Session H12: Geophysical Fluid Dynamics: Atmospheric (5:45pm - 6:30pm CST)

Topological flow data analysis for identification of atmospheric blockings

In this talk, we propose a new objective algorithm to detect atmospheric blocking by extracting topological features of geo-potential height data at 500 hPa. It provides a unique symbolic representation, named COT representation, and a discrete graph structure, called a Reeb graph, to each data based on the mathematical classification theory of streamline topology for structurally stable Hamiltonian vector fields. By comparing the results with those detected by the conventional algorithms, we show that the proposed algorithm finds atmospheric blocking events more simply and effectively with fewer meteorological parameters. Moreover, it can determine the morphological types of blocking events, which has been considered to be a difficult task so far. A long-term statistical investigations with using the algorithm not only reveals the locality and seasonality of atmospheric blocking, but it is also informative for the climate predictions.   

Cold-cored tropical cyclone

A tropical cyclone is a warm-cored storm. The warm core powers its intense winds. Upon landfall, the warm core decays, and consequently, so do the attendant winds. This fortunate occurrence limits the destruction largely to the coastal region near landfall. Unfortunately, however, some landfalling tropical cyclones transition to extratropical cyclones. Powered by a baroclinically-unstable environment, an extratropical cyclone can reintensify and inflict substantial damage thousands of kilometers inland. Thus, accurate forecasting of whether or not a tropical cyclone will transition to an extratropical cyclone carries immense consequences. To objectively identify the completion of the transition, the prevailing forecasting methods invoke a telltale signature: the birth of a cold core in the lower troposphere. We show computationally and argue theoretically that a landfalling tropical cyclone engenders such a cold core purely from internal dynamics without any recourse to an external, unstable environment. That is, a ``cold-cored tropical cyclone'' is but the natural state of a tropical cyclone past landfall.   

Effect of particle inertia on the alignment of small ice crystals in turbulent clouds

Small non-spherical particles settling in a quiescent fluid tend to orient so that their broad side faces down, because this is a stable fixed point of their angular dynamics at small particle Reynolds number. Turbulence randomises the orientations to some extent, and this affects the reflection patterns of polarised light from turbulent clouds containing ice crystals. An overdamped theory predicts that turbulence-induced fluctuations of the orientation are very small when the settling number Sv (a dimensionless measure of the settling speed) is large. At small Sv, by contrast, the overdamped theory predicts that turbulence randomises the orientations. This overdamped theory neglects the effect of particle inertia. Therefore we consider here how particle inertia affects the orientation of small crystals settling in turbulent air. We find that it can significantly increase the orientation variance, even when the Stokes number St (a dimensionless measure of particle inertia) is quite small. We identify different asymptotic parameter regimes where the tilt-angle variance is proportional to different inverse powers of Sv. Our theory predicts how the degree of alignment depends on particle size, shape, and turbulence intensity.   

Not Participating

We propose a new framework for the mathematical properties of atmospheric blocking events . The occurrence of blockings is associated with conditions featuring anomalously high instability. Longer-lived blockings are very rare and have typically higher instability. In the case of Atlantic blockings, predictability is reduced at the onset and decay of the blocking event, while it is higher in the mature phase. Blockings are realised when the trajectory of the system is in the neighbourhood of a specific class of unstable periodic orbits (UPOs). UPOs corresponding to blockings have, indeed, a higher degree of instability compared to UPOs associated with zonal flow. The analysis of UPOs elucidates that the model features a very severe violation of hyperbolicity, due to the presence of a substantial variability in the number of unstable dimensions: atmospheric states can differ a lot in term of their predictability. The lack of robustness associated with the violation of hyperbolicity might be a basic cause contributing to the difficulty in representing blockings in numerical models and in predicting how their statistics will change as a result of climate change. This corresponds to fundamental issues limiting our ability to construct very accurate numerical models of the atmosphere   

Multi-Point Monin--Obukhov Similarity of Turbulence Cospectra in the Convective Atmospheric Boundary Layer

The shear-stress and temperature-flux cospectra in the convective atmospheric surfce layer are predicted using the multi-point Monin-Obukhov similarity theory (MMO). The surface layer contains the convective layer ($z \gg -L$) and the convective-dynamic layer ($z \ll -L$). The former consists of the convective range ($k \ll 1/z$) while the latter consists of the convective range ($k \ll -1/L$) and the dynamic range ($-1/L \ll k\ll 1/z$), where $z$, $k$, and $L$ are the height from the ground, the horizontal wavenumber, and the Obukhov length, respectively. We predict the cospectra for both ranges. They have the same scaling in the convective range for both $z \ll -L$ and $z \gg -L$. The shear-stress cospectrum and the vertical temperature-flux cospectrum have $k^0$ scaling in both the convective and dynamic ranges. The horizontal temperature-flux cospectrum has $k^{-1/3}$ and $k^{-1}$ scaling in the convective and dynamic ranges respectively. The predicted scaling exponents are in general agreement with high-resolution large eddy simulation results. However, the horizontal temperature-flux cospectrum is found to change sign from the dynamics range (negative) to the convective range (positive), which is shown to be caused by the the temperature--pressure-gradient interaction.   

Daily stability patterns of atmospheric boundary layer in Coastal region

Physical processes that affect the coastal environment are often unique because of the dominant effect of the interactions between the land and the sea. Atmospheric stability plays a vital role in local and meso-scale atmospheric circulation and transportation processes in coastal region, yet relatively little is known about stability condition in this region. A coast separates two drastically different surfaces, where the atmospheric boundary layer in this region is horizontally inhomogeneous with rapid temporal forcing conditions. This works presents diurnal variation of turbulent characteristics and the atmospheric stability during different seasons of a year in Qatar coastal region of Qatar in the Persian Gulf. The micrometeorological data are collected from a measurement station in at the coastal site (26.08N, 51.36E) from September 2015 to August 2016 using sonic anemometers and a weather station. The results identify two stability patterns of `orderly' and `disheveled' daily patterns. An `orderly' day is identified as days when at least, 90{\%} of the time the wind comes from the land (180º - 360º direction). Days that are not accepted as `orderly', are classified into `disheveled'. The atmospheric stability analyze indicates that the `orderly' days show a clear `U' shape variation of the Obukhov stability parameter z/L, with an average value of 0.105. While the stability in `disheveled' days represent a chaotic trend with a~right-skewed \quad distribution of more unstable cases, average z/L$=$ -0.098. Similar orderly and disheveled trends are observed for turbulent kinetic energy and shear stress in these days.   

Large-Eddy Simulation of Atmospheric Boundary-Layer Gusts for Small Unmanned Air Systems

The Urban Air Mobility(UAM) concept involves safe and efficient transportation of goods and passengers over populated urban spaces using Unmanned Air Systems(UAS). Physical damage to goods and injuries to passengers, caused by unanticipated wind gusts is a major challenge for UAM and UAS. Predicting its occurrence in real-time continues to be a challenge for such low-altitude operating applications, especially in an urban terrain setting. Large Eddy Simulations(LES) have always helped researchers to closely study and understand the turbulent characteristics in different flow fields. In this work we aim to use LES with the intent to better understand and capture turbulent flow events observed in Atmospheric Boundary Layer(ABL) like gusts, over a terrain covered by urban canopy. Resolving the flow properties close to the roughness sub-layer (flow region in which the roughness has a direct influence) and inside the roughness canopy, might yield a better understanding of the phenomenon and hence is of great importance to us. Previous studies by researchers on topics like flow around low-rise buildings immersed in ABL, flow over a flat terrain covered by roughness blocks and many others, provide us with a good starting point and we would like to build upon this further for our work.   

Climate and atmospheric turbulence analysis during Shamal events in Persian Gulf

The impact of low-level jets (LLJs) on events such as dust/salt storms has demonstrated the need for a clear understanding of the physical mechanisms underlying such LLJs. Shamal is a prominent LLJ blowing over Persian Gulf that is characterized by wind speeds of 30 m/s, or more, at an altitude of 300--700 m, and much lower speed at surface level, with a Northerly-Northwesterly direction. This is mainly a thermal phenomenon where the vertical temperature profile forms an inversion. A “Shamal day” is commonly defined as a day with a meteorological in Northerly-Northwesterly direction and wind speed over 8.75 m/s in at least 3 hours of the day. This work analyzes the climate and turbulence characteristics during Shamal events that consists of at least two consecutive Shamal days. Micrometeorological data was obtained at a coastal site of Qatar (26.08N, 51.36E) from September 2015 to August 2016. Three sonic anemometers and a weather station placed on a 9 m tower recorded the wind velocity, direction, temperature, humidity, and pressure data. It is observed that during a Shamal event, temperature and humidity decrease while pressure increases. General climate and turbulence characteristics of the coastal cite presented and compared with that of the Shamal events, with one day before and after each event. Turbulence kinetic energy and shear stress, heat flux, Richardson number, and Monin-Obukhov length are analyzed for characterization of turbulent mixing processes in Shamal days.   

Analysis of Gravity Wave Data from Atmospheric Soundings

Methods to improve weather and climate predictions necessitate better observations and understanding of energy and momentum transfer in the atmosphere, particularly the phenomenon known as \textit{gravity waves}. Gravity waves were observed by analyzing data collected from radiosondes launched from Oklahoma State University's Unmanned Aircraft Flight Station. Multiple different sources have been found which generate gravity waves, including those formed by low-level jets. Due to conduction caused by the sun heating the Earth, a propagating wind is created displacing air parcels which are the beginning production of a gravity wave. Currently, the wind observations are analyzed using a combination of the Morlet wavelet transform and the hodograph methods to identify gravity waves. Further analysis detects the location, frequency, propagation direction, and amplitude which can be used to distinguish the cause. This research accomplishes this by comparing examples from atmospheric background readings to assist in identifying the source of the gravity waves, whether that be from convection, topography, or wind shear. This research is supported by NSF.   

Data Collection from Atmospheric Soundings for Gravity Wave Detection

The discovery of atmospheric gravity waves~and their~capability to transfer energy through the atmosphere has accelerated the interest in weather balloon soundings and the data~collected to locate and characterize the generation of gravity waves.~By examining the formation of the gravity waves, which may be caused by wind shear, convection, or topography,~one is~able to get an improved understanding of how~they~might~impact atmospheric conditions,~with the interest in~using the information to~improve~weather and climate preditions. This research is focused on~using weather balloons and radiosondes to take atmospheric readings while in flight.~From sounding~data~taken~at~sunrise, the observations indicate that the production of gravity waves~might~be~similar~to those previously observed during~total solar eclipses. This presentation will discuss the experimental observation process and the methodology used to identify gravity waves of various types and how these observations are confirmed.~   

Atmospheric Observations from High Altitude Solar Balloons

Solar balloons provide a potential platform for atmospheric observations for both terrestrial and extraterrestrial environments. Unlike traditional helium balloons, a solar balloon generates lift by absorbing solar radiation, which raises the temperature inside the envelope. Using these balloons, we are able to collect data on atmospheric conditions at variable altitudes for multi-hour durations. As the air in the envelope is heated, the buoyant force exerted on the balloon will cause it to rise in the atmosphere. Changes in atmospheric conditions are measurable through the resultant changes in the solar balloon's flight dynamics. A recently funded NASA PSTAR project plans to demonstrate the ability to use solar balloon-based infrasound measurements to study seismic activity and atmospheric conditions in Oklahoma. This will serve as a technology demonstration and Earth analog for the proposed mission to study Venus using solar balloons as an aerial platform for collecting atmospheric and infrasonic measurements. In preparation for these analog missions, scheduled for the summer of 2021, we have begun launching balloons with various data packages. This presentation will report on the preliminary solar balloon launches and measurements.