Browsing by Author "Johnson, Richard H., advisor"
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Item Open Access A study of the relationship between thunderstorm processes and cloud-top ice crystal size(Colorado State University. Libraries, 2008) Lindsey, Daniel T., author; Johnson, Richard H., advisorSatellite observations and numerical models are used to understand the physical mechanisms responsible for thunderstorms with varying cloud-top ice crystal sizes. Geostationary Operational Environmental Satellite (GOES) data are used to create a three-year climatology of cloud-top 3.9 µm reflectivity, a quantity which is closely correlated with particle size. Maximum mean values are found over the High Plains and Rocky Mountain regions of the U.S., suggesting that convection over that region tends to generate smaller anvil ice crystals than areas throughout much of the eastern U.S. To correct for preferred forward scattering by the cloud-top ice crystals, an effective radius retrieval using GOES is developed. Forward radiative transfer simulations are run for a wide range of cloud-top ice crystal sizes and sun-cloud-satellite scattering angles. The output is used to generate a lookup table, so that GOES-measured radiances may be used along with sun-satellite geometry to obtain an estimate for particle size. Validation of the retrieval shows that the assumed scattering properties perform quite well. To help explain the geographical variation in cloud-top ice crystal size, a composite analysis is performed in the High Plains region by averaging environmental conditions for days which produced both small and large ice crystal storms. Small ice is found to occur with relatively high based storms and steep mid-level lapse rates. Additionally, observational evidence from a pyrocumulonimbus event is presented to show the effect of low-level cloud condensation nuclei (CCN) on cloud-top ice crystal size. Model simulations using the Colorado State University Regional Atmospheric Modeling System (RAMS) are performed to help understand the physical mechanisms responsible for cloud-top ice crystal size. Through a series of sensitivity tests, it is found that larger low-level CCN concentrations lead to smaller anvil ice. In addition, as cloud-base temperature decreases (and cloud-base height increases), storm-top ice crystals get smaller. A weaker updraft strength is found to have very little effect on ice crystal size.Item Open Access Analysis of the diurnal cycle in Taiwan during the terrain-influenced monsoon rainfall experiment(Colorado State University. Libraries, 2012) Ruppert, James Howard, author; Johnson, Richard H., advisor; Chandrasekar, V., committee member; Fletcher, Steven J., committee member; Maloney, Eric D., committee memberThe diurnal cycle is investigated in Taiwan during the summer monsoon ("Mei- yu" or plum rain) season using enhanced observations from the 2008 Terrain-influenced Monsoon Rainfall Experiment (TiMREX). The diurnal cycle of an undisturbed period is compared with that of a disturbed period in an aim to 1) better understand the variability of the diurnal cycle as a function of large-scale forcing, 2) describe the complex relationships between rainfall and orographically modified flow, and 3) determine the governing environmental characteristics that distinguish disturbed and undisturbed periods. The study is performed using a regional reanalysis generated by employing three-dimensional variational data assimilation techniques, 0.5° 6-h forecasts from the NCEP GFS (National Centers for Environmental Prediction Global Forecast System), and multiple observation platforms (from TiMREX datasets and others). The undisturbed period (UNDIST) was characterized by southwesterly monsoon flow at low levels, zonal flow in the upper troposphere, suppressed daily-mean rainfall, and unimpeded insolation. Accordingly, pronounced diurnal land-sea breeze (LSB) and mountain-valley (MV) circulations strongly controlled rainfall patterns, which exhibited patterns consistent with low-Froude-number (Fr) flow diverting around the mountainous island of Taiwan. Maximum daytime onshore/upslope flows were associated with enhanced rainfall along the coastal plains and foothills of Taiwan (as opposed to the high peaks), until the nighttime transition brought offshore/downslope flows and development of offshore rainfall where nocturnal density currents converged with the impinging southwesterly monsoon flow. During the disturbed period (DIST), the positioning of a prominent upper- tropospheric trough put Taiwan in a favorable area for large-scale ascent and convective organization, while a shallow, northerly cold intrusion (the Mei-yu front) provided a low- level triggering mechanism for vigorous deep convection. Although the amplitude of diurnal LSB/MV circulations was suppressed during this period (in association with reduced insolation), rainfall diurnal variability was noteworthy, suggesting heightened sensitivity of rainfall to diurnal flows. Consistent with moist conditions and higher-Fr flow, rainfall during this period was maximized over the high mountain peaks. Analysis of vertical profiles of vertical motion and apparent heat sources and moisture sinks for UNDIST demonstrates a predominance of shallow vertical circulations and bottom-heavy convection. In contrast, vigorous deep convection was the dominant rainfall mode during DIST. That the environment was more conducive for vigorous deep convection during DIST explains the increased sensitivity of rainfall to diurnal flows. Common to both periods was an afternoon transition from shallow to deep convection to stratiform rainfall (heating above the freezing level and cooling below; consistent with previous studies). The evolution of rainfall prior to, during, and following DIST exhibited a similar transition. This reflects the "self-similar" nature of tropical convective rainfall systems across spatial and temporal scales.Item Open Access Cumulus moistening, the diurnal cycle, and large-scale tropical dynamics(Colorado State University. Libraries, 2015) Ruppert, James H., author; Johnson, Richard H., advisor; Maloney, Eric D., committee member; van den Heever, Sue, committee member; Chandrasekar, V., committee memberObservations and modeling techniques are employed to diagnose the importance of the diurnal cycle in large-scale tropical climate. In the first part of the study, soundings, radar, and surface flux measurements collected in the Indian Ocean DYNAMO experiment (Dynamics of the Madden–Julian Oscillation, or MJO) are employed to study MJO convective onset. According to these observations, MJO onset takes place as follows: moistening of the low–midtroposphere is accomplished by cumuliform clouds that deepen as the drying by large-scale subsidence and horizontal advection simultaneously wane. This relaxing of subsidence is tied to decreasing column radiative cooling, which links back to the evolving cloud population. A new finding from these observations is the high degree to which the diurnal cycle linked to air-sea and radiative fluxes invigorates clouds and drives column moistening each day. This diurnally modulated cloud field exhibits pronounced mesoscale organization in the form of open cells and horizontal convective rolls. Based on these findings, it is hypothesized that the diurnal cycle and mesoscale cloud organization represent two manners in which local convective processes promote more vigorous day-to-day tropospheric moistening than would otherwise occur. A suite of model tests are carried out in the second part of the study to 1) test the hypothesis that the diurnal cycle drives moistening on longer timescales, and 2) better understand the relative roles of diurnally varying sea surface temperature (SST) and direct atmospheric radiative heating in the diurnal cycle of convection. Moist convection is explicitly represented in the model, the diurnal cycle of SST is prescribed, and cloud-interactive radiation is simulated with a diurnal cycle in shortwave heating. The large-scale dynamics are parameterized using the spectral weak temperature gradient (WTG) technique recently introduced by Herman and Raymond. In this scheme, external (i.e., large-scale) vertical motion wwtg is diagnosed based on the internal diabatic heating in the model. wwtg is then used to advect model temperature and humidity. wwtg opposes domain-averaged temperature anomalies via adiabatic warming and cooling, thereby yielding a feedback between the model diabatic heating and the large-scale column moisture source associated with large-scale vertical motion. With a control simulation that successfully replicates a regime of shallow convection similar to nature, it is found through sensitivity tests that the diurnal cycle in tropospheric radiative heating is the dominant driver of both diurnal column moisture variations and nocturnal rainfall in this regime, the latter of which agrees with previous findings by Randall et al. The diurnal cycle in SST and surface fluxes, in turn, drives the daytime convective regime, which is distinct from the nocturnal regime by its rooting in the boundary layer. A simulation in which the diurnal cycle is stretched to 48 h amplifies an important nonlinear feedback at work in the diurnal cycle, which owes to the high-amplitude diurnal cycle in column relative humidity RH. This diurnal cycle in RH limits the amount of evaporation, and hence evaporative cooling, that takes place in the cloud layer. By throttling down the diabatic cooling, the diurnal cycle throttles down the daily-mean moisture sink driven by large-scale subsidence, such that the environment drifts toward a more moist state, all else being equal. When the diurnal cycle is not present, this nonlinear moisture source is weaker, and the environment drier. This feedback rectifies diurnal moistening onto longer timescales, thereby linking the diurnal cycle to longer timescales. These findings suggest that 1) the diurnal cycle of moist convection, as observed in DYNAMO, cannot be ruled out as an column moisture source important to MJO initiation, and 2) that proper representation of the diurnal cycle is prerequisite to accurate representation of large-scale climate, at least within the regime studied herein.Item Open Access Quasi-stationary, extreme-rain-producing convective systems associated with midlevel cyclonic circulations(Colorado State University. Libraries, 2008) Schumacher, Russ Stanley, author; Johnson, Richard H., advisorObservations and numerical simulations are used to investigate the atmospheric processes responsible for initiating, organizing, and maintaining quasi-stationary mesoscale convective systems (MCSs) that form in association with midlevel mesoscale convective vortices or cutoff lows. Six events were identified in which an MCS remained nearly stationary for 6-12 hours and produced excessive rainfall that led to significant flash flooding. Examination of individual events and composite analyses reveals that the MCSs formed in thermodynamic environments characterized by very high relative humidity at low levels, moderate convective available potential energy, and very little convective inhibition. In each case, the presence of a strong low-level jet (LLJ) led to a pronounced reversal of the wind shear vector with height. Convection was initiated by lifting associated with the interaction between the LLJ and the midlevel circulation. One of these events was examined in detail using numerical simulations. This MCS, which occurred on 6-7 May 2000 in eastern Missouri, produced in excess of 300 mm of rain in 9 hours and led to destructive flash flooding. Simulations indicate that the MCS was long-lived despite the lack of a cold pool at the surface. Instead, a nearly stationary low-level gravity wave helped to organize the convection into a quasi-linear system that was conducive to extreme local rainfall amounts. Additionally, the convective system acted to reintensify the midlevel MCV and also caused a distinct surface low pressure center to develop in both the observed and simulated system. To further understand the important processes in these MCSs, idealized simulations using a low-level lifting mechanism and a composite thermodynamic profile are employed. These simulations successfully replicate many of the features of the observed systems. The low-level environment is nearly saturated, which is not conducive to the production of a strong surface cold pool; yet the convection quickly organizes into a quasi-linear system that produces very heavy local rainfall. As in the May 2000 case, a low-level gravity wave was responsible for this organization. The upstream development of new convective cells is shown to result from the interaction of the reverse-shear flow with these waves.Item Open Access Surface and synoptic features of leading and parallel stratiform mesoscale convective systems(Colorado State University. Libraries, 2010) Steinweg-Woods, Jesse Matthew, author; Johnson, Richard H., advisor; Rutledge, S. A. (Steven Allan), committee member; Ramirez, Jorge A., committee memberThis study investigates the surface wind, pressure, and temperature patterns associated with mesoscale convective systems meeting the classification of leading or parallel stratiform. Cases were surveyed over the period of 2002 to 2007 inside the state of Oklahoma. Thirteen leading stratiform cases and ten parallel stratiform cases were identified. The synoptic environment these cases developed in was also investigated and compared to previous studies. The Oklahoma Mesonetwork is utilized to examine the surface features associated with these cases. Mesoscale contributions were isolated through filtering of these data in order to focus on fine-scale features. Streamlines, surface pressures, potential temperatures, and equivalent potential temperatures were analyzed through this method. The study concluded that leading and parallel stratiform systems had similar synoptic environments to previous studies, with some minor differences. Leading stratiform systems were more dependent on low-level jet influence than parallel stratiform systems were. Both system types also formed in the warm sector of a frontal system along its boundary, which typically was a warm or stationary front. These systems also tended to be located in the outer edge of the right entrance region of jet streaks. Leading stratiform systems had a surface pressure pattern very different from trailing stratiform systems. It was found that, in leading stratiform systems, a mesohigh formed behind the main convective region, and a mesolow formed ahead of the stratiform region. Parallel stratiform results were more inconclusive due to a lack of quality cases, but one case showed a surface pressure pattern almost identical to asymmetrical trailing stratiform systems. Maxima and minima of potential and equivalent potential temperatures tended to be located in the same areas, with lower temperatures near mesohighs and higher temperatures near mesolows. It is possible that a leading inflow jet is causing the surface pressure to decrease ahead of the stratiform region in leading stratiform systems. This needs to be confirmed in future study by investigating the vertical wind profile of these systems and validate the presence of this jet.Item Open Access 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 memberAs 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.Item Open Access Wake vortices and tropical cyclogenesis downstream of Sumatra over the Indian Ocean(Colorado State University. Libraries, 2015) Fine, Caitlin Marie, author; Johnson, Richard H., advisor; Schubert, Wayne H., committee member; Kirby, Michael J., committee memberA myriad of processes acting singly or in concert may contribute to tropical cyclogenesis, including convectively coupled waves, breakdown of the inter-tropical convergence zone (ITCZ), or upper-level troughs. This thesis investigates the role that topographic effects from the island of Sumatra may play in initiating tropical cyclogenesis (TC genesis) in the eastern Indian Ocean. If easterly flow is split by the mountains of Sumatra, counter-rotating lee vortices may form downstream. Because Sumatra straddles the equator, though the wake vortices rotate in opposite directions, they will both be cyclonic when winds are easterly upon Sumatra, and may intensify further into tropical cyclones. The phenomenon of cross-equatorial cyclone pairs, or "twin" tropical cyclones, in the Indian Ocean originating from Sumatra was first noted by Kuettner (1989). TC genesis appears to be particularly favored during the pre-onset phase of the Madden Julian Oscillation (MJO), when easterly flow encroaches upon Sumatra and the resulting cyclonic wake vortices encounter convectively coupled waves and enhanced moisture associated with the MJO in the Indian Ocean. Operational analysis data from the Year of Tropical Convection (YOTC) and Dynamics of the Madden Julian Oscillation (DYNAMO) campaigns were used to evaluate the impacts of Sumatra's topography upon the flow. The YOTC data encompass two years, from May 2008 to April 2010, while the special observing period of DYNAMO was conducted from October to December 2011. This research also presents three case studies of twin tropical cyclones west of Sumatra in the Indian Ocean, which were all determined to originate from Sumatran wake vortices and occurred between October and December of 2008, 2009, and 2011. Multiple cyclonic wake vortices and vorticity streamers were observed downstream of Sumatra during periods of easterly flow, most frequently between October and December. Froude numbers calculated for the region upstream of Sumatra with regard to easterly flow between October and December favored flow blocking and splitting, more so for Sumatra's northern tip due to the higher terrain there. Correlations between zonal wind and relative vorticity are more significant near Sumatra's northern tip than near and downstream of the island's southern tip. Cyclonic vorticity was maximized at the level of Sumatra's topography for most easterly wind days west of both the north and south ends of the island, suggesting that topography was contributing to vorticity generation. Thirteen tropical cyclones in the Indian Ocean during the YOTC and DYNAMO campaigns were determined to develop from cyclonic wake vortices downstream of Sumatra, including three tropical cyclone pairs. Over 75% of these tropical cyclones formed between October and December. In four cases, wake vortices were generated by anomalously easterly low-level flow that preceded the active phase of the Madden Julian Oscillation. These vortices proceeded to encounter the MJO convective envelope, which is frequently accompanied by convectively coupled waves and may have altered the environment to be more moist and favorable for tropical cyclogenesis. In many cases, equatorial westerly winds, which may have been related to westerly wind bursts from the MJO or to convectively coupled equatorial Rossby waves, intensified low-level cyclonic circulations. It is suggested that diabatic heating in the vicinity of twin tropical cyclones may disturb the atmosphere enough to invigorate extant convectively coupled Kelvin waves, or contribute to the formation of a Kelvin wave. The research presented herein describes the interaction of the flow with steep topography on Sumatra and its role in tropical cyclogenesis over the Indian Ocean, a mechanism for TC genesis that has heretofore received little attention.