Comment on acp-2022-572

Abstract. The Balloon Lidar Experiment (BOLIDE) was the first high-power lidar flown and operated successfully on board a balloon platform. As part of the PMC Turbo payload, the instrument acquired high-resolution backscatter profiles of polar mesospheric clouds (PMCs) from an altitude of ∼ 38 km during its maiden ∼ 6 d flight from Esrange, Sweden, to northern Canada in July 2018. We describe the BOLIDE instrument and its development and report on the predicted and actual in-flight performance. Although the instrument suffered from excessively high background noise, we were able to detect PMCs with a volume backscatter coefficient as low as 0.6×10-10 m−1 sr−1 at a vertical resolution of 100 m and a time resolution of 30 s.

Responses of polar mesospheric cloud brightness to stratospheric gravity waves at the South Pole and Rothera, Antarctica

We made the first lidar observations of polar mesospheric clouds (PMC) at the South Pole and in the southern hemisphere with an Fe Boltzmann temperature lidar in the 1999-2000 and 2000-2001 austral summer seasons. Strong PMC activities were observed at the South Pole and extensive data were collected. Here we summarize the lidar observation results including the interannual, seasonal and diurnal variations of PMC altitude, brightness and occurrence probability. In particular, our data show that PMC at the South Pole are a few kilometers higher than in the northern hemisphere and PMC at the South Pole exhibit seasonal trends in both altitude and brightness. We explore the possible causes through the study of atmospheric thermal structure and upwelling wind by using NCAR TIME-GCM model and then presenting a PMC altitude model. Our initial conclusion is that these hemispheric differences and seasonal trends in PMC altitudes are the combination results of the hemispheric differences in thermal structure and upwelling wind, which are caused by the Earth orbital eccentricity and inter-hemisphere difference in gravity wave forcing.

The particle size distribution of Polar Mesospheric Clouds (PMC) is closely related to the fundamental processes of cloud formation and evolution. Still, despite substantial observational efforts, specific details about the particle size distribution have remained obscure. In this study, we aim at deriving more constraints on PMC size distributions by combining optical measurements from two satellite instruments observing a common PMC volume. We use a special set of 2D tomographic limb observations from the Optical Spectrograph and Infrared Imager System (OSIRIS) on the Odin satellite from 2010 to 2011 in the latitude range 78° N to 80° N and compare these to simultaneous PMC observations from the nadir-viewing Cloud Imaging and Particle Size (CIPS) instrument on the AIM satellite. A key goal is to find the assumption on the mathematical shape of the particle size distribution that should be applied to a vertically resolving limb-viewing instrument to reach consistent size results compared to the column-integrated ice distribution as seen by a nadir-viewing instrument. Our results demonstrate that viewing geometry and sampling volume of each instrument must be carefully considered and that the same size distribution assumption cannot simultaneously describe a column-integrated and a local height-resolved size distribution. In particular, applying the standard Gaussian assumption, used by many earlier PMC studies, to both limb and nadir observation leads to an overestimate of particle sizes seen by OSIRIS by about 10 nm as compared to CIPS. We show that the agreement can be improved if a Log-normal assumption with a broad distribution width around σ = 1.42 is adopted for OSIRIS. A reason for this broad distribution best describing the OSIRIS observations we suggest the large retrieval volume of the limb measurement. Gravity waves and other small-scale processes can cause horizontal variations and a co-existence of a wide range of particle populations in the sampling volume. Horizontal integration then leads to apparently much broader size distributions than encountered in a small horizontal sampling volume.

Kjellstrand et al. (2022), https://10.1029/2021JD036232 describes the evolution and dynamics of a strong, large‐scale Kelvin‐Helmholtz instability (KHI) event observed in polar mesospheric clouds (PMCs) on 12 July 2018 by high‐resolution imagers aboard the PMC Turbulence (PMC Turbo) stratospheric long‐duration balloon experiment. The imaging provides evidence of KH billow interactions and instabilities that are strongly influenced by gravity waves at larger scales. Specific features include initially separated regions of KHI, secondary convective and KH instabilities of individual billows, and “tubes” and “knots” that arise where billow cores are mis‐aligned or discontinuous along their axes. This study describes a direct numerical simulation of KH billow interactions in a periodic domain seeded with random initial noise that enables excitation of multiple KH billows exhibiting variable phase structures that capture multiple features of the observed KHI dynamics. Variable KH billow phases along their axes yield initial vortex tubes having diagonal alignments that link adjacent, but mis‐aligned, billow cores. Weak initial vortex tubes and billow cores having nearly orthogonal alignments amplify, interact strongly, and drive intense vortex knots at these sites. These vortex tube and knot (T&K) dynamics excite “twist waves” that unravel the initial vortex tubes, and drive increasingly strong vortex interactions and a cascade of energy and enstrophy to successively smaller scales in the turbulence inertial range. The implications of T&K dynamics are much more rapid and intense breakdown and decay of the KH billows, and significantly enhanced energy dissipation rates, where these interactions occur.

The Polar Mesospheric Cloud (PMC) Turbulence experiment performed optical imaging and Rayleigh lidar PMC profiling during a 6‐day flight in July 2018. A mosaic of seven imagers provided sensitivity to spatial scales from ∼20 m to 100 km at a ∼2‐s cadence. Lidar backscatter measurements provided PMC brightness profiles and enabled definition of vertical displacements of larger‐scale gravity waves (GWs) and smaller‐scale instabilities of various types. These measurements captured an interval of strong, widespread Kelvin‐Helmholtz instabilities (KHI) occurring over northeastern Canada on July 12, 2018 during a period of significant GW activity. This paper addresses the evolution of the KHI field and the characteristics and roles of secondary instabilities within the KHI. Results include the imaging of secondary KHI in the middle atmosphere and multiple examples of KHI “tube and knot” (T&K) dynamics where two or more KH billows interact. Such dynamics have been identified clearly only once in the atmosphere previously. Results reveal that KHI T&K arise earlier and evolve more quickly than secondary instabilities of uniform KH billows. A companion paper by Fritts et al. (2022), https://doi.org/10.1029/2021JD035834 reveals that they also induce significantly larger energy dissipation rates than secondary instabilities of individual KH billows. The expected widespread occurrence of KHI T&K events may have important implications for enhanced turbulence and mixing influencing atmospheric structure and variability.

Polar mesospheric cloud (PMC) imaging and lidar profiling performed aboard the 5.9‐day PMC Turbo balloon flight from Sweden to northern Canada in July 2018 revealed a wide variety of gravity wave (GW) and instability events occurring nearly continuously at approximately 82 km. We describe one event exhibiting GW breaking and associated vortex rings driven by apparent convective instability. Using PMC Turbo imaging with spatial and temporal resolution of 20 m and 2 s, respectively, we quantify the GW horizontal wavelength, propagation direction, and apparent phase speed. We identify vortex rings with diameters of 2–5 km and horizontal spacing comparable to their size. Lidar data show GW vertical displacements of ±0.3 km. From the data, we find a GW intrinsic frequency and vertical wavelength of 0.009 ± 0.003 rad s −1 and 9 ± 4 km, respectively. We show that these values are consistent with the predictions of numerical simulations of idealized GW breaking. We estimate the momentum deposition rate per unit mass during this event to be 0.04 ± 0.02 m s −2 and show that this value is consistent with the observed GW. Comparison to simulation gives a mean energy dissipation rate for this event of 0.05–0.4 W kg −1 , which is consistent with other reported in situ measurements at the Arctic summer mesopause.

Morphology of polar mesospheric clouds as seen from space

Middle‐ultraviolet (210‐ to 252‐nm) images have revealed the transpolar structure of polar mesospheric clouds (PMCs) at a spatial resolution of 3 km. The ultraviolet and visible imaging and spectrographic imaging instrument on the Midcourse Space Experiment (MSX) satellite collected over 27,000 mid‐UV images of PMCs during 26 passes over the north and south polar regions during the summer seasons of 1997, 1998, and 1999. A Lomb periodogram analysis of PMC radiance projected to an 83‐km altitude reveals periodic structures with wavelengths ranging from ∼100 to ∼3000 km. In either hemisphere, more PMCs have features with wavelengths shorter than 1000 km than longer, and a crude spectrum of the PMC structures suggests a spectral peak between 500 and 1000 km. There is little evidence of structures having wavelengths short of ∼100 km, and the longer wavelengths generally have more spectral “power” than the shorter wavelengths. PMC structures do not remain stable over time periods of weeks, but may retain similar structural features for at least as long as 24 hours. The clouds may be considered markers of gravity waves, which carry energy from the lower atmosphere to the mesosphere and modulate the appearance of PMCs.

[1] Southern Hemisphere (SH) polar mesospheric clouds (PMCs), also known as noctilucent clouds, have been observed to be more variable and, in general, dimmer than their Northern Hemisphere (NH) counterparts. The precise cause of these hemispheric differences is not well understood. This paper focuses on one aspect of the hemispheric differences: the timing of the PMC season onset. Observations from the Aeronomy of Ice in the Mesosphere satellite indicate that in recent years the date on which the PMC season begins varies much more in the SH than in the NH. Using the Canadian Middle Atmosphere Model, we show that the generation of sufficiently low temperatures necessary for cloud formation in the SH summer polar mesosphere is perturbed by year-to-year variations in the timing of the late-spring breakdown of the SH stratospheric polar vortex. These stratospheric variations, which persist until the end of December, influence the propagation of gravity waves up to the mesosphere. This adds a stratospheric control to the temperatures in the polar mesopause region during early summer, which causes the onset of PMCs to vary from one year to another. This effect is much stronger in the SH than in the NH because the breakdown of the polar vortex occurs much later in the SH, closer in time to the PMC season.

The solar occultation for ice experiment

Gravity wave momentum fluxes in the MLT—Part II: Meteor radar investigations at high and midlatitudes in comparison with modeling studies

Micrometeorites that ablate in the lower thermosphere and upper mesosphere are thought to recondense into nanometer‐sized smoke particles and then coagulate into larger dust particles. Previous studies with one‐dimensional models have determined that the meteoric dust size distribution is sensitive to the background vertical velocity and have speculated on the importance of the mesospheric meridional circulation to the dust spatial distribution. We conduct the first three‐dimensional simulations of meteoric dust using a general circulation model with sectional microphysics to study the distribution and characteristics of meteoric dust in the mesosphere and upper stratosphere. We find that the mesospheric meridional circulation causes a strong seasonal pattern in meteoric dust concentration in which the summer pole is depleted and the winter pole is enhanced. This summer pole depletion of dust particles results in fewer dust condensation nuclei (CN) than has traditionally been assumed in numerical simulations of polar mesospheric clouds (PMCs). However, the total number of dust particles present is still sufficient to account for PMCs if smaller particles can nucleate to form ice than is conventionally assumed. During winter, dust is quickly transported down to the stratosphere in the polar vortex where it may participate in the nucleation of sulfate aerosols, the formation of the polar CN layer, and the formation of polar stratospheric clouds (PSCs). These predictions of the seasonal variation and resulting large gradients in dust concentration should assist the planning of future campaigns to measure meteoric dust.

Abstract. We study short period gravity waves (20–120 min) in the equatorial Mesosphere and Lower Thermosphere (MLT) using a Medium Frequency (MF) radar at Pameungpeuk (7.4° S, 107.4° E), Indonesia. In particular, we study local time and seasonal variation of the gravity wave variance and its relation to tropical convection. The gravity wave variance at 88 km enhances between 20:00 LT and 07:00 LT, with a peak at 02:00–03:00 LT. The enhancement is mainly observed during February–April and September–October and shows inter-annual variability. Convective activity over the same location persists from 16:00–21:00 LT with a peak activity ~18:00 LT and enhances between November–April. Time delay between the peak of convection and that of gravity wave activity ranges 1–15 h, which is consistent with theoretical calculations and previous reports based on reverse ray tracing analysis.

Abstract. Gravity waves (GWs) as well as solar tides are a key driving mechanism for the circulation in the Earth's atmosphere. The propagation of gravity waves is strongly affected by tidal waves as they modulate the mean background wind field and vice versa, which is not yet fully understood and not adequately implemented in many circulation models. The daylight-capable Rayleigh–Mie–Raman (RMR) lidar at Kühlungsborn (54∘ N, 12∘ E) typically provides temperature data to investigate both wave phenomena during one full day or several consecutive days in the middle atmosphere between 30 and 75 km altitude. Outstanding weather conditions in May 2016 allowed for an unprecedented 10-day continuous lidar measurement, which shows a large variability of gravity waves and tides on timescales of days. Using a one-dimensional spectral filtering technique, gravity and tidal waves are separated according to their specific periods or vertical wavelengths, and their temporal evolution is studied. During the measurement period a strong 24 h wave occurs only between 40 and 60 km and vanishes after a few days. The disappearance is related to an enhancement of gravity waves with periods of 4–8 h. Wind data provided by ECMWF are used to analyze the meteorological situation at our site. The local wind structure changes during the observation period, which leads to different propagation conditions for gravity waves in the last days of the measurement period and therefore a strong GW activity. The analysis indicates a further change in wave–wave interaction resulting in a minimum of the 24 h tide. The observed variability of tides and gravity waves on timescales of a few days clearly demonstrates the importance of continuous measurements with high temporal and spatial resolution to detect interaction phenomena, which can help to improve parametrization schemes of GWs in general circulation models.