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Browsing by Author "Birner, Thomas, advisor"

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    Connections between climate sensitivity and large-scale extratropical dynamics
    (Colorado State University. Libraries, 2019) Davis, Luke L. B., author; Thompson, David W. J., advisor; Birner, Thomas, advisor; Randall, David A., committee member; Pinaud, Olivier, committee member
    The response of the extratropical storm tracks to anthropogenic forcing is one of the most important but poorly understood aspects of climate change. The direct, thermodynamic effects of climate change are relatively well understood, but their two-way interactions with large-scale extratropical dynamics are extremely difficult to predict. There is thus continued need for a robust understanding of how this coupling evolves in space and time. The dry dynamical core represents one of the simplest possible numerical models for studying the response of the extratropical storm tracks to climate change. In the model, the extratropical circulation is forced by relaxing to a radiative equilibrium profile using linear damping. The linear damping coefficient plays an essential role in governing the structure of the circulation. But despite decades of research with the dry dynamical core, the role of the damping coefficient in governing the circulation has received relatively little scrutiny. In this thesis, we systematically vary the damping rate and the equilibrium temperature field in a dry dynamical core in order to understand how the amplitude of the damping influences extratropical dynamics. Critically, we prove that the damping rate is a measure of the climate sensitivity of the dry atmosphere. The key finding is that the structure of the extratropical circulation is a function of the climate sensitivity. Larger damping timescales – which are equivalent to higher climate sensitivities – lead to a less dynamically active extratropical circulation, equatorward shifts in the jet, and a background state that is almost neutral to baroclinic instability. They also lead to increases in the serial correlation and relative strength of the annular modes of climate variability. It is argued that the climate sensitivity of the dry atmosphere may be identifiable from its dynamical signatures, and that understanding the response of the circulation to climate change is critically dependent on understanding its climate sensitvity.
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    Origins and impacts of tropopause layer cooling in tropical cyclones
    (Colorado State University. Libraries, 2020) Rivoire, Louis, author; Birner, Thomas, advisor; Knaff, John A., advisor; Bell, Michael M., committee member; Davis, Christopher A., committee member; Kummerow, Christian D., committee member; Venayagamoorthy, Subhas K., committee member
    Remote sensing data from GPS radio occultation reveal temperatures lower than climatological average over a layer several kilometers deep near the tropopause above tropical cyclones (TCs). This signal, here referred to as tropopause layer cooling (TLC), occurs primarily during TC intensification and on spatial scales of the order of 1000 km. TLC has been hypothesized to be the result of: 1) Adiabatic expansion in cloud tops that overshoot the local level of neutral buoyancy. 2) Long wave radiative effects near the cloud top. 3) Adiabatic expansion in the TC secondary circulation. The relative role of these mechanism has not been quantified yet, perhaps pertaining to the large uncertainties and relative lack of vertical resolution of observational data sets and numerical modeling studies near the tropopause. Given the complex relationships between the thermal structure of the upper troposphere and the TC secondary circulation, determining which mechanisms are at play is paramount. TLC is also expected to destabilize the upper troposphere to convection and allow clouds to reach higher altitudes, likely leading to subtle but consequential changes in the secondary circulation and associated latent heating vertical distribution. Low temperatures near the tropopause can lead to in situ formation of cirrus clouds, which impact the radiative budget in the tropical tropopause layer. Lastly, low temperatures above convective systems have been linked to dehydration of the stratosphere, prompting the question of the role of TCs on the climate. Mechanism 1 is discussed in light of existing literature and suggested to be of marginal importance. Mechanisms 2 and 3 are examined using a combination of observational and theoretical analysis, and numerical modeling. Radiative heating rates calculated using cloud properties retrieved by the A-train suggest that mechanism 2 may explain up to half of TLC in the inner core, but only marginal amounts of TLC at larger radii. While reanalysis data sets suggest that mechanism 3 may explain TLC, numerical simulations of TCs with higher resolution suggest that mechanism 3 does not act in a way consistent with the secondary circulation as is typically pictured, and may need to be revisited. Other mechanisms involving processes which violate gradient wind balance near the tropopause need to be formulated. Finally, feedbacks between TLC, cloud structure, and TC dynamics are examined using parcel theory and idealized simulations. Parcel theory predicts that the TC thermal structure exerts a positive feedback on cloud top height during intensification, especially when convective entrainment is taken into account. While idealized simulations capture this general behavior, they exhibit other complex, transient behaviors which indicate breaking points in the interaction between clouds and their thermal environment.
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    Quantifying deep convective influence on the tropical tropopause layer (TTL)
    (Colorado State University. Libraries, 2011) Paulik, Laura C., author; Birner, Thomas, advisor; Stephens, Graeme, committee member; Heald, Colette, committee member; Krueger, David, committee member
    The transition between the troposphere and the stratosphere is best described as a layer containing both tropospheric and stratospheric characteristics. In the tropics, this region is known as the Tropical Tropopause Layer (TTL). The TTL roughly spans the altitude range of 12-18 km, bounded from above by the cold point tropopause (CPT) and from below by the main convective outflow level. This region is unique in that it is subject to both tropospheric and stratospheric processes (e.g. deep convective transport/the stratospheric circulation). Processes in the TTL set the boundary condition for atmospheric constituents entering the stratosphere. This thesis aims to better quantify deep convective influence on the TTL using two approaches. The first approach investigates TTL ozone using the Southern Hemisphere Additional Ozonesondes (SHADOZ) dataset. Low ozone concentrations in the TTL are indicative of deep convective transport from the boundary layer. A new diagnostic, the "ozone mixing height" is introduced that identifies the maximum altitude in a vertical ozone profile up to which reduced ozone concentrations, typical of transport from the boundary layer are observed. Deep convective temperature and stratification signals in the TTL are quantified using this diagnostic. The second approach collocates deep convective clouds identified by CloudSat 2B-CLDCLASS with COSMIC GPS temperature profiles. Results suggest the convective temperature signal is large-scale and persistent in time; however, it is only the convective events that penetrate into the upper half of the TTL that have a significant impact on TTL temperature. Finally, CloudSat 2B-CLDCLASS data is used in conjunction with the SHADOZ dataset revealing that deep convective cloud top heights appear to be well approximated by the level of neutral buoyancy.
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    Seasonal to multi-decadal variability of the width of the tropical belt
    (Colorado State University. Libraries, 2013) Davis, Nicholas Alexander, author; Birner, Thomas, advisor; Thompson, David, committee member; Venayagamoorthy, Karan, committee member
    An expansion of the tropical belt has been extensively reported in observations, reanalyses, and climate model simulations, but there is a great deal of uncertainty in estimates of the rate of widening as different diagnostics give a wide range of results. This study critically examines robust diagnostics for the width of the tropical belt to explore their seasonality, interannual variability, and multi-decadal trends. These diagnostics are motivated by an exploration of two simple models of the Hadley circulation and subtropical jets. The width based on the latitudes of the maximum tropospheric dry bulk static stability, measuring the difference in potential temperature between the tropopause and the surface, is found to be closely coupled to the width based on the subtropical jet cores on all timescales. In contrast, the tropical belt width and Northern Hemisphere edge latitudes based on the latitudes at which the vertically-averaged streamfunction vanishes, a measure of the Hadley circulation's poleward edges, lags those of the other diagnostics by approximately one month. The tropical belt width varies by up to ten degrees latitude among the diagnostics, with trends in the tropical belt width ranging from -0.5 to 2.0 degrees per decade over the 1979-2012 period. Nevertheless, in agreement with previous studies nearly all diagnostics exhibit a widening trend, although the streamfunction diagnostic exhibits a significantly stronger widening than either the jet or dry bulk stability diagnostics. Finally, GPS radio occultation observations are used to assess the ability of the reanalyses to reproduce the tropical belt width, finding that they better situate the latitudes of maximum bulk stability versus those of the subtropical jets.
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    The dynamics of Hadley circulation variability and change
    (Colorado State University. Libraries, 2017) Davis, Nicholas Alexander, author; Birner, Thomas, advisor; Randall, David A., committee member; Barnes, Elizabeth A., committee member; Venayagamoorthy, Subhas K., committee member; Randel, William J., committee member
    The Hadley circulation exerts a dominant control on the surface climate of earth's tropical belt. Its converging surface winds fuel the tropical rains, while subsidence in the subtropics dries and stabilizes the atmosphere, creating deserts on land and stratocumulus decks over the oceans. Because of the strong meridional gradients in temperature and precipitation in the subtropics, any shift in the Hadley circulation edge could project as major changes in surface climate. While climate model simulations predict an expansion of the Hadley cells in response to greenhouse gas forcings, the mechanisms remain elusive. An analysis of the climatology, variability, and response of the Hadley circulation to radiative forcings in climate models and reanalyses illuminates the broader landscape in which Hadley cell expansion is realized. The expansion is a fundamental response of the atmosphere to increasing greenhouse gas concentrations as it scales with other key climate system changes, including polar amplification, increasing static stability, stratospheric cooling, and increasing global-mean surface temperatures. Multiple measures of the Hadley circulation edge latitudes co-vary with the latitudes of the eddy-driven jets on all timescales, and both exhibit a robust poleward shift in response to forcings. Further, across models there is a robust coupling between the eddy-driving on the Hadley cells and their width. On the other hand, the subtropical jet and tropopause break latitudes, two common observational proxies for the tropical belt edges, lack a strong statistical relationship with the Hadley cell edges and have no coherent response to forcings. This undermines theories for the Hadley cell width predicated on angular momentum conservation and calls for a new framework for understanding Hadley cell expansion. A numerical framework is developed within an idealized general circulation model to isolate the mean flow and eddy responses of the global atmosphere to radiative forcings. It is found that it is primarily the eddy response to greenhouse-gas-like forcings that causes Hadley cell expansion. However, the mean flow changes in the Hadley circulation itself crucially mediate this eddy response such that the full response comes about due to eddy-mean flow interactions. A theoretical scaling for the Hadley cell width based on moist static energy is developed to provide an improved framework to understand climate change responses of the general circulation. The scaling predicts that expansion is driven by increases in the surface latent heat flux and the width of the rising branch of the circulation and opposed by increases in tropospheric radiative cooling. A reduction in subtropical moist static energy flux divergence by the eddies is key, as it tilts the energetic balance in favor of expansion.
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    Transport-radiation feedbacks of ozone in the tropical tropopause layer
    (Colorado State University. Libraries, 2017) Charlesworth, Edward, author; Birner, Thomas, advisor; Ravishankara, A. R., committee member; Oprea, Iuliana, committee member
    The tropical tropopause layer (TTL) is a region in the atmosphere that shows an interesting combination of tropospheric and stratospheric characteristics over the extent of several kilometers. For example, the TTL shows both convectively-driven tropospheric dynamics and the beginning of the mechanically-driven Brewer-Dobson circulation. The TTL is also important for climate due to its role as the gateway for most air that enters the stratosphere. In this work, a single-column model is used to investigate why a tropical tropopause layer of the observed vertical extent exists. This is done through computations of radiative convective equilibrium temperatures and interactive photochemical equilibrium ozone concentrations. The model uses only a basic simulation of ozone chemistry, convection, and stratospheric upwelling, but the results show that such a simplified expression of critical processes can produce temperature and ozone profiles that are very similar to observations. It is found that vertical transport of ozone by the Brewer-Dobson circulation and its associated effects on radiative heating rates is of first-order importance in producing the observed temperature structure of the tropical tropopause layer, within this simple modeling context. Adiabatic cooling due to stratospheric upwelling is found to be equally important to generate the tropical tropopause layer. With these combined processes, it is suggested that the even the lowest upwelling velocities on the order of observed upwelling can produce a TTL. With regards to climate change through the strengthening Brewer-Dobson circulation, this model suggests that an increase in upwelling from 0.5 to 0.6 mm/s should cool the cold point tropopause by 3.5 K and loft it by half a kilometer.
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    Tropical tropopause layer variability associated with the Madden-Julian oscillation during DYNAMO
    (Colorado State University. Libraries, 2015) Dagg, Erin L., author; Birner, Thomas, advisor; Johnson, Richard H., advisor; Schubert, Wayne H., committee member; Kirby, Michael, committee member
    As the transition region between the troposphere and stratosphere, the tropical tropopause layer (TTL) has importance as the gateway to the stratosphere for atmospheric tracers such as water vapor.  This has implications for Earth's radiative budget and climate.  Observations in this region show time variations across multiple scales that are not fully understood, including the intraseasonal variability of the Madden-Julian oscillation (MJO). In this study, we investigate the evolution of TTL properties and their vertical structure during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign from October-December 2011.  This time period is particularly interesting in that two prominent MJO passages were seen over the tropical Indian Ocean.   We focus analysis on two equatorial sites.  Gan Island, Maldives (0.7° S, 73.2° E) provides a better understanding of the response of the TTL to MJO dynamics in the region of initiation.  Manus Island, Papua New Guinea (2.1° S, 147.4° E) observations portray a later stage of the MJO during its eastward propagation.  We use multiple datasets, including high vertical resolution, three-hourly atmospheric soundings over the three-month period.  CALIPSO satellite data is additionally used in determining the presence of thin cirrus clouds and their impact on radiative heating rates.  Characteristics of the broadscale structure of the MJO are analyzed, as well as higher-frequency variations of the flow near the TTL accompanying an increase in MJO-related deep convective clouds. Spectral filtering is used to isolate low-frequency variability, Kelvin wave activity, and higher-frequency gravity wave perturbations.  A 7-20 day bandpass of the temperature and zonal wind fields reveals strong Kelvin wave signals in late October and early December.  This Kelvin wave response to large-scale convection exhibits a downward phase velocity consistent with an eastward-propagating energy source below.  The descending cold phase between 100-150 hPa coincides with a lowering of the cold point tropopause and an increase in cirrus cloud frequency preceding the active phase of the MJO.  The wave signals dissipate before reaching Manus Island, suggesting that the MJO may have decoupled from convection.  Further analysis shows lower stratospheric gravity wave activity does not appear to be modulated by the MJO, but is generally stronger at Manus Island due to its proximity to the west Pacific warm pool.
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    Vertical and horizontal mixing in the tropical tropopause layer
    (Colorado State University. Libraries, 2015) Glanville, Anne Alexandra, author; Birner, Thomas, advisor; Randall, David A., committee member; Oprea, Iuliana, committee member
    Nearly all air enters the stratosphere through a single layer in the tropics. The tropical tropopause layer (TTL) is a transition region between the troposphere and stratosphere and its roles include regulating stratospheric chemistry and surface climate. Multiscale dynamics existing in the TTL range from transient convection to the hemispheric wave-driven circulation and the relative influences of these processes still remain unclear. This study pays special attention to vertical and horizontal mixing which are associated with breaking gravity waves and Rossby waves, respectively. Our study quantifies the roles of these dynamics by taking advantage of the conservative nature of water vapor in the lower stratosphere. Unable to change concentration in the lowermost stratosphere after passing through the cold point, water vapor becomes a tracer for total transport and its signal is known as the tape recorder. This tape recorder is studied using observations, reanalysis data, a chemistry-climate model (CCM), and simple idealized modeling. Modifying past methods, we are able to capture the seasonal cycle of effective transport in the TTL and we introduce seasonally-dependent dynamics to a one-dimensional model and perform a parameter-sweep to test all possible dynamical combinations. Simulating with unrealistic annual mean transports results in bimodality where either vertical advection or vertical mixing dominate. The solutions that depend on amplified vertical advection disappear when seasonally-dependent transports are used. Overall, all datasets show that vertical mixing is as important to TTL transport as vertical advection itself even during boreal winter when advection peaks. The reanalysis and CCM have increased effective transport compared to observations, however, they rely on different dynamics. The reanalysis has amplified vertical mixing while the CCM has amplified vertical advection. This hints at the possible influence of spurious diffusion from data assimilation and its role in amplifying TTL transport.
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    Wave-mean flow positive feedbacks associated with sudden stratospheric warmings
    (Colorado State University. Libraries, 2014) Sjoberg, Jeremiah P., author; Birner, Thomas, advisor; Eykholt, Richard, committee member; Garcia, Rolando, committee member; Schubert, Wayne, committee member; Thompson, David, committee member
    Sudden stratospheric warmings - most often characterized by zonal mean zonal wind easterlies at 60°N, 10 hPa - represent the largest dynamical perturbations to the wintertime polar stratosphere. Despite this, the predictability of sudden warmings remains low, in part because the forcing of these warming events involves a nonlinear positive feedback between planetary scale waves and the zonal wind of the stratosphere. In the wave-mean flow positive feedback, wave forcing decelerates the mean flow, allowing enhanced upward wave propagation, which then further decelerates the mean flow, etc., until the mean flow no longer supports wave propagation. This positive feedback process is crucial for the initiation of such events. Because the associated low predictability stems from poorly resolving initiation, this dissertation focuses on increasing mechanistic understanding of the wave-mean flow positive feedback associated with sudden stratospheric warmings. A simple model of wave-mean flow interaction is the first tool utilized here. In the original form of the model, constant bottom boundary wave forcing, set by geopotential height perturbations, results in a zonal wind state that oscillates between positive values (westerlies) and negative values (easterlies). We present a reformulation of the bottom boundary condition which allows for specification of the upward wave activity flux. Unlike with the original bottom boundary condition, we may precisely set the wave amplitudes propagating into the model domain. With this reformulated model, steady incoming wave fluxes lead to a steady zonal wind response. The oscillating state from the original model is found to rely on a representation of the positive feedback that is too strong. Transient forcing experiments in the reformulated simple model support previous results that there is a preferential wave forcing time scale on the order of 10 days for sudden stratospheric warmings. Forcing the model near this preferential time scale most efficiently drives the positive feedback. Lower stratospheric wave fields in reanalysis data show supporting evidence for these preferential wave forcing time scales prior to sudden stratospheric warmings. Pulses of wave activity flux are also analyzed in reanalysis data, and a set of pulses which are a novel proxy for strong wave-mean flow positive feedback are found. The zonal wind near these pulses display the expected characteristics of the positive feedback: strong precedent zonal winds and strong subsequent wind decelerations. This proxy is thus a useful diagnostic for the wave-mean flow positive feedback. A general circulation model forced by idealized planetary scale topography is employed to perform high order experiments. By stepwise increasing the height of the topography, we find that the frequency of sudden stratospheric warmings within the model increases nonlinearly to a maximum at moderate topographic heights and then strongly jumps down to a lower, steady value for still higher topography. Analyzing the proxy for positive feedback here reveals that the positive feedback is strongest in the range of topographic heights associated with the largest occurrence of sudden warmings, and also that preferential wave forcing time scales on the order of 10 days are upheld.