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About Ahmad Talaei

My research focuses on atmospheric Gravity Waves (GWs) which are known to be important drivers of mesoscale fluctuations throughout Earth atmosphere. They are generated by a variety of sources like mountains, oceans, tropical convection, severe weather events, etc. As they propagate upward, their amplitudes increase and they speed up. This growth in amplitude cannot last forever and most of the large-amplitude gravity waves ultimately break in ~100 to 150 km height region (lower thermosphere). The dissipated energy and momentum is transferred to the atmospheric mean circulation. This results in large changes in upper atmospheric wind and temperature structures. Indeed, GWs play central roles in transferring energy and momentum from troposphere/stratosphere to mesosphere/lower thermosphere
 
1. What is an atmospheric gravity wave?
The atmospheric gravity (or buoyancy) wave is not the new concept in physics as it’s one of the oldest branches in fluid dynamics. However, observation of moving irregularities in the higher altitudes especially in ionosphere and invoking the atmospheric gravity waves as a cause of irregularities is almost new in physics. As a comprehensive work, Liller and Whipple published a paper in 1954 about measurements of wind velocities from the motions of five photographic persistent meteor trains vs altitude in the range from 81 to 113 km. The results certainly looked irregular and much of the efforts to explain the abnormalities were initially directed toward magneto-ionic, hydro-magnetic, electrostatically induced motions of ionization and the turbulences mostly things coming from the Sun. However, it was concluded that the theory does not accurately represent the properties of meteor at those heights.
 
There were some meteorological works that had previously dealt with atmospheric waves in the presence of gravity and derived a dispersion relation for such waves and showed their compatibility with speed of propagation at those altitudes (things coming from earth). However, with meteorological sources in mind, many questions raised with no answers as:
a. What would be the source of those presumed waves?
b. How do the waves grow with height as they propagated away from their source?
c. How did the noisy spectrum of meteor altitude give way to the relatively quiescent wave field in some layers? And so on.
 
Hines was the first person who added gravity and pressure gradient of the neutral gas to his hydro-magnetic formulation (called initially acoustic-gravity-hydromagnetic system) and represented a noisy system as combination of waves with meteorological sources and hence completely answered to all those questions by the significant property of gravity waves: the energy, if carried by gravity waves can propagate upward for the downward phase progression.
 
Consider an atmosphere at rest (no background wind) and an air parcel of mass m in equilibrium with its environment at height z. Any small vertical disturbance applied to the air parcel would displace the parcel slightly upward to the distance z+δz, which then would be restored by gravity force and will move a little bit less than its initial height to distance z-δz due to the its inertia. At this moment, the buoyancy force due to the gradient of the atmosphere mass density will restore the parcel upward and so on. This "organized oscillation" of the parcel in a big scale produces waves by the term of gravity force and so called gravity waves. The frequency of this oscillation is proportional to the gravity constant and the gradient of density over density itself. The significant feature of the gravity waves is: if wave fronts are moving upward, the wave energy must be propagated downward.
 
2. What are atmospheric gravity wave sources?
Living on a spherical spinning planet creates many interesting events required to make the planet so perfect a place for life. However, the physics behind these events are astonishingly complicated so need simple (and sometimes unreal) explanations at the beginning. Now, as a simple example for the source of those vertical disturbances mentioned above, assume that the diameter of earth is shrinked to 1 m. From conservation of angular momentum and assuming that all the Earth mass is included within 1 m diameter sphere (!) it has to rotate about 1.9e9 times each second. Such a dynamical sphere, spinning very fast, has potential to create the mentioned disturbances in atmosphere continuously and even more if we consider the mountains on its surface (like a rocking paddle at the top of water-tank acted to produce the waves). Now, let’s start with the gravity waves generated by mountains.

The orographic gravity waves are stationary waves that remain fixed over the generating mountains. Having a stationary wave (not a standing wave but a wave that stays fixed in a space for a long time without moving) is a little bit hard to understand. You may think it's like the disturbances generated by the water flowing over the rocks in a running streams. As long as there exists a water flow, you see a wave-shape water flow over the rocks that these waves stay there without displacement. The amplitude of these waves, obviously, depends on the water flow. In the same way, waves are generated over the mountains if there exist a wind (perpendicular to the mountain ridge line). Now, if the waves appear stationary, then it must be moving against the wind at the same speed as the wind but in the opposite direction. This way, we can attach a phase speed (intrinsic or by an observer drifting with the background wind) to these waves and then show their dynamical effects in our calculations. Therefore, the mountains are one source for vertical disturbances of atmosphere and they produce waves which transport energy from lower atmosphere up to the middle and upper atmosphere.

In general, any vertical process that can disturb the planetary atmosphere (like Earth tropical convection, mountain, ocean, severe weather events, adjustment of balanced flows and/or instability, aurora heating etc.) once that disturbance reaches the stably stratified layer, it gains jet-like wave motion propagating upward to the mesosphere, ionosphere and so on. A stably stratified fluid bears positive vertical temperature gradient or the fluid density increases with depth so it displays a negative vertical density gradient. Except for a relatively thin layer in contact with Earth's surface, the planetary boundary layer, the atmosphere is almost always stably stratified. For example, stratosphere is a layer of atmosphere which is stratified in temperature with warmer layers higher up and cooler layers farther down. Any stably stratified fluid can creates those mentioned "organized oscillations" of parcels for any vertical disturbance.
 
As the waves propagate upward their amplitudes increases and they speed up. It’s like running from sea to beach with the maximum power. As the water level reaches downer, the runner gains more speed and then at the coast side the person achieves the highest speed. Indeed, the energy is stored and transferred by waves but in different layers it shows different features.
 
Another example is cracking the bullwhip. The decrease in linear density of whip causes the amplitude of wave initiated near the handle to grow as the wave moves down the whip. By the time the impulse reaches the end of the whip the perturbation velocity of the whip end exceeds the speed of the sound causing the loud crack sound. The analogous effect in atmosphere (density decrease with height) can explain the wave growth with altitude as they propagated away from their sources. This growth in amplitude cannot last forever and at some critical conditions, the waves evanesce or break and then transfer energy and momentum to atmospheric mean circulation.
 
The measurements of Hydroxyl (OH) airglow emission make is possible to look at these waves activities in mesosphere/mesopause. Our data set has been obtained using an Advanced Mesospheric Temperature Mapper (AMTM), a new instrument designed at Utah State University (USU). The AMTM is a ground-based infrared imaging system that measures selected emission lines in the mesospheric OH airglow emission in order to create intensity and temperature maps around altitude. It provides high spectral sensitivity over a large  field of view which covers about  area of the mesosphere. As gravity waves propagate upward, they pass through the OH emitting layer in the mesosphere and hence significantly modify the emission profile of the OH bands. This creates wavelike pattern in the emission layer. The AMTM captures these patterns as gravity wave events in the upper mesosphere.
 
3. Why atmospheric gravity waves are important?
First of all I have to emphasize that the primary mechanism through which energy and momentum are transferred from lower atmosphere to the upper atmosphere and ionosphere is through the generation and propagation of "waves". These waves redistribute ionospheric plasma and also drive the neutral density to the thermosphere. These significantly alter the state of the thermosphere/ionosphere system. This change will affect the prevalence of the resulting ionospheric irregularities.
 
These waves are generated by a variety sources like tides, planetary waves, gravity waves etc. They transport momentum vertically (this momentum transfer is also crucial to the large-scale momentum balance of the stratosphere and mesosphere). Some parts of wave's spectrum achieve convective or shear instability, spawning additional waves or turbulence. Other parts of the wave spectrum are ultimately dissipated by molecular diffusion in 100 to 150 km height region, and some fraction of those waves penetrate all the way to the base of the exosphere (500-600 km).
 
Determining the role of atmospheric gravity waves in coupling between the lower atmosphere and thermosphere and also their dissipation (which drives the mean circulation) will provide vital information not only on the variability of Earth’s near-space environment, but also on terrestrial climate variability and change. Here, I have listed some significant effects of GWs in atmosphere and space:
A. These waves redistribute ionospheric plasma and also drive the neutral density to the thermosphere.
B. These significantly alter the state of the thermosphere/ionosphere system.
C. GWs influences climate change scenarios
D. GWs effects global circulation models
 
4. What's my job?
Well, I actually analyze the image data. During the past years, I developed and tested a program to analyze the speed distribution of mesospheric gravity waves for an extensive amount of images in a month (with a time interval of 34.5 sec between each image, the program analyzes about 79,360 images). This was based on an analysis technique initially developed by a Japanese student, Takahashi Matsuda who visited our lab in the Fall 2014 from the National Institute for Polar Research. The image data I used were continuously taken by AMTM. One of which is operational at the South Pole, Antarctica and a second AMTM has operated at Bear Lake Observatory (BLO), Utah for the summer months. The project was very successful and I received a Blood Howard scholarship and Keith Taylor Fellowship to perform this research. The goal is to increase the temporal capability of the program to include a much broader range of data-collecting times and hence to investigate the seasonal and year to year variability of gravity waves activates which will provide the novel measure of momentum transfer over a much larger time scale.
 
In order to evaluate the horizontal image data, first the stars are removed from the each raw image as well as the dark counts and offset values. The images are calibrated and unwrapped then. Now, the images, if subtracted by the motion of background wind, contain the accurate information about the atmospheric gravity waves only. The series of de-trended images are the inputs for our program. The program gives the horizontal wave parameters like wavelength, period, phase speed and propagation direction directly with applying the standard 3D Fast Fourier Transform (FFT) on the AMTM images. These parameters are obtained based on the pixel brightness of the OH mesosphere emission. Only interested range of wavelengths and periods are selected in order to get the horizontal distribution of gravity-waves phase speed.
 
During the past year, I also developed a new mathematical solution which yields an acoustic-gravity wave equation for a compressible and non-stationary atmosphere without the application of the Boussinesq approximation which has recognized limitations. This research was submitted to J. Geophys. Res., in March and is currently under review [Talaei and M. J. Taylor, 2016]. I proved that such a commonly used approximation does not yield growth in the amplitude of wave solution. In contrast, the new solution introduces a term in the amplitude of the vertical velocity of acoustic-gravity waves which supports the growth in amplitudes as waves propagate opposite to the wind direction. Importantly, this could play a significant role in explaining the directional filtering of atmospheric waves in a realistic atmosphere containing strong and highly variable winds. The resultant wave equation is, indeed, a significant improvement to the well-known Taylor-Goldstein equation, the starting point for most recent analyses of the effects of wind shear on gravity waves. The solution may be particularly important for improved gravity wave propagation studies in the upper mesosphere and thermosphere/ionosphere regions.

I plan to determine the momentum transfer by atmospheric gravity waves using the new mathematical solution. In order to do this, I will need atmospheric wind and temperature data plus the gravity wave activities (parameters) in upper atmosphere. Fortunately, I have a unique opportunity here at USU to do this job. Indeed, AMTM located in BLO provides the gravity waves parameters (e.g., horizontal wavelength and period) around  altitude and the Na Light Detection and Ranging (LIDAR) operated by Dr. Titus Yuan provides background atmospheric parameters (e.g., vertical wind and temperature profile) from  to  altitude. The opportunity is unique because both AMTM and LIDAR are looking in the same volume of sky and thus provide actual atmospheric properties for the wave propagation through the mesosphere, mesopause and lower thermosphere. The idea is to combine the horizontal structure of atmospheric waves with the vertical profile of atmospheric wind in order to estimate the momentum transfer by gravity waves and then quantitatively examine the moving irregularities at higher altitudes. The result could also improve our knowledge about coupling between the lower atmosphere and thermosphere.



Honors and Awards

  • Received Howard L. Blood Scholarship, Utah State University, 2016
  • Received 2016 Gene Adams Endowed Scholarship for academic performance and personal integrity, Utah State University
  • Received 2016 Special Recognition Blood Scholarship for research achievements in fluid theory and atmospheric dynamics, Utah State University
  • Received Keith Taylor Fellowship, Utah State University, 2015
  • Received Howard L. Blood Scholarship, Utah State University, 2013

Education

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Present PhD, Utah State University ‐ Physics
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