High-latitude Summer Mesopause Research Articles

The Role of Inertial Instability in Cross-Hemispheric Coupling

AbstractRecent studies suggest linkages between anomalously warm temperatures in the winter stratosphere, and the high-latitude summer mesopause. The summer temperature anomaly is manifested in the decline of polar mesospheric clouds. The 2-day wave is a strong-amplitude and transient summer feature that interacts with the background state so as to warm the high-latitude summer mesopause. This wave has been linked to a low-latitude phenomenon called inertial instability, which is organized by breaking planetary waves in the winter stratosphere. Hence, inertial instability has been identified as a possible nexus between the disturbed winter stratosphere, and summer mesopause warming. We investigate a sustained occurrence of inertial instability during 19 July–8 August 2014. During this period, stratospheric winter temperatures warmed by about 10 K, while a steep decline in polar mesospheric clouds was reported between 26 July and 6 August. We present, for the first time, wave driving associated with observed inertial instability. The effect of inertial instability is to export eastward momentum from the winter hemisphere across the equator into the summer hemisphere. Using a primitive equation model, we demonstrate that the wave stresses destabilize the stratopause summer easterly jet. The reconfigured wind profile excites the wavenumber-4 component of the 2-day wave, leading to enhanced warming of the summer mesopause. This work supports previous numerical investigations that identified planetary wave–driven inertial instability as a source of the 2-day wave.

On the relationship between seasonal occurrence of northern hemispheric polar mesosphere summer echoes and mean mesopause temperatures

The occurrence of polar mesospheric summer echoes (PMSE) has been closely associated with the seasonal cooling of the high‐latitude summer mesopause region wherein the echoes are embedded. Indeed, a “threshold” temperature has been suggested, with PMSE appearing as mesosphere temperatures fall below some critical value. A careful comparison between seasonally varying mean mesopause temperatures and PMSE occurrence statistics, however, suggests that the probability curve of PMSE occurrence is displaced from the seasonal curve of low summer mesopause temperatures by 1 to 2 weeks, with the temperature decrease leading the PMSE occurrence curve in springtime and the temperature increase leading the PMSE dropout in the fall. A similar displacement can also be seen in the occurrence statistics for polar mesospheric clouds (PMC), a closely related phenomenon observed by satellites. This lack of a direct correlation between mesopause temperature and PMSE occurrence suggests that PMSE occurrence is governed by more than just low mesospheric temperatures. The most likely possibility is that summertime mesospheric water vapor maximizes somewhat later than the minimum in mesospheric temperature, providing a more hospitable environment for PMSE generation. This possibility is supported by a model of the supersaturation region [Garcia, 1989], which shows a comparable lag between the seasonal mesospheric temperature minimum and the maximum in water vapor mixing ratio.

Dynamical and radiative forcing of the summer mesopause circulation and thermal structure: 1. Mean solstice conditions

We present here a simple eigenvalue model which illustrates the sensitivity of the mean circulation and thermal structure near the high‐latitude summer mesopause to dynamical and radiative processes under solstice conditions. Dynamical forcing is provided by gravity wave energy and momentum fluxes and their divergence which contribute both a zonal body force and implied turbulent heating and transport in the mesopause region. These profiles are chosen to be consistent with measurements of gravity wave fluxes and energy densities at high latitudes. Radiative forcing represents the effects of solar heating, non‐LTE CO2 cooling, and LTE O3 cooling below 70 km and is parameterized as a height‐dependent relaxation toward an equilibrium temperature profile accounting for both radiative and chemical influences. Gravity wave momentum flux divergence forces a mean vertical motion related to the meridional gradient of the density‐weighted momentum flux through the “downward control principle” and a mean meridional circulation required to balance the zonal body force. The model exhibits strong sensitivity to dynamical forcing and implies significant constraints on wave influences at mesopause altitudes. For plausible wave fluxes and energy densities, the model yields a mean vertical motion of ∼0.05 m s−1, a mean meridional jet of ∼10 to 15 m s−1, and a mesopause temperature of ∼130 K at Arctic latitudes, all of which are in good agreement with observations. The eigenvalue solution also yields estimates of the induced turbulent heating and vertical diffusion that may pose useful guidelines in modeling studies. Seasonal variations in the mesopause structure are the subject of a companion paper.

A study of the vertical motion field near the high-latitude summer mesopause during MAC/SINE

We present the results of high-resolution observations of the vertical velocity field obtained with the EISCAT and SOUSY VHF radars near the high-latitude summer mesopause during the MAC/SINE campaign in northern Norway in 1987. The data reveal an energetic motion field with maximum amplitudes of ~ 10 m/s and characteristic periods of ~5–30 min. Motions exhibit a high degree of vertical coherence and a quasi-periodic structure, with typical durations of 5–10 cycles. Estimates of the mean vertical velocity are downward at lower levels and are near zero or positive at greater heights. The mean vertical velocity variance is found to be ~5 m 2/s 2, consistent with other high-latitude measurements. Frequency spectra computed for each radar are found to exhibit considerable variability, while vertical wavenumber spectra are seen to be somewhat variable in amplitude and to have slopes approaching −3 at lower wavenumbers. These results are suggestive of an energetic spectrum of gravity wave motions near the mesopause that has a large vertical flux of wave energy, that may have observed wave frequencies differing significantly from intrinsic frequencies due to Doppler shifting by large horizontal winds, and that is consistent with the separability of the frequency and wavenumber dependence of the motion spectrum and with gravity wave saturation at sufficiently small vertical scales.

A theory of enhanced saturation of the gravity wave spectrum due to increases in atmospheric stability

In this paper we consider a vertical wavenumber spectrum of vertically propagating gravity waves impinging on a rapid increase in atmospheric stability. If the high-wavenumber range is saturated below the increase, as is usually observed, then the compression of vertical scales as the waves enter a region of higher stability results in that range becoming supersaturated, that is, the spectral amplitude becomes larger than the saturation limit. The supersaturated wave energy must then dissipate in a vertical distance of the order of a wavelength, resulting in an enhanced turbulent energy dissipation rate. If the wave spectrum is azimuthally anisotropic, the dissipation also results in an enhanced vertical divergence of the vertical flux of horizontal momentum and enhanced wave drag in the same region. Estimates of the enhanced dissipation rates and radar reflectivities appear to be consistent with the enhancements observed near the high-latitude summer mesopause. Estimates of the enhanced mean flow acceleration appear to be consistent with the wave drag that is needed near the tropopause and the high-latitude summer mesopause in large-scale models of the atmosphere. Thus, this process may play a significant role in determining the global effects of gravity waves on the large-scale circulation.

On the statistical correlation between polar mesospheric cloud occurrence and enhanced mesospheric radar echoes

Using data from the Poker Flat MST radar and the Solar Mesosphere Explorer Satellite (SME), we demonstrate the existence of a statistically significant correlation between the occurrence of Polar Mesospheric Clouds (PMC) and enhanced VHF radar echoes near the high‐latitude summer mesopause. We propose three physical conditions which could explain the coexistence of strong summertime echoes and PMC. First, both the enhanced echoes and the PMC are most likely to occur in the presence of very low mesopause temperatures and associated steep temperature gradients. Second, the presence of PMC ice particles could induce a strong gradient in the electron density profile, which would produce enhancements in the radar echo strength. Third, it is likely that heavy water cluster ions, indicative of PMC particle nucleation, can alter the ambipolar diffusion coefficient, thus allowing the electron density fluctuations to extend down to much smaller vertical scales. Under these conditions, the turbulent power at the 3 meter scale required for the occurrence of the MST radar echoes could be greatly enhanced. Finally, we discuss the possibility of using MST radars as instruments for PMC detection.

Noctilucent clouds: Simulation studies of their genesis, properties and global influences

The formation, evolution and properties of noctilucent clouds are studied using a timedependent one-dimensional model of ice particles at mesospheric altitudes. The model treats ice crystals, meteoric dust, water vapor and air ionization as fully interactive cloud elements. For ice particles, the microphysical processes of nucleation, condensation, coagulation and sedimentation are included; the crystal habits of ice are also accounted for. Meteoric dust is analyzed in the manner of Hunten et al. (1980). The simulated particle sizes range from 10 Å to 2.6μm. The chemistry of water vapor and the charge balance of the mesosphere are also analyzed in detail. Based on model calculations, including numerous sensitivity tests, several conclusions are reached. Extremely cold mesopause temperatures (<140K) are necessary to form noctilucent clouds; such temperatures only exist at high latitudes in summer. A water vapor concentration of 4–5 ppmv is sufficient to form a visible cloud. However, a subvisible cloud can exist in the presence of only 1 ppmv of H 2O. Ample cloud condensation nuclei are always present in the mesosphere; at very low temperatures, either meteoric dust or hydrated ions can act as cloud nuclei. To be effective, meteoric dust particles must be larger than 10–15 Å in radius. When dust is present, water vapor supersaturations may be held to such low values that ion nucleation is not possible. Ion nucleation can occur, however, in the absence of dust or at extremely low temperatures (<130K). While dust nucleation leads to a small number (<10cm −3) of large ice particles (>0.05 μm radius) and cloud optical depths (at 550 nm) ∼10 −4, ion nucleation generally leads to a large number (∼10 3cm −3) of smaller particles and optical depths ∼10 −5). However, because calculated nucleation rates in noctilucent clouds are highly uncertain, the predominant nucleus for the clouds (i.e., dust or ions) cannot be unambiguously established. Noctilucent clouds require several hours-up to a day-to materialize. Once formed, they may persist for several days, depending on local meteorological conditions. However, the clouds can disappear suddenly if the air warms by 10–20 K. The environmental conditions which exist at the high-latitude summer mesopause, together with the microphysics of small ice crystals, dictate that particle sizes will be ≲ 0.1 μm radius. The ice crystals are probably cubic in structure. It is demonstrated that particles of this size and shape can explain the manifestations of noctilucent clouds. Denser clouds are favored by higher water vapor concentrations, more rapid vertical diffusion and persistent upward convection (which can occur at the summer pole). Noctilucent clouds may also condense in the cold “troughs” of gravity wave trains. Such clouds are bright when the particles remain in the troughs for several hours or more; otherwise they are weak or subvisible. Model simulations are compared with a wide variety of noctilucent cloud data. It is shown that the present physical model is consistent with most of the measurements, as well as many previous theoretical results. Ambient noctilucent clouds are found to have a negligible influence on the climate of Earth. Anthropogenic perturbations of the clouds that are forecast for the next few decades are also shown to have insignificant climatological implications.

Physical Sciences: Detection of Water Cluster Ions at the High Latitude Summer Mesopause

A SOUNDING rocket was launched recently from the Andoya range (69° 17′ N), chiefly to study the ion composition around the cold summer mesopause, and its relation to ambient temperature, ionization production and ionization loss rates. The most striking result was the large size of the water cluster ions detected and the extremely rapid change in the ion composition around 85 km. The rocket, a Nike Cajun launched on August 8, 1971, at 1211 GMT, reached an altitude of 98.6 km and carried the following experiments.

Optical characteristics of mesospheric aerosol distributions in relation to noctilucent clouds

It has not hitherto been possible to establish whether the appearance of noctilucent clouds is due to an increased concentration or locally increased size of solid particles at the mesopause, or to a combination of these mechanisms. In this contribution the possibility of a relationship between the optical characteristics of noctilucent clouds and ice sublimation is discussed on the basis of present-day knowledge of the structure and photochemistry of the mesosphere. The first part is devoted to a discussion of the character of the sublimation process at these altitudes. It is found that such a process is possible in conditions prevailing at the high-latitude summer mesopause. The time scale of sublimation and evaporation is slow compared with horizontal translation velocities observed in this region. Below the mesopause evaporation becomes rapid compared with the settling speed of submicron particles. In the second part the scattering properties of monodisperse and polydisperse particle distributions, and the possible changes following a sublimation process are reviewed on the basis of numerical computations of scattering functions of coated spherical particles. It is found that the optical consequences of sublimation depend critically upon the presence and relative concentration of particles obeying the Rayleigh scattering law in the visible spectrum. The characteristic time scale of brightness changes by sublimation is at least 10 to 100 minutes. Different optical methods for further investigation of the nature of mesospheric aerosols are summarized and a method is proposed for the detection of scattering layers in the mesosphere by means of ultraviolet intensity and polarization measurements from rockets and satellites. DOI: 10.1111/j.2153-3490.1968.tb00354.x

Optical characteristics of mesospheric aerosol distributions in relation to noctilucent clouds

It has not hitherto been possible to establish whether the appearance of noctilucent clouds is due to an increased concentration or locally increased size of solid particles at the mesopause, or to a combination of these mechanisms. In this contribution the possibility of a relationship between the optical characteristics of noctilucent clouds and ice sublimation is discussed on the basis of present-day knowledge of the structure and photochemistry of the mesosphere. The first part is devoted to a discussion of the character of the sublimation process at these altitudes. It is found that such a process is possible in conditions prevailing at the high-latitude summer mesopause. The time scale of sublimation and evaporation is slow compared with horizontal translation velocities observed in this region. Below the mesopause evaporation becomes rapid compared with the settling speed of submicron particles. In the second part the scattering properties of monodisperse and polydisperse particle distributions, and the possible changes following a sublimation process are reviewed on the basis of numerical computations of scattering functions of coated spherical particles. It is found that the optical consequences of sublimation depend critically upon the presence and relative concentration of particles obeying the Rayleigh scattering law in the visible spectrum. The characteristic time scale of brightness changes by sublimation is at least 10 to 100 minutes. Different optical methods for further investigation of the nature of mesospheric aerosols are summarized and a method is proposed for the detection of scattering layers in the mesosphere by means of ultraviolet intensity and polarization measurements from rockets and satellites. DOI: 10.1111/j.2153-3490.1968.tb00354.x

On the spatial distribution of some hydrogen components in the mesosphere and lower thermosphere

On the basis of a photochemical oxygen-hydrogen atmosphere model the concentrations of OH, HO 2 , H 2 , H 2 O and H were computed as a function of height, latitude and season. The computations were made for two values of the hydrogen/air mixing ratio, differing by a factor of about 100. Atomic hydrogen was found to be the major hydrogen constituent above about 85 km, while water vapor takes up almost all hydrogen at levels below about 75 km. In the shallow layer between these two regimes molecular hydrogen enters as the major hydrogen component. The seasonal and latitudinal variation was found to be relatively small. The effect of the air motion is discussed. An ascent of 1 cm/s near the high latitude summer mesopause seems to be necessary to keep the moisture content high enough for ice clouds to form. DOI: 10.1111/j.2153-3490.1965.tb01426.x