Browsing by Author "Randall, David, advisor"

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Open Access
Comparison of convective clouds observed by spaceborne W-band radar and simulated by cloud-resolving atmospheric models
(Colorado State University. Libraries, 2014) Dodson, Jason B., author; Randall, David, advisor; Birner, Thomas, committee member; Maloney, Eric, committee member; Chandrasekar, V., committee member
Deep convective clouds (DCCs) play an important role in regulating global climate through vertical mass flux, vertical water transport, and radiation. For general circulation models (GCMs) to simulate the global climate realistically, they must simulate DCCs realistically. GCMs have traditionally used cumulus parameterizations (CPs). Much recent research has shown that multiple persistent unrealistic behaviors in GCMs are related to limitations of CPs. Two alternatives to CPs exist: the global cloud-resolving model (GCRM), and the multiscale modeling framework (MMF). Both can directly simulate the coarser features of DCCs because of their multi-kilometer horizontal resolutions, and can simulate large-scale meteorological processes more realistically than GCMs. However, the question of realistic behavior of simulated DCCs remains. How closely do simulated DCCs resemble observed DCCs? In this study I examine the behavior of DCCs in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) and Superparameterized Community Atmospheric Model (SP-CAM), the latter with both single-moment and double-moment microphysics. I place particular emphasis on the relationship between cloud vertical structure and convective environment. I also emphasize the transition between shallow clouds and mature DCCs. The spatial domains used are the tropical oceans and the contiguous United States (CONUS), the latter of which produces frequent vigorous convection during the summer. CloudSat is used to observe DCCs, and A-Train and reanalysis data are used to represent the large-scale environment in which the clouds form. The CloudSat cloud mask and radar reflectivity profiles for CONUS cumuliform clouds (defined as clouds with a base within the planetary boundary layer) during boreal summer are first averaged and compared. Both NICAM and SP-CAM greatly underestimate the vertical growth of cumuliform clouds. Then they are sorted by three large-scale environmental variables: total preciptable water (TPW), surface air temperature (SAT), and 500hPa vertical velocity (W500), representing the dynamical and thermodynamical environment in which the clouds form. The sorted CloudSat profiles are then compared with NICAM and SP-CAM profiles simulated with the Quickbeam CloudSat simulator. Both models have considerable difficulty representing the relationship of SAT and clouds over CONUS. For TPW and W500, shallow clouds transition to DCCs at higher values than observed. This may be an indication of the models' inability to represent the formation of DCCs in marginal convective environments. NICAM develops tall DCCs in highly favorable environments, but SP-CAM appears to be incapable of developing tall DCCs in almost any environment. The use of double moment microphysics in SP-CAM improves the frequency of deep clouds and their relationship with TPW, but not SAT. Both models underpredict radar reflectivity in the upper cloud of mature DCCs. SP-CAM with single moment microphysics has a particularly unrealistic DCC reflectivity profile, but with double moment microphysics it improves substantially. SP-CAM with double-moment microphysics unexpectedly appears to weaken DCC updraft strength as TPW increases, but otherwise both NICAM and SP-CAM represent the environment-versus-DCC relationships fairly realistically.
Open Access
Downdraft impacts on tropical convection
(Colorado State University. Libraries, 2013) Thayer-Calder, Katherine, author; Randall, David, advisor; Johnson, Richard, committee member; Maloney, Eric, committee member; Strout, Michelle, committee member
Downdrafts are an integral part of the convective cycle, and have been observed and documented for more than a hundred years. But many questions still surround convective downdrafts and their most difficult to observe properties. These questions have made the parameterization of convective downdrafts in global climate models (GCMs) very difficult. Designers of parameterizations have resorted to a wide range of assumptions and unverified hypotheses in their models of convective downdrafts. In the last ten years, computing resources have advanced to a point where large domain, high resolution cloud resolving models (CRMs) can easily be run for long simulations. This study uses several simulations with 1 km horizontal resolution from the System for Atmospheric Modeling (SAM) v6.8.2 to examine convective downdrafts. We look at Radiative-Convective Equilibrium (RCE), a 21 day case from TOGA-COARE, Weak Temperature Gradient (WTG) simulations with varied shear profiles, and Lagrangian Parcel data to consider many difficult to observe properties of downdrafts. We consider a variety of assumptions and questions that arise in the development of convective parameterizations. Our results show that downdrafts are an important mass flux in all simulations, and that cold pools organize convective systems and enhance updraft Convective Available Potential Energy (CAPE). We examine the ability for downdrafts to help couple deep convection to high relative-humidity regions in the tropics, and find that entrainment is likely a more important process in this relationship. We discuss the impact of downdrafts in maintaining boundary layer quasi-equilibrium, and find that, in our simulations, environmental entrainment has a larger impact on low-level most static energy. Finally, we show results from Lagrangian parcel data that illuminate our downdrafts as existing in an unsaturated state, with increasing buoyancy as they descend. We show that many of our downdrafts have positive buoyancy perturbations, suggesting the presence of warm downdrafts and under-shooting bottoms in heavily precipitating tropical systems.
Open Access
Effects of warming and stratospheric aerosol injection on tropical cyclone distribution and frequency in a high-resolution global circulation model
(Colorado State University. Libraries, 2024) Feder, Andrew, author; Randall, David, advisor; Hurrell, James, committee member; Rugenstein, Jeremy, committee member
Tropical cyclones (TCs) occur stochastically in any given TC season, with varying numbers and intensities within basins over time. Nevertheless, they arise out of fundamental laws of thermodynamics and fluid physics, and in recent years, as global circulation models (GCMs) have increased in spatial resolution, increasingly realistic TCs and TC distributions have emerged from them. Where prior research on TC climatologies has relied on proxies like Potential Intensity (PI) and synthetic storm models, the cyclones emerging from the dynamics of newer GCMs can now be analyzed directly, using native model variables. Such direct analysis may be particularly useful in studying possible global storm distributions under radically altered future climates, including high-emissions warming scenarios, and even those shaped by climate interventions. These interventions include various directed changes in global albedo, such as Stratospheric Aerosol Injection (SAI), with only limited precedent in the historical period. GCMs simulating realistic climate intervention scenarios, have not as of yet paired storm-resolving resolution with realistic intervention scenario construction. This has left gaps in our understanding as to how interventions might affect global storm/TC distributions, and whether ameliorating warming in this way could also substantially lessen related natural disaster risk profiles. In this paper, we utilize a new high-resolution model configuration to conduct experiments examining the effects of SAI, on tropical cyclones and global storm physics more broadly. These experiments are constructed based on prior work on SAI using the GLENS GCM ensemble (Tilmes et al. 2020; Danabasoglu 2019a,b). Our analysis centers on 3 10-year experiments conducted using 30-km grid spacing. These include a recent-past calibration run; the Intergovernmental Panel on Climate Change climate pathway SSP 8.5 (IPCC 2021), for the years 2090-2099, with no SAI; and SSP 8.5, with SAI having begun in 2020 to maintain a global temperature rise of no more than 1.5° C, also simulated for the years 2090-2099. With the resulting data sets, we deploy a novel TC-tracking algorithm to analyze resulting changes in storm tracks and properties. Based on our results for these different scenarios, we find that SAI, while in some ways restoring global storm patterns to a pre-warming state, may also create unique basin-scale TC distribution features and pose novel related hazards.
Open Access
Linear and nonlinear properties of numerical methods for the rotating shallow water equations
(Colorado State University. Libraries, 2015) Eldred, Chris, author; Randall, David, advisor; Birner, Thomas, committee member; Schubert, Wayne, committee member; Estep, Don, committee member; Lauritzen, Peter, committee member; Bleck, Rainer, committee member
The shallow water equations provide a useful analogue of the fully compressible Euler equations since they have similar conservation laws, many of the same types of waves and a similar (quasi-) balanced state. It is desirable that numerical models posses similar properties, and the prototypical example of such a scheme is the 1981 Arakawa and Lamb (AL81) staggered (C-grid) total energy and potential enstrophy conserving scheme, based on the vector invariant form of the continuous equations. However, this scheme is restricted to a subset of logically square, orthogonal grids. The current work extends the AL81 scheme to arbitrary non-orthogonal polygonal grids, by combining Hamiltonian methods (work done by Salmon, Gassmann, Dubos and others) and Discrete Exterior Calculus (Thuburn, Cotter, Dubos, Ringler, Skamarock, Klemp and others). It is also possible to obtain these properties (along with arguably superior wave dispersion properties) through the use of a collocated (Z-grid) scheme based on the vorticity-divergence form of the continuous equations. Unfortunately, existing examples of these schemes in the literature for general, spherical grids either contain computational modes; or do not conserve total energy and potential enstrophy. This dissertation extends an existing scheme for planar grids to spherical grids, through the use of Nambu brackets (as pioneered by Rick Salmon). To compare these two schemes, the linear modes (balanced states, stationary modes and propagating modes; with and without dissipation) are examined on both uniform planar grids (square, hexagonal) and quasi-uniform spherical grids (geodesic, cubed-sphere). In addition to evaluating the linear modes, the results of the two schemes applied to a set of standard shallow water test cases and a recently developed forced-dissipative turbulence test case from John Thuburn (intended to evaluate the ability the suitability of schemes as the basis for a climate model) on both hexagonal-pentagonal icosahedral grids and cubed-sphere grids are presented. Finally, some remarks and thoughts about the suitability of these two schemes as the basis for atmospheric dynamical development are given.
Open Access
Skillful long-range forecasts of North American heat waves from Pacific storm propagation
(Colorado State University. Libraries, 2017) Jenney, Andrea, author; Randall, David, advisor; Barnes, Elizabeth, committee member; Anderson, Georgiana Brooke, committee member
Extreme heat poses major threats to public health and the economy. Long- range predictions of heat waves offer little improvement over climatology despite the continuing improvements of weather forecast models. Previous studies have hinted at possible relationships between tropical West Pacific convection and subsequent anomalous near-surface air temperature and rainfall over the North American Plains. We show that the later stages of propagation of the Boreal Summer Intraseasonal Oscillation (BSISO) can be used to skillfully hindcast a number of Great Plains heat waves between 1948 and 2016 with a three-month lead time. Possible teleconnection mechanisms are investigated, with the most likely being related to a BSISO-induced reduction in Plains spring rainfall and subsequent land-atmosphere feedbacks. Our results are the first to demonstrate that a West Pacific weather event can be used to skillfully forecast US Plains heat waves with a lead time of three months.
Open Access
The impact of reforestation in the northeast United States on precipitation and surface temperature
(Colorado State University. Libraries, 2013) Clark, Allyson, author; Randall, David, advisor; Denning, A. Scott, committee member; Binkley, Dan, committee member
Since the 1920s, forest coverage in the northeastern United States has recovered from disease, clearing for agricultural and urban development, and the demands of the timber industry. Such a dramatic change in ground cover can influence heat and moisture fluxes to the atmosphere, as measured in altered landscapes in Australia, Israel, and the Amazon. In this study, the impacts of recent reforestation in the northeastern United States on summertime precipitation and surface temperature were quantified by comparing average modern values to 1950s values. Weak positive (negative) relationships between reforestation and average monthly precipitation and daily minimum temperatures (average daily maximum surface temperature) were found. There was no relationship between reforestation and average surface temperature. Results of the observational analysis were compared with results obtained from reforestation scenarios simulated with the BUGS5 global climate model. The single difference between the model runs was the amount of forest coverage in the northeast United States; three levels of forest were defined - a grassland state, with 0% forest coverage, a completely forested state, with approximately 100% forest coverage, and a control state, with forest coverage closely resembling modern forest coverage. The three simulations were compared, and had larger magnitude average changes in precipitation and in all temperature variables. The difference in magnitudes between the model simulations observations was much larger than the difference in the amount of reforestation in each case. Additionally, unlike in observations, a negative relationship was found between average daily minimum temperature and amount of forest coverage, implying that additional factors influence temperature and precipitation in the real world that are not accounted for in the model.
Open Access
The spatial scale of convective aggregation in cloud-resolving simulations of radiative-convective equilibrium
(Colorado State University. Libraries, 2017) Patrizio, Casey, author; Randall, David, advisor; Thompson, David, advisor; Kirkpatrick, Allan, committee member
A three-dimensional cloud-resolving model (CRM) was used to investigate the preferred separation distance between humid, rainy regions formed by convective aggregation in radiative--convective equilibrium without rotation. We performed the simulations with doubly-periodic square domains of widths 768 km, 1536 km and 3072 km over a time period of about 200 days. The simulations in the larger domains were initialized using multiple copies of the results in the small domain at day 90, plus a small perturbation. With all three domain sizes, the simulations evolved to a single statistically steady convective cluster surrounded by a broader region of dry, subsiding air by about day 150. In the largest domain case, however, we found that an additional convective cluster formed when we the simulation was run for an extended period of time. Specifically, a smaller convective cluster formed at around day 185 at a maximum radial distance from the larger cluster and then re-merged with the larger cluster after about 10 days. We explored how the aggregated state was different in each domain case, before the smaller cluster formed in the large domain. In particular, we investigated changes in the radial structure of the aggregated state by calculating profiles for the water, dynamics and radiation as a function of distance from the center of the convective region. Changes in the vertical structure were also investigated by compositing on the convective region and dry, subsiding region at each height. We found that, with increasing domain size, the convective region boundary layer became more buoyant, the convective cores reached deeper into the troposphere, the mesoscale convective updraft became weaker, and the mesoscale convective region spread out. Additionally, as the domain size was increased, conditions in the remote environment became favorable for convection. We describe a physical mechanism for the weakening of the mesoscale convective updraft and associated broadening of the convective region with increasing domain size, which involves mid-level stable layer enhancement as a result of the deeper convection. Finally, a simple analytical model of the aggregated state was used to explore the dependency of the convective fractional area on the domain size. The simple model solutions that had net radiative cooling and surface evaporation in the convective region were consistent with the simulation results. In particular, the solutions captured the broadening of the convective region, the weakening of the convective region updraft, as well as the positive and declining gross moist stability (GMS) that occurred with increasing domain size in the simulations. Furthermore, the simple model transitioned from positive to negative GMS at a domain length of about 7000 km because the convective region boundary layer became progressively more humid with increasing domain size. This suggests that the spatial scale of the aggregated RCE state in the simulations would be limited to a length scale of about 7000 km, as convectively-active areas are commonly observed to have positive GMS. This work additionally suggests that the processes that influence the water vapor content in the convective region boundary layer, such as convectively-driven turbulent water vapor fluxes, are important for determining the spatial scale of the aggregated RCE state.
Open Access
Understanding the role of ocean dynamics in climate variability
(Colorado State University. Libraries, 2021) Patrizio, Casey R., author; Thompson, David, advisor; Randall, David, advisor; Rugenstein, Maria, committee member; Rugenstein, Jeremy, committee member; Small, Richard, committee member
The ocean plays a key role in regulating Earth's mean climate, both because of its massive heat capacity, but also its heat transport by slow-moving circulations and other dynamics. In principle, fluctuations in such ocean heat transport can influence the variability in the climate, by impacting the sea-surface temperature (SST) variability and in turn the atmospheric variability through surface heat exchange, but this is incompletely understood, particularly in the extratropics. The goal of this dissertation is to clarify the role of ocean dynamics in climate variability, first focusing on the role of ocean dynamics in SST variability across the global oceans (Chapters 1 and 2), and then on the impact of midlatitude ocean-driven SST anomalies on the atmospheric circulation (Chapter 3). In Chapter 1, the contributions of ocean dynamics to ocean-mixed layer temperature variance are quantified on monthly to multiannual timescales across the globe. To do so, two methods are used: 1) a method in which monthly ocean heat transport anomalies are estimated directly from a state-of-the-art ocean state estimate spanning 1992-2015; and 2) a method in which they are estimated indirectly using the energy budget of the mixed layer with monthly observations of SSTs and air-sea heat fluxes between 1980-2017. Consistent with previous studies, both methods indicate that ocean dynamics contribute notably to mixed layer temperature variance in western boundary current regions and tropical regions on monthly to interannual timescales. However, in contrast to previous studies, the results also suggest that ocean dynamics reduce the variance of Northern Hemisphere mixed layer temperatures on timescales longer than a few years. In Chapter 2, the role of ocean dynamics in midlatitude SST variability is further understood using Hasselmann's model of climate variability, wherein midlatitude SST anomalies are driven entirely by atmospheric processes. Motivated by the results of Chapter 1, here Hasselmann's climate model is extended to include the forcing and damping of SST variability by ocean processes, which are estimated indirectly from monthly observations. It is found that the classical Hasselmann model driven only by observed surface heat fluxes generally produces midlatitude SST power spectra that are too red compared to observations. Including ocean processes in the model reduces this discrepancy by decreasing the low-frequency SST variance and increasing the high-frequency SST variance, leading to a whitening of the midlatitude SST spectra. This happens because ocean forcing increases the midlatitude SST variance across many timescales but is outweighed by ocean damping at timescales > 2 years, particularly away from the western boundary currents. It is also shown that the whitening of midlatitude SST variability by ocean dynamical processes operates in NCAR's Community Earth System Model (CESM). In the final chapter, the atmospheric circulation response to midlatitude ocean-forced SST anomalies is explored. In particular, the extended Hasselmann model is used to isolate the oceanic and atmospheric-forced components of the observed SST variability in the Kuroshio-Oyashio Extension (KOE) region. The associated atmospheric circulation anomalies are diagnosed by lagged-regression of monthly sea-level pressure (SLP) anomalies onto the KOE-averaged SST anomalies, and their oceanic and atmospheric-forced components. Consistent with previous studies, a large-scale SLP pattern is found to lag the KOE SST anomalies by one month. Here it is shown that this pattern is linked to the oceanic-forced component of the SST variability, but not the atmospheric-forced component. The results hence suggest that the midlatitude ocean dynamical processes in the North Pacific influence the variability of the large-scale atmospheric circulation.