Atmospheric Science Papers (Blue Books)

Permanent URI for this collectionhttps://hdl.handle.net/10217/100334

Much of this digital collection of Blue Books comes from CSU's Department of Atmospheric Science. Included are student theses and dissertations and project reports dating from 1959 to 2007. The works focus on different areas of atmospheric science research such as climate change, severe weather, climatology, solar radiation, remote sensing, wind forecasting, and air quality.

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Open Access
Forecasting summer/fall El Niño-southern oscillation events at 6-11 month lead times
(Colorado State University. Libraries, 2004-06) Seseske, Stacey A., author
Accurately predicting El Niño-Southern Oscillation (ENSO) events is an important yet challenging task, especially at the extended range of 6-11 months. This research offers a new methodology for forecasting extended range ENSO events, utilizing global data and a statistical model. Most ENSO forecasts utilize data local to the Pacific Ocean basin. This research uses an all-subsets technique to select from an inclusive pool of global predictors that are able to capture useful ENSO precursor signals beyond the Pacific basin. A multiple linear regression using the best five predictors produced a December 1 forecast for the June-July-August (6-8 month forecast) and the September-October-November (9-11 month forecast) Sea Surface Temperature Anomaly SSTA) in the Niño 3-4 region (5°N-5°S, 120-170°W). The performance of each forecast was then compared to the analogous 1 December ENSO-CLIPER (Knaff and Landsea 1911) forecast which is held as a benchmark for specifying ENSO forecast skill. Results for the 6-8 month forecast (1 December to JJA) show that the scheme presented in this research (herein referred to as SG) explains more variance than that of the ENSO-CLIPER scheme. The SG scheme accounts for 58 percent of the variance for the period 1952-2002, while the ENSO-CLIPER model explains only 14 percent for the same period. The 9-11 month forecast (1 December to SON) shows that SG only improves slightly upon ENSO-CIOPER. The SG forecast explained 3;6 percent of the variance for the 1950-2002 period, while ENSO-CLIPER explained 25 percent. However, by combining the two schemes (SG + ENSO-CLIPER) it is possible to explain over 50 percent of the variance in the SON SSTA timeseries 9-11 months in advance.
Open Access
Intraseasonal oscillations and the Southeast Asian monsoon onset
(Colorado State University. Libraries, 2001) Loechl, Erica M., author
The role of intraseasonal oscillations as a triggering mechanism for the Southeast Asian monsoon (SEAM) is investigated through a detailed study of the 1998 SEAM and expanded analysis to include the 20 years from 1979-1998. Spectral analysis of outgoing longwave radiation (OLR) at each gridpoint over the South China Sea (SCS) (10-20° N, 110-120° E) revealed the presence of intraseasonal oscillations with periods of 12-24 days and 30-60 days during the monsoon season. The SCS is an area of maximum variability for both of these oscillations during the SEAM onset. The monsoon onset was defined as the first day of sustained convection (based on analysis of OLR) and the commencement of southerly and westerly winds over the SCS. The range of SEAM onset dates for the 20 year period was from 10 May - 15 June. The onset timing was considered normal if it occurred between 11 May - 31 May. Analysis of band-pass filtered OLR, 850 and 200 mb winds suggest the 1998 SEAM onset was triggered by the nearly simultaneous arrival of the 12-24 day and 30-60 day oscillations into the SCS. The main component of the 12-24 day oscillation developed in the northwest Pacific near Japan and propagated southwestward into the SCS. The 30-60 day oscillation is comprised of the eastward-moving Madden-Julian oscillation {MJO) and the northward-moving monsoon trough. Lag regression analysis revealed circulation and convection patterns of the intraseasonal oscillations for the 20 years to be similar to the patterns seen in 1998. However, the oscillations appeared to be stronger during the monsoon onsets compared to the entire monsoon seasons. Significant midlatitude influence on the SCS during the onset was observed in the regressions in the form of an upper and lower level wave train that extended from the SCS into the northern Pacific. The northern part of the SCS (15-20° N) served as a "transition-zone" between midlatitudes and the tropics. Regressions based on this area displayed characteristics of both midlatitude and tropical influence during the SEAM onset. The interannual variability of the onset timing and intraseasonal oscillations was assessed by catagorizing each year according to the onset timing, presence of intraseasonal oscillations at the onset, and ENSO phase. Years where both the 12-24 day and 30-60 day oscillation were present at the onset tended to have normal onsets. Years where only the 30-60 day mode was present at the SEAM onset had the earliest onsets and years where neither oscillation was present at the SEAM onset had the latest onsets. It appears that the existence of intraseasonal oscillations assists the monsoon onset and can cause it to happen earlier than it would otherwise occur. However, monsoon onsets will occur despite the absence of such oscillations - they simply will occur later. A direct link between the onset timing and ENSO events was not obvious from this study.
Open Access
Trace gases, aerosols, and solar radiation in the stratosphere -- explored and unexplored problem areas
(Colorado State University. Libraries, 1973) Reiter, Elmar R., author; American Institute of Aeronautics and Astronautics, publisher
The Climatic Impact Assessment Program (CIAP) of the Department of Transportation is--among other things--faced with the task of providing a detailed review of the "undisturbed" stratosphere as it exists before the commencement of SST traffic. CIAP Monograph No. 1, presently in preparation, deals with this "natural stratosphere." Material for a rough draft of this volume was compiled by approximately 90 scientists during a workshop in Ft. Lauderdale from November 28 through December 1, 1972. This short review paper, based upon the draft of CIAP Monograph No. 1, points out the highlights of our present knowledge of the stratosphere, especially with respect to the O3, NOx and H2O problems, and also explores some of the deficiencies in this knowledge which will have to be overcome before a complete assessment of human impact on this atmospheric region can be made.
Open Access
Paper III. Clear air turbulence models and forecasting for Project Topcat, second phase, September 1-30, 1963
(Colorado State University. Libraries, 1964-05) Reiter, Elmar R., author
A short account is given of clear-air turbulence (CAT) forecasting models and procedures used in "Project TOPCAT", from September 1 through October 3, 1963. Some preliminary results from the project are communicated.
Open Access
Wie entstehen die tropischen Hurrikans?
(Colorado State University. Libraries, 1963) Riehl, Herbert, author; Umschau Verlag, publisher
Open Access
A case study of radioactive fallout
(Colorado State University. Libraries, 1963) Reiter, Elmar R., author; American Meteorological Society, publisher
During September 1961 a series of balloon ascents made from Flin Flon, Canada, carrying scintillation counters sensitive to gamma radiation, revealed the existence of shallow stable atmospheric layers carrying radioactive debris, presumably from the Russian test series during the same month. The debris layers encountered on 14 and 15 September have been studied in particular. The debris detected over Flin Flon on 14 September, 2221 GCT, at 650 mb had undergone strong sinking motion. One may conclude that it came from the region immediately underneath the tropopause shortly prior to 13 September, 12 GCT, entering the middle troposphere through the stable layer underneath the jet core, sometimes referred to as a "jet-stream front." Beginning with 17 September a distinct area of radioactive fallout begins to appear at the surface over the eastern United States. Some of this debris seems to be identical with the one detected over Flin Flon, and it apparently was transported by the same jet stream. Part of the fallout seems to be associated with a small collapsing cold dome travelling ahead of this jet stream.
Open Access
Nature and observation of high-level turbulence, especially in clear air
(Colorado State University. Libraries, 1964) Reiter, Elmar R., author; American Institute of Aeronautics and Astronautics, publisher
Open Access
Die vertikale struktur des strahlstromkernes aus forschungsflügen des Project Jet Stream
(Colorado State University. Libraries, 1962) Reiter, Elmar R., author; Dt. Wetterdienst, publisher
Open Access
An investigation of the radiative boundary conditions during the development of the southwest monsoon Saudi Arabian heat low
(Colorado State University. Libraries, 1981-07) Smith, Eric A., author; Sakkal, M. Marwan, author; Ackerman, Steven A., author; Cox, Stephen K., author; Vonder Haar, Thomas H., author
Progress report no. 1 on the cooperative research project between the Department of Atmospheric Science at Colorado State University and the Faculty of Meteorology and Environmental Science at King Abdul-Aziz University in accordance with the CID-ARME TED Project of the University of Arizona. Period covered February 15 to August 15, 1981.
Open Access
Surface and dual-Doppler radar analysis of 23-24 June 1985 OK PRE-STORM heat bursts
(Colorado State University. Libraries, 1990-11) Bernstein, Ben Carter, author
Doppler radar data are used to investigate heat bursts which accompanied a mesoscale convective system that traversed the OK PRE-STORM mesonetwork on 23-24 June 1985. The three dimensional structure of the precipitation and wind fields are discussed in an effort to uncover the cause of strong downdrafts which lead to the formation of heat bursts. Surface mesonetwork data are presented to depict the changes in temperature, dew point, relative humidity, Be, wind speed and wind direction which characterize them. Digitized WSR-57 radar data are used to detail the progression of the MCS, and a sounding is used to give an indication of the vertical structure of the temperature, dew point and wind fields in the vicinity of the heat bursts. The 23-24 June MCS formed along a dry line in western Kansas. As the system matured, an area of stratiform precipitation developed behind a line of convective towers which moved southeast with time. Southwesterly flow at upper levels caused the stratiform precipitation to extend northeastward, allowing it to become isolated from the convective line. A broad surface mesohigh was observed beneath the core of the stratiform region. This feature \Vas flanked by sharp pressure gradients and mesolows to its northwest, north and northeast. It was within these mesolows that the heat bursts occurred. Time series of surface mesonetwork data show that heat bursts are characterized by sudden dramatic rises in temperature and falls in dew point. Strong, gusty winds and modest falls in Be also accompanied the bursts. These changes are attributed to the removal of a shallow, moist stable layer at the surface by an intrusion of warm , dry air from aloft. A 600 km2 area of winds greater than 15 m s-1 was detected near the surface by the CP-3 Doppler radar. This wind maximum developed in an area of low reflectivity on the northeastern edge of the stratiform extension. These features moved southeast with time, and passed over one station in the mesonetwork, PAM-12. Their passage corresponded in time with the occurrence of heat bursts at that station. Dual-Doppler syntheses for 5-10 minutes preceding the onset of the heat bursts reveal an anvil-like structure in the precipitation field over PAM-12. Stratiform precipitation was falling just to the west of the station, and a strong reflectivity gradient existed at the edge of this precipitation . A strong mesoscale inflow entered the anvil cloud region at mid-levels, descended along the base of the anvil and reached the surface in the area of the strong reflectivity gradient. Downdrafts reaching 3 m s-1 were observed there. The winds detected in this region are used· to infer winds which may have existed in the precipitation free portion of the stratiform anvil. It is proposed that this lateral inflow jet warmed dry adiabatically as it descended, deformed the surface stable layer and caused a dramatic warming and drying at PAM-12.
Open Access
Geostrophic adjustment in a stratified atmosphere
(Colorado State University. Libraries, 1980-08) Fulton, Scott R., author; Schubert, Wayne H., author
The geostrophic adjustment process in a compressible atmosphere with arbitrary vertical stratification is studied as an initial value problem. The governing equations are the adiabatic quasi-static equations on an f-plane linearized about a motionless basic state. A rigid lid upper boundary condition is assumed which permits the use of a discrete eigenfunction expansion in the vertical. Using Fourier transforms in the horizontal a general solution is obtained for both the transient and final states. The general solution is evaluated for several simple experiments with axisymmetric initial conditions in the mass and vorticity fields which have horizontal variations on the tropical cloud cluster scale. These experiments assume a basic state characterized by constant static stability in log-pressure coordinates and a Coriolis parameter corresponding to 20°N latitude. Results are presented which illustrate the nature of the transient adjustment process. Comparison of the initial and final states indicates that the inclusion of vertical structure does not alter the basic conclusion from previous barotropic studies that in the tropics the mass field tends to adjust to the wind field. However, it is found that the extent of this adjustment depends strongly on the vertical structure of the initial conditions. These results are interpreted in term s of the projection of the initial conditions onto the vertical modes.
Open Access
Simple ice phase parameterization
(Colorado State University. Libraries, 1979-12) Stephens, Mark Argyle, author
A two variable ice parameterization was developed for use in three-dimensional models of cumulonimbus clouds and mesoscale squall lines. Bulk water techniques were employed to simulate the growth and decay of snow crystals and of graupel in order to keep the use of computer resources to a minimum. An externally specified concentration of ice crystals was used to initiate snow. Graupel was assumed to follow the Marshall-Palmer distribution with a constant total concentration. Microphysical growth processes for snow included initiation from the vapor at liquid water saturation, riming, melting, vapor deposition and conversion of rimed crystals into graupel. The graupel microphysical processes that were modeled included raindrop freezing by contact with snow crystals, accretion of raindrops, vapor deposition, riming of cloud droplets and melting. Both types of ice were allowed to precipitate. Sensitivity tests and internal consistency checks on the parameterization were done using a one-dimensional, time-dependent cloud model. Results suggested that the parameterization should simulate adequately the ice phase evolution in higher dimensional models. The parameterization is most suitable for modeling studies in which the major emphasis is on exploring the dynamic consequences of the ice phase rather than exploratory studies in cloud microphysics. Several deficiencies of the parameterization were commented on, specifically: the use of an externally specified snow concentration and its influence on the conversion of snow into graupel. Comments were also made on how local changes in the snow concentration brought about by seeding, ice multiplication and aggregation could be handled in higher dimensional models.
Open Access
Water vapor and temperature inversions near the 0⁰C level over the tropical western Pacific
(Colorado State University. Libraries, 1994) Hart, Kenneth A., author
During the Intensive Observation Period (IOP) [1 November 1992 - 28 February 1993] of the Tropical Ocean-Global Atmosphere Coupled-Ocean Atmosphere Response Experiment (TOGA COARE ). several periods of water vapor and temperature inversions near the 0°C level were observed. Satellite and radiosonde data from TOGA COARE are used to document the large-scale conditions and thermodynamic and kinematic structures present during three extended periods in which moisture and temperature inversions near the freezing level were very pronounced. Observations from each case are synthesized into schematics which represent typical structures of the inversion phenomena. Frequency distributions of the inversion phenomena along with climatological humidity and temperature profiles are calculated for the four-month IOP. Specific humidity or q inflections were observed simultaneously over large domains for extended periods of time (in excess of two days) in the midtroposphere. Local maxima in humidity were found to commonly occur near the freezing level or about 560 mb in the tropical region under study. The humidity inversions often occur in conjunction with temperature inversions (or stable layers) also at the 0°C level. The stable layer is usually positioned slightly above the moisture inversion. Two of the cases examined were observed during the active phase of the Madden-Julian Oscillation or MJO in which strong, low-level westerlies and upper-level easterlies were observed. IR satellite images of the TOGA COARE region reveal that the soundings which reflected moisture and temperature inversions at the freezing level ascended in generally clear and undisturbed conditions. During the three cases under examination, deep convection was observed in latitudinally oriented bands which formed a “double-ITCZ" structure about the equator. Frequency distributions of specific humidity inversions observed during the IOP reflect a maximum number of occurrences at altitudes just below the melting level. Distributions of stable temperature layers show a double-peak distribution - a low-level peak associated with tradewind-like or shallow cumulus clouds and a secondary peak corresponding to pressures near the 0°C level. The recurring moisture and temperature inversions near the freezing level were common enough to be evidenced in IOP averages. Climatological profiles of relative humidity averaged for the entire IOP reveal a nearly constant humidity average between 750 and 500 mb which contradicts the linear relation between relative humidity and pressure prescribed by Manabe and Wetherald (1967). The average relative humidity profiles resembled others reported in recent studies using data from the tropical western Pacific. Several mechanisms for the formation of the inversions near the 0°C level are proposed. Temperature inversions may be the remnants of stratiform rain regions which are known to produce isothermal layers just below the melting level (Findeisen 1940). Another possibility is that the temperature inversions may be the result of melting-induced, positive potential vorticity (PV) anomalies. A third explanation involves the effects of gravity waves produced by tropical cloud systems. Moisture inversions may be caused by the preferential detrainment of water vapor from cumulus clouds near stable layers. Heavily-raining congestus clouds were frequently observed by COARE ship-based radars to top out near the freezing level which may have left higher concentrations of water vapor there.
Open Access
Opposing mesoscale flows in a broken midlatitude squall line
(Colorado State University. Libraries, 1993) Miner, Barbara D., author
During the period 14-15 June 1985 a broken line of convection with one primary gap (echo-free) region developed along a cold front passing through the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM) domain. Radar and satellite data are presented to provide an overview of the life cycle of the line. Observations from the OK PRE-STORM mesonetwork and upper air soundings are used to document the occurrence of the gap and an associated surface mesolow. The convective line initially developed as two mesoscale convective systems (MCSs) , one in northeast Kansas, the other in the Texas panhandle, along a weak cold front. As the two MCSs matured, convection developed between the wo similar to the broken-line squall line formation described by Bluestein and Jain (1985). Despite strong low-level convergence and strong moisture convergence, an echo-free region remained between the two MCSs throughout the life cycle of the line. The upper level flow pattern along the line of convection showed strong upper level outflow from the two MCSs converging over the echo-free region and strong subsidence in that region from 250 mb to 650 mb. It is hypothesized that the strong mid- and upper level subsidence was the main factor in the lack of convection in the echo-free region. During the mature phase of the line, a surface mesolow developed within the echo-free region. Calculations are made using a form of the hypsometric equation to determine if the presence of the surface mesolow could have been produced by the mid and upper level subsidence found in that region. At 0300 UTC the mesolow was 2 mb lower than the surrounding areas. Calculations show that subsidence warming in the column could account for a drop in pressure of .75 mb. The results of the study show that while strong low-level convergence existed all along the front throughout most of its life cycle, mid- and upper level outflow from the existing MCSs prevented convection in the echo-free region. The resulting subsidence contributed to the formation of the surface mesolow in the echo-free region. This study shows the need for the evaluation of upper-level forcing mechanisms when forecasting the development of thunderstorms along fronts and convergence zones.
Open Access
Studies of the relationship between submicron marine aerosol and initial marine stratus properties
(Colorado State University. Libraries, 1993-12) Jensen-Leute, Tara L., author; Kreidenweis, Sonia M., author
A systematic study of the relationship between submicron aerosols and the marine stratus cloud properties has been undertaken. The first part of the study included participation in the Atlantic Stratocumulus Transition Experiment - Marine Aerosol and Gas Exchange (ASTEX/MAGE) cooperative research experiment Measurements of submicron marine aerosol were collected using the Differential Mobility Particle Sizing (DMSP) system for determining the typical chemical composition and aerosol size distribution of marine aerosol. The second part of the investigation involved cloud process simulation with the Colorado State University dynamic cloud chamber. Marine aerosol distribution measurements were taken over a 25 day period from June 1 to June 25, 1992. Analysis of the data showed that the distributions were generally bimodal in clean air masses with total number concentrations ranging from 100 to 900 particles cm-3, while distributions were generally monomodal in polluted air masses with total number concentrations ranging from 800 to 1400 particles cm-3. Using the '"typical" thermodynamic and aerosol characteristics observed during the field project, the Colorado State University dynamic cloud chamber was used to conduct a well controlled study of the effects of submicron aerosol on the formation of marine stratus type clouds. Selected size distributions of ammonium sulfate were injected into the chamber and exposed to adiabatic expansions that simulated typical marine updraft velocities. Observations from the experiments were compared to model predictions from a one dimensional cloud model as well as other published modeling results. The dynamic cloud chamber, as configured for this study, was shown to be suitable for use in making stratus cloud simulations at updraft velocities greater than 1.0 m s-1. Mean diameter, liquid water content and dispersion coefficient values appeared to be comparable to the model predictions. Nucleated aerosol fraction trends agreed with model results. Details of the design, implementation and data interpretation are presented.
Open Access
Examination of deep stable layers in the intermountain region of the western United States
(Colorado State University. Libraries, 1986-12) Wolyn, Paul G., author; Mckee, Thomas B., author
The definition of a deep stable layer sed in this report is 65% of the lowest 1.5km of the 1200 GMT sounding having a lapse rate of 2.5°Ckm-1 or less. Deep stable layers are associated with one important group of days which can potentially cause poor regional air quality in the intermountain region of the western United States. At Grand Junction, CO, Salt Lake City, UT, Winnemucca, NV, and Boise, ID they cause low daytime convective boundary layer heights and can allow for light winds near the surface even if moderate or strong synoptic scale winds aloft are present. A climatology of deep stable layer days showed that at the four intermountain region stations most of the days with deep stable layers occurred in December and January. Using a strict deep stable layer definition and episode criteria, episodes of three days or longer occurred on the average at least once every two years at Salt Lake City and Winnemucca, and at least once a year at Boise and Grand Junction. An analysis of the mixing volumes for five consecutive Decembers at the four intermountain region stations shows that all the deep stable layer days had low mixing volumes. A deep stable layer episode, which occurred from December 6 to December 23, 1980 at the four intermountain region stations, was examined in-depth to study the life cycle of a deep stable layer episode and to study the importance of different meteorological factors to the initiation, continuation, and termination of the episode. The initiation of the episode is associated with the movement of a warm ridge aloft into the region and is accompanied by a descending region of rapid warming and strong stability. Synoptic-scale warm air advection and subsidence are both important mechanisms for causing the warming aloft. Weak incoming solar radiation resulting in modest surface heating is important to prevent the destruction of the descending stable region. When the region of rapid warming descended to 0.5km-1.5km it formed a capping stable layer. In this part of the episode called the continuation phase, a disturbance was able to weaken the deep stable layer but not terminate it. The longwave radiative effects of fog may be important in this phase of the episode. The termination of the episode is associated with the destruction of the warm ridge aloft and the movement of disturbances into the region. Surface heating may be important for aiding in the termination of the episode. The presence of a thick fog layer can require a stronger disturbance to terminate the episode.
Open Access
Surface pressure features and precipitation structure of PRE-STORM mesoscale convective systems
(Colorado State University. Libraries, 1992) Loehrer, Scot M., author
The surface pressure features accompanying 16 mesoscale convective systems (MCSs) have been documented using data from the May-June 1985 Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM). The general synoptic-scale environmental conditions as well as the detailed mesoscale aspects of the systems are examined. Radar data are examined to show the reflectivity structure of each MCS. Also, the upper-air data are examined to show the system-relative airflow structure associated with these systems. The general synoptic-scale conditions were very similar to those shown by Maddox (1983) found in conjunction with the genesis region of mesoscale convective complexes (MCCs). There was a generally weak surface front , most often quasi-stationary in nature , and low-level warm advection by a low-level southerly jet which also advected in very moist air. Also, a weak 500 mb short wave feature was often found in conjunction with these systems. In the mature-to-dissipating stages of 12 of the 16 cases. the radar reflectivity and surface pressure structures were found to be very similar. Composite depictions of the pressure features are developed based on these similarities. At some time during the mature-to-dissipating stage for each case, the radar reflectivity structure became asymmetric (Houze et al. 1990) in nature with a leading bow-shaped convective line with a region of enhanced stratiform precipitation found to the rear of the far northern portion of the convective line. This structure is quite similar to that shown by Pedgley (1962) for MCS cases in England. Two of the four other MCSs that did not develop these particular structures were cold-frontal systems positioned directly along a cold front. Atmospheric conditions in these regions did not allow the development of these common structures. One system was too short-lived to have developed into the common structure of the 12 systems. There were four paths that the 12 systems took to the development of this asymmetric structure. First, there were systems which initially had a disorganized pattern of convection, but towards the end of their existence developed a small convective line on their southern end. Any convection on the northern end became stratiform, leading to the asymmetric structure (2 cases). Second, there were convective lines which were initially symmetric in nature but slowly developed a region of enhanced stratiform precipitation on their northern ends (4 cases). Third, there were cases where a back-building convective line led to a region at the southern end of the system lacking stratiform precipitation (3 cases). Finally, there were cases consisting initially of intersecting convective lines, one oriented east-west and the other oriented northeast-southwest extending to its south. An enhanced stratiform area developed to the northwest of the apex of the two lines and then the east-west line dissipated (3 cases). The surface pressure structure shows a fairly weak pre-squall low ahead of the convective line. A mesohigh was typically associated with the convective line as well as much of the stratiform precipitation region. A wake low was found at the back edge of the enhanced stratiform precipitation region. An intense pressure gradient was typically found along the back edge of and extending into the enhanced stratiform precipitation region. For the cases containing special soundings, the system-relative upper-air flow structure showed a rapidly descending, strong rear inflow jet in the region of the wake low. In summary, a remarkable and unexpected result from this study is that although MCSs observed over the mesonetwork during PRE-STORM originated in a variety of ways, they tended to develop a common cloud and precipitation structure during their mature-to-dissipating stages, a pattern characterized here by the term "asymmetric" after Houze et al. (1990). This repeatable structure also had a repeatable surface pressure pattern associated with it (as described above). These findings have led to the development of an updated model for the surface pressure pattern accompanying squall lines, one containing both symmetric and asymmetric structures. This model is consistent with but extends past models based on a small number of case (Fujita 1955, Pedgley 962, Johnson and Hamilton 1988).
Open Access
Mesoscale boundary layer development over mountainous terrain
(Colorado State University. Libraries, 1985-11) Bader, David C., author
The diurnal evolution of the mesoscale boundary layer ( ~ 50 km) over gently sloping terrain upwind of a high mountain barrier is described using both observational data and results from numerical model simulations . Atmospheric sounding data are presented from two nights , one during the summer and the second during the winter, when the mesoscale nocturnal boundary layer development was observed . Subsequently, a series of two and three-dimensional numerical model experiments are presented which identify the important physical processes responsible for the observed features. A conceptual model of mesoscale boundary layer evolution is then presented that accounts for the principle dynamic mechanisms discerned from analysis of the observational and simulated cases. Observational data from both nights reveal that the nocturnal boundary layer (NBL) extends 200-500 m above the valley ridgetops and plateaus and cools 1-4 K. Found within this layer are topographically induced thermal winds of 1-5 m s-l which either produce mesoscale circulations or influence the prevailing synoptic wind fields. The summer night data reveal that when the prevailing geostrophic wind is less than 7-10 m s-l at 500 mb and contains ro component into the high barrier, a complete mountain-plain circulation forms over the plateaus and ridges and down valley winds in ·the underlying valleys are strong. Contrasted with this is the winter case in which a much stronger geostrophic wind ( ~ 15 m s-l at 500 mb) directed into the barrier inhibited the mountain-plain circulation . In this case, there was a wide mesoscale variability in nocturnal boundary layer structure . One numerical experiment realistically simulated the diurnal evolution of the mesoscale boundary layer through a complete cycle encompassing the evening and morning transition periods. Additional simulations reveal that surface cooling and the shear between low-level thermal f lows and the overlying winds produce a 300 -500 m deep NBL 3-5 h after sunset. The nocturnal stable layer contains decoupled wind regions and can form a blocked wind region upwind of the barrier. In all cases, the nocturnal thermal wind component was 4-5 m s-l directed away from the barrier. It is also demonstrated that full three-dimensional model configuration are necessary to adequately describe the evolution of three-dimensional boundary layer structure.
Open Access
Precipitating convective cloud downdraft structure: a synthesis of observations and modeling
(Colorado State University. Libraries, 1985) Knupp, Kevin Robert, author
This study represents a comprehensive investigation in which observations are integrated with three-dimensional cloud model results to examine the kinematic, dynamic and thermodynamic structure of downdrafts associated with precipitating convection. One particular downdraft type, the low-level precipitation-associated downdraft, is investigated in considerable detail. It is shown that this downdraft exhibits significant structural, dynamic and thermodynamic properties which differ appreciably from other independent downdrafts within precipitating convective clouds. General airflow and trajectory patterns within low-level downdrafts are typically convergent from~ 0.8 km upwards to downdraft top, typically less than S km AGL. Observed mass flux profiles often increase rapidly with decreasing height as a result of strong buoyancy forcing below the melting level. Such patterns indicate that strong cooling by melting and evaporation within statically unstable low levels generates low perturbation pressure by virtue of buoyantly-induced pressure perturbations. Cloud model results verify this process and indicate that pressure perturbations are strongest during downdraft developing stages. Maximum modeled pressure reductions up to 2 mb are located within downdrafts and precipitation about 0.6 km below the 273 K level approximately 10 min after heavy precipitation(~ 2 g kg -1 ) enters low levels. The magnitude of this buoyantly-produced pressure reduction is influenced by temperature, static stability, relative humidity and precipitation characteristics. Model results and related calculations indicate that cooling provides the impetus for downdraft formation. Melting, in particular is generally found to make significant contribution to total cooling in cases having relatively shallow(< 2 km) PBL. Cooling by evaporation becomes increasingly important as PBL depth increases. Inflow to the low-level downdraft, although vertically continuous, can be separated into two branches. The up-down branch originating within the PBL initially rises up to 4 km and then descends within the main precipitation downdraft. The midlevel branch, most pronounced during early downdraft stages, originates from above the PBL and transports low-valued e to low levels. Pressure forces important along e both branches act to lift stable air along the up-down branch, and provide downward forcing of positively-buoyant air in the upper regions of both branches. Two primary conclusions are drawn from the results of this study: (1) Downdrafts are driven at low levels within regions of strong static instability by strong cooling provided by melting and evaporation. Cloud level entrainment effects make secondary contributions. (2) Precipitation size and phase (e.g. melting) are probably the most important controlling parameters for downdraft strength.
Open Access
Spectral methods for limited area models
(Colorado State University. Libraries, 1984-11) Fulton, Scott R., author
This study investigates the usefulness of Chebyshev spectral methods in limited area atmospheric modeling. Basic concepts of spectral methods and properties of Chebyshev polynomials are reviewed. Chebyshev spectral methods are illustrated by applying them to the linear advection equation in one dimension. Numerical results demonstrate the high accuracy obtained compared to finite difference methods. The nonlinear shallow water equations on a bounded domain in two dimensions are then considered as a more realistic prototype model. Characteristic boundary conditions based on Reimann invariants are developed, and contrasted with wall conditions and boundary conditions based on the assumption of balanced flow. Chebyshev tau and collocation methods are developed for this model. Results from one-dimensional tests show the superiority of the characteristic conditions in most situations. Results from two-dimensional tests are also presented. Comparison of the tau and collocation methods shows that each has its own advantages and both are practical. Time differencing schemes for Chebyshev spectral methods are studied. The stability condition obtained with explicit time differencing, often thought to be "severe", is shown to be less severe than the corresponding condition for finite difference methods. Numerical results and asymptotic estimates show that time steps may in fact be limited by accuracy rather than stability, in which case simple explicit time differencing is practical and efficient. ·,Two modified explicit schemes are reviewed, and implicit time differencing is also discussed. The results of this study indicate that Chebyshev spectral methods are a practical alternative to finite difference methods for limited area modeling, especially when high accuracy is desired. Spectral methods require less storage than finite difference methods, are more efficient when high enough accuracy is desired, and are at least as easy to program.