10−6 and 6.1×10−6 km−1, respectively. The multichannel capability of POAM II also makes it possible to study the wavelength dependence of the measured slant optical depth for the clouds with largest extinction. The results of this analysis suggest an upper limit to the modal particle radii for these clouds of approximately 70 nm.

POAM II observations of polar mesospheric clouds in the Southern Hemisphere

The POAM II instrument has been taking data for approximately three years. PMCs have been detected in all three Southern Hemisphere seasons. The POAM II PMC data set has been analyzed using a simple geometric cloud model. We find mean cloud altitudes deduced from these data are 82–83 km with a mean thickness of 2.6 km. The means of the 448.1 nm peak slant optical depth were determined to range from 1.2×10 −3 to 1.9×10 −3 for the three seasons, corresponding to cloud extinction coefficients of 3.9×10 −6 to 6.1×10 −6 km −1, respectively. The wavelength dependence of the measured slant optical depth for the clouds with largest extinction suggest an upper limit to the modal particle radii for these clouds of approximately 70 nm.

Mesospheric clouds and the physics of the mesopause region

During the last decade, new satellite observations of polar mesospheric clouds and ground‐based radar and lidar sounding of the high‐latitude summertime mesosphere have stimulated new interest in this highly unusual atmospheric region. Computer simulations indicate that the ice condensation model for mesospheric cloud formation is correct in broad outline. This review contains a brief historical account of early mesospheric cloud studies with an emphasis on the two main theories: cosmic dust and ice condensation models. Current topics are described, including wave‐cloud interactions, the dynamics and momentum budget of the high‐latitude polar mesosphere, new satellite observations, polar mesospheric summer radar echoes, heterogeneous chemistry, and implications of apparent secular changes in cloud brightness for global change of the atmosphere as a whole. There are major uncertainties in our theoretical understanding of the ice formation process, particularly in how the ice particles are nucleated. Future progress will require new research efforts in three separate directions: observations (ground based, rocket, and satellite), laboratory experiments, and theoretical modeling.

Satellite observations of polar mesospheric clouds by the solar backscattered ultraviolet spectral radiometer: Evidence of a solar cycle dependence

Measurements from space of the Earth's ultraviolet albedo are affected by scattering of sunlight from polar mesospheric clouds (PMC). We have examined 8 years of solar backscattered ultraviolet (SBUV) albedo data and find evidence of an annual occurrence of PMC in the summertime polar cap regions of both hemispheres. We have devised a cloud selection algorithm based upon the expected spectral properties of PMC scattering and show that the albedo residuals selected by this algorithm, at least above some threshold brightness, possess the basic properties of PMC with regard to their brightness levels, their seasonal characteristics, and latitude variation. Because of the uniform sampling of the polar caps, particularly in the southern hemisphere, it is possible to examine year‐to‐year variations in PMC activity. We find that a significant trend is present in the PMC occurrence frequency values over the period 1978 to 1986. The increase (up to a factor of 10 at some brightness levels) in the occurrence frequency from solar maximum to solar minimum conditions indicates an anticorrelation with solar activity, an effect that also appears to be present in noctilucent cloud sightings over the past several decades. Garcia [1989] suggested that PMC should be modulated by changes in the solar Lyman‐alpha (121.6 nm) flux through its strong photodissociation control of upper mesospheric water vapor. Solar Lyman‐alpha underwent a significant (50%) variation during this period. Our results confirm this expectation, and thus indirectly support the current theories of PMC formation. However, other influences such as long‐term changes of mesopause temperature or dynamics cannot be ruled out. We have identified two kinds of hemispherical asymmetries: the first, that PMC in the northern hemisphere is significantly brighter than in the south is consistent with previous results derived from Solar Mesosphere Explorer data; and the second, that the solar cycle response in the south is more pronounced than in the north. We show that if PMC scattering is not taken into account in the SBUV ozone retrieval algorithm, systematic errors of order 10% can occur in derived ozone concentration in the 40–50 km region of the summer polar cap.