B. P. Williams

The Deep Propagating Gravity Wave Experiment (DEEPWAVE): An Airborne and Ground-Based Exploration of Gravity Wave Propagation and Effects from Their Sources throughout the Lower and Middle Atmosphere

AbstractThe Deep Propagating Gravity Wave Experiment (DEEPWAVE) was designed to quantify gravity wave (GW) dynamics and effects from orographic and other sources to regions of dissipation at high altitudes. The core DEEPWAVE field phase took place from May through July 2014 using a comprehensive suite of airborne and ground-based instruments providing measurements from Earth’s surface to ∼100 km. Austral winter was chosen to observe deep GW propagation to high altitudes. DEEPWAVE was based on South Island, New Zealand, to provide access to the New Zealand and Tasmanian “hotspots” of GW activity and additional GW sources over the Southern Ocean and Tasman Sea. To observe GWs up to ∼100 km, DEEPWAVE utilized three new instruments built specifically for the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) Gulfstream V (GV): a Rayleigh lidar, a sodium resonance lidar, and an advanced mesosphere temperature mapper. These measurements were supplemented by in situ probes, dropson...

Quantifying gravity wave momentum fluxes with Mesosphere Temperature Mappers and correlative instrumentation

An Advanced Mesosphere Temperature Mapper and other instruments at the Arctic Lidar Observatory for Middle Atmosphere Research in Norway (69.3°N) and at Logan and Bear Lake Observatory in Utah (42°N) are used to demonstrate a new method for quantifying gravity wave (GW) pseudo-momentum fluxes accompanying spatially and temporally localized GW packets. The method improves on previous airglow techniques by employing direct characterization of the GW temperature perturbations averaged over the OH airglow layer and correlative wind and temperature measurements to define the intrinsic GW properties with high confidence. These methods are applied to two events, each of which involves superpositions of GWs having various scales and character. In each case, small-scale GWs were found to achieve transient, but very large, momentum fluxes with magnitudes varying from ~60 to 940 m2 s−2, which are ~1–2 decades larger than mean values. Quantification of the spatial and temporal variations of GW amplitudes and pseudo-momentum fluxes may also enable assessments of the total pseudo-momentum accompanying individual GW packets and of the potential for secondary GW generation that arises from GW localization. We expect that the use of this method will yield key insights into the statistical forcing of the mesosphere and lower thermosphere by GWs, the importance of infrequent large-amplitude events, and their effects on GW spectral evolution with altitude.

Lidar studies of atmospheric dynamics near polar mesopause

Global change, space weather, and their possible adverse impacts on human activities are not only of scientific interest, but also of great public concern. Since the Arctic middle and upper atmosphere exhibits significant sensitivity to internal and external perturbations, systematic studies at high latitudes have become a scientific priority. Several international research programs are being conducted at the Arctic Light Detection and Ranging Observatory for Middle Atmosphere Research (ALOMAR), which was established at Andoya, Norway (69°N, 16°E) in 1994 to perform regular Arctic light detection and ranging (lidar) observations in tandem with other radio and optical instrumentation, as well as with in situ rocket and balloon measurements [von Zahn, 1997].

Momentum flux estimates accompanying multiscale gravity waves over Mount Cook, New Zealand, on 13 July 2014 during the DEEPWAVE campaign

Observations performed with a Rayleigh lidar and an Advanced Mesosphere Temperature Mapper aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V research aircraft on 13 July 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) measurement program revealed a large-amplitude, multiscale gravity wave (GW) environment extending from ~20 to 90 km on flight tracks over Mount Cook, New Zealand. Data from four successive flight tracks are employed here to assess the characteristics and variability of the larger- and smaller-scale GWs, including their spatial scales, amplitudes, phase speeds, and momentum fluxes. On each flight, a large-scale mountain wave (MW) having a horizontal wavelength ~200–300 km was observed. Smaller-scale GWs over the island appeared to correlate within the warmer phase of this large-scale MW. This analysis reveals that momentum fluxes accompanying small-scale MWs and propagating GWs significantly exceed those of the large-scale MW and the mean values typical for these altitudes, with maxima for the various small-scale events in the range ~20–105 m2 s−2.

Large-amplitude mesospheric response to an orographic wave generated over the Southern Ocean Auckland Islands (50.7°S) during the DEEPWAVE project

The Deep Propagating Gravity Wave Experiment (DEEPWAVE) project was conducted over New Zealand and the surrounding regions during June and July 2014, to more fully understand the generation, propagation, and effects of atmospheric gravity waves. A large suite of instruments collected data from the ground to the upper atmosphere (~100 km), with several new remote-sensing instruments operating on board the NSF Gulfstream V (GV) research aircraft, which was the central measurement platform of the project. On 14 July, during one of the research flights (research flight 23), a spectacular event was observed as the GV flew in the lee of the sub-Antarctic Auckland Islands (50.7°S). An apparent “ship wave” pattern was imaged in the OH layer (at ~83.5 km) by the Utah State University Advanced Mesospheric Temperature Mapper and evolved significantly over four successive passes spanning more than 4 h. The waves were associated with orographic forcing generated by relatively strong (15–20m/s) near-surface wind flowing over the rugged island topography. The mountain wave had an amplitude T′~ 10 K, a dominant horizontal wavelength ~40 km, achieved a momentum flux exceeding 300m s , and eventually exhibited instability and breaking at the OH altitude. This case of deep mountain wave propagation demonstrates the potential for strong responses in the mesosphere arising from a small source under suitable propagation conditions and suggests that such cases may be more common than previously believed.

Investigation of a mesospheric gravity wave ducting event using coordinated sodium lidar and Mesospheric Temperature Mapper measurements at ALOMAR, Norway (69°N)

New measurements at the ALOMAR observatory in northern Norway (69°N, 16°E) using the Weber sodium lidar and the Advanced Mesospheric Temperature Mapper (AMTM) allow for a comprehensive investigation of a gravity wave (GW) event on 22 and 23 January 2012 and the complex and varying propagation environment in which the GW was observed. These observational techniques provide insight into the altitude ranges over which a GW may be evanescent or propagating and enable a clear distinction in specific cases. Weber sodium lidar measurements provide estimates of background temperature, wind, and stability profiles at altitudes from ~78 to 105 km. Detailed AMTM temperature maps of GWs in the OH emission layer together with lidar measurements quantify estimates of the observed and intrinsic GW parameters centered near 87 km. Lidar measurements of sodium densities also allow more precise identification of GW phase structures extending over a broad altitude range. We find for this particular event that the extent of evanescent regions versus regions allowing GW propagation can vary largely over a period of hours and significantly change the range of altitudes over which a GW can propagate.

Effects of a large mesospheric temperature enhancement on the hydroxyl rotational temperature as observed from the ground

The rotational temperature obtained from the rotational population distribution in the bands of the hydroxyl airglow has been shown to be a suitable proxy for the temperature at a height of 87 km [She and Lowe, 1998]. In this paper we examine in detail simultaneous observations on November 2–3, 1997, at Fort Collins, Colorado (41°N, 105°W), with both a sodium temperature lidar and the Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) OH mesospheric temperature mapper during which significant differences between the hydroxyl and lidar temperatures occur. The large differences are associated with a major temperature enhancement in the region of the peak of the hydroxyl emission. We model the effect on the shape of the emission rate profile of the hydroxyl airglow caused by the large temperature enhancement observed on this night by the lidar. As a result of the temperature sensitivity of the processes that give rise to the airglow, the profile shows major distortions from its normal shape. These distortions in turn lead to hydroxyl rotational temperatures that differ significantly from the 87-km lidar observations. The mean rotational temperature deduced in this way agrees well with the observed values. Such deviations in the temperature are expected to be rare, occurring only when a large temperature enhancement occurs near the peak of the airglow emission profile.

Seasonal climatology of the nighttime tidal perturbation of temperature in the midlatitude mesopause region

We present a seasonal-average climatology of the nocturnal variation of temperature and sodium density in the mesopause region (80–110km) based on 300 nights of observation from 1990–1997 by the sodium resonance lidar in Fort Collins, Colorado (40.6°N, 105°W). Waves with downward phase progression were observed during all seasons, with properties (coherence over many nights, observed half-period, etc.) consistent with the upward-propagating semidiurnal tide. The semidiurnal tide was strong (10K at 90km) and stable from November to March with a 30 km vertical wavelength. In April and September, the vertical wavelengths were longer and the amplitude increased rapidly with height above 95km. From May–August, there is a change in wave structure and low amplitudes at 90 km, possibly due to the diurnal tide. The Thermosphere-lonosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) predicts a dominant semidiurnal tide in good agreement with observations, except for a difference in the phase at equinox.

Does Strong Tropospheric Forcing Cause Large‐Amplitude Mesospheric Gravity Waves? A DEEPWAVE Case Study

The DEEPWAVE (deep-propagating wave experiment) campaign was designed for an airborne and ground-based exploration of gravity waves from their tropospheric sources up to their dissipation at high altitudes. It was performed in and around New Zealand from 24 May till 27 July 2014, being the first comprehensive field campaign of this kind. A variety of airborne instruments was deployed onboard the research aircraft NSF/NCAR Gulfstream V (GV) and the DLR Falcon. Additionally, ground-based measurements were conducted at different sites across the southern island of New Zealand, including the DLR Rayleigh lidar located at Lauder (45.04 S, 169.68 E). We focus on the intensive observing period (IOP) 10 on the 4 July 2014, when strong WSW winds of about 40 m/s at 700 hPa provided intense forcing conditions for mountain waves. At tropopause level, the horizontal wind exceeded 50 m/s and favored the vertical propagation of gravity waves into the stratosphere. The DLR Rayleigh Lidar measured temperature fluctuations with peak-to-peak amplitudes of about 20 K in the mesosphere (60 km to 80 km MSL) over a period of more than 10 hours. Two research flights were conducted by the DLR Falcon (Falcon Flight 04 and 05) during this period with straight transects (Mt. Aspiring 2a) over New Zealand´s Alps at three different flight-levels around the tropopause (approx. 11 km MSL). These research flights were coordinated with the GV (Research Flight 16) where the largest mountain wave amplitudes at flight-level (approx. 13 km MSL) were measured during DEEPWAVE. Additionally a first analysis of Falcons in-situ flight-level data revealed amplitudes in the vertical wind larger than 4 m/s at all altitudes in the vicinity of the highest peaks of the Southern Alps. Here, we present a comprehensive picture of the gravity wave characteristics and propagation properties during this interesting gravity wave event. We use the airborne observations combined with a comprehensive set of ground-based measurements consisting of 13 radiosoundings (1.5 hourly interval) together with the DLR Rayleigh lidar. To cover the altitude range from the troposphere to the mesosphere, high-resolution (1 hourly) ECMWF analyses and forecasts are used to estimate the propagation conditions of the excited mountain waves. The goal of our investigation is to find out whether the large amplitude mesospheric gravity waves are caused by the strong tropospheric forcing.

Evidence of solar cycle effect in the mesopause region (2002): Observed temperatures in 1999 and 2000 at 98.5km over Fort Collins, CO (41oN, 105oW)

Abstract Based on 417 nights of observation of temperatures in the mesopause region over Fort Collins, CO (41°N,105°W) from Spring of 1990 to March of 1999, climatological temperatures between 83 and 105 km were recently published. In the same article, a ∼5 K cooler temperature climatology with the effect of the observed episodic warming (peaked near 1993 attributable to Mt. Pinatubos eruption) removed was also published. It was further established that in both the cases, the maximum and minimum annual temperature variations in the mesopause region occur near 85.5 and 98.5 km , respectively, over Fort Collins, CO. With only minimal annual and semi-annual variations, the time series of temperatures at 98.5 km may serve as a qualitative proxy for the long-term temperature change in the region, relatively free from model bias. This paper reports the nightly mean temperatures at 98.5 km between April 1999 and December 2000, during which the flux of Solar Cycle 23 approached its maximum. When these temperatures are compared to the 8-yr climatology with episodic warming removed, one finds that the mean temperatures for 1999 and 2000 are higher than the climatology mean by 3.6 and 16.8 K , respectively. We suggest that the observation of these higher mean temperatures as the solar max approaches is a signature of the solar cycles effect on the temperatures in a midlatitude mesopause region.