Modeling Strong Wind Shears In The Upper Atmosphere

Offshore windmills creating wind shear (Credit: NOAA)

From comfortable breezes to powerful tornados and hurricanes, winds are our most direct experience with the atmosphere. Winds not only blow near the Earth surface but also higher up—like the head/tail winds we see reported when taking an international flight.

Actually winds can be much stronger at higher altitudes, for example, winds with speed larger than 400km/hour (250mph) have been measured in the lower thermosphere (around 100 km or 60 miles altitude). To put things in perspective, the highest wind speed of hurricane Irma, a category 5 major hurricane, was 295 km/h (185 mph). Wind can also be highly variable, changing both spatially and temporally, and wind shear is a quantity measuring the spatial change. Wind shear is important for the stability and species transport of the atmosphere. Quantifying winds and wind shears are essential for atmosphere studies.

Wind measurements using rockets, radar, and lidar have revealed that the horizontal wind shear in the vertical direction can also be extremely large in the upper atmosphere, with a prominent peak located between 100-110 km altitude (and 90-100km at high latitudes in the summer hemisphere), right above the mesopause region. The peak values can be as large as 100 m/s/km (wind changes by 360 km/h within one kilometer, or 360 mph within one mile). In the lower atmosphere, radiosonde measurements have identified a wind shear peak immediately above the tropopause at equatorial and mid-latitudes, with values over 40 m/s/km.

One common feature shared by the mesopause and the tropopause is that both regions are statically stable: the air temperature is low while the temperature increase with altitude is large due to heating by solar extreme ultraviolet absorption in the thermosphere and by ultraviolet absorption by ozone in the stratosphere, respectively. The static stability is measured by buoyancy frequency. Buoyancy frequency right above the mesopause is the largest in the entire Earth atmosphere, and the buoyancy frequency also attains a peak value immediately above the tropopause.

Larger static stability makes it possible for the vertical shear of horizontal wind to reach larger values. This is because the vertical shear rate cannot grow to arbitrarily large values in a stably stratified flow. When the shear becomes large, instability (termed shear instability or dynamic instability) sets in and can lead to turbulence mixing, which in turn limits the shear rate. The threshold for the shear instability has been shown to be proportional to the static stability. Therefore, an atmosphere region with stronger static stability, such as mesopause and tropopause, can sustain larger wind shear.

Modeling Mesoscale Wind Shear

However, processes responsible for driving the large wind shears are not well understood, nor could previous global models reproduce the observed large wind shears. It has been hypothesized that the large wind shears could be caused by atmospheric gravity waves, which are waves excited by weather systems and topography and can propagate to the middle and upper atmosphere. Gravity waves can perturb wind, temperature, and density of the atmosphere and the magnitude of the perturbation can become very large at higher altitudes when the air becomes very thin.

Previous studies using an idealized two-dimensional mesoscale model with simple and prescribed gravity waves have demonstrated that winds and shears can indeed reach large values in the mesopause region. However, since their horizontal scales ranging from tens to thousands of kilometers, the gravity waves are usually poorly resolved or not resolved at all in most global models that extend into the lower thermosphere (often referred to as whole atmosphere models, which often have horizontal resolution coarser than 100 km). Probably because of the poor representation of gravity waves, previous global models could not reproduce the observed large wind shear in the mesopause region.

Thanks to increasing computational power and enhanced numerical algorithms, now whole atmosphere models with high spatial resolution are being developed. One such development is the Whole Atmosphere Community Climate Model (WACCM) developed at the National Center for Atmospheric Research (NCAR). WACCM extends from the Earth surface to ~145 km altitude, and its high-resolution version has a quasi-uniform horizontal resolution of ~25km and a vertical resolution of 0.1 scale height (500-700 m). Simulations using WACCM can produce rather realistic gravity waves. A recent study examined the wind shears in the simulations and found that the vertical shear of horizontal wind reaches peak values around the mesopause and the tropical and midlatitude tropopause, consistent with the spatial structure of static stability.

This simulation below is from the same model (WACCM with high-resolution). The first half of the video shows the wind near Earth surface where gravity waves are generally weak and not evident. The second half shows the wind in the lower thermosphere, where gravity waves are very active and are actually the dominant feature.

The study analyzed the magnitude, latitudinal dependence, and statistics of the large wind shears in both regions, and found that they compare well with available observations. This study also found that smaller scale processes, which are likely gravity waves, significantly contribute to the large shears, and may play a dominant role in producing the largest shears. The mesopause is known to be important for controlling exchanges of key species (e.g., atomic oxygen and nitric oxides) between the thermosphere and mesosphere that determines the compositional and thermal structures of the space environment, while the tropopause is known to play a key role in the transport of water vapor and ozone between the stratosphere and the troposphere. The implications of the gravity waves and the large winds for transport and mixing are explored and found to be potentially important.

This study, Large Wind Shears and Their Implications for Diffusion in Regions With Enhanced Static Stability: The Mesopause and the Tropopause was recently published in the Journal of Geophysical Research: Atmospheres.

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