Browsing by Author "Yuh-Lang Lin, Committee Chair"

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The Dynamics of Orographic Precipitation: A Mesoscale Modeling Perspective
(2003-04-24) Chiao, Sen; Yuh-Lang Lin, Committee Chair
This research investigates the local circulation associated with a heavy orographic rainfall event occurred during 19-21 September 1999 (MAP IOP-2B). The near-surface flow was dominated by a barrier jet originally from the Adriatic Sea and a southerly jet from the Gulf of Genoa. A significant westward turning occurred when the southeasterly and southerly flow approached the south side of the Alps. Precipitation was mainly concentrated on the windward slopes, especially near the Lago Maggiore area. This event was simulated with a 5-km horizontal grid spacing using the Penn State/NCAR MM5 model. The MM5 simulation reproduced the basic features such as the timing and location of the deep trough and the associated precipitation evolution, though the total amount of precipitation is slightly higher than that measured by rain gauges. Sensitivity experiments have been conducted to investigate the effects of upstream orography, western flank of the Alps, earth's rotation, boundary layer friction, and model horizontal resolution. The results demonstrate that the juxtaposed low-level convergence and the orographic uplifting of a potentially unstable impinging flow were the major causes of the heavy rainfall. The westward turning of the southeasterly and southerly flow was caused by boundary friction, rotational as well as orographic blocking. The boundary layer friction reduced the total amount of the rainfall and altered its distribution by weakening the wind near the surface. The precipitation distribution and amount over the southern upslopes of the Alps were not directly related to either coastal Apennines Mountains or the west flank of the Alps. The 1.67 km horizontal grid spacing simulation indicates that heavy rainfall tended to concentrate in the vicinity of individual mountain peaks. The total amount of rainfall was over-predicted along the windward slopes due to the strong upward motion that occurred on the upslopes. The results imply that the dynamical forcing manifested as vertical motion increases rapidly as resolution increases. It is speculated that the rainfall over-prediction problem might be caused by the inaccurate or unrealistic microphysical processes over complex topography.
An Evaluation of the Influence of Several Convective Parameterization Schemes on a Real Time Turbulence Model (RTTM) in Weakly-Forced Environments
(2003-12-01) Slusser, Sarah Winters; Mike Kaplan, Committee Co-Chair; Yuh-Lang Lin, Committee Chair; Lawrence Carey, Committee Chair
The purpose of this study is to improve the convective forecasts of weakly forced environments within the Real Time Turbulence Model (RTTM). Focusing on the southeastern portions of the United States, due to the amount of NASA flight missions conducted in that region, four cases are simulated using the Kain-Fritsch, Grell, and Kain-Fritsch 2 cumulus parameterization schemes. Three of the four cases have weak synoptic forcing with limited, sparse precipitation coverage and one case has strong synoptic forcing with concentrated, heavy precipitation coverage. It is usually within the weak synoptic forcing environments that the currently used Kain-Fritsch scheme greatly overestimates the precipitation amounts. Verification of model precipitation against satellite, radar and surface observations was performed in order to statistically assess the impact of the Kain-Fritsch, Grell, and Kain-Fritsch 2 schemes for the four cases. Then, a comparison of the performance of the schemes and an examination of their influence on the local environment is conducted. We found that the Kain-Fritsch 2 scheme more accurately depicts the precipitation coverage in the weak forcing environments due to modifications made to the cloud model and trigger function, which involve alterations to the CAPE calculation and precipitation efficiency relationship and the inclusion of a minimum environmental entrainment rate and variable cloud radius. By strengthening the cloud model's dependence on deep layer relative humidity the modifications reduce the amount of precipitation produced in a weakly forced environment while having a minimal effect on a strongly forced environment.
The Impact of Superimposed Synoptic to Meso-Gamma Scale Motions on Extreme Snowfall over Western Maryland and Northeastern West Virginia during the 2003 Presidents' Day Winter Storm
(2005-06-30) Kiefer, Michael Thomas; Gary Lackmann, Committee Member; Michael Kaplan, Committee Member; Yuh-Lang Lin, Committee Chair
During the second Presidents' Day winter storm of 15-18 February 2003, snowfall totals exceeding 100 cm were reported across a relatively small region of western Maryland and northeastern West Virginia (hereafter the region of interest). This numerical modeling study considers the role of two juxtaposed low-level jet/front systems in influencing the development and/or modification of meso-alpha to meso-gamma scale circulations producing these extreme snowfall totals. The goal in each chapter is to link each mechanism to the interaction of the aforementioned low-level jet/front systems, the maritime and continental, in order to synthesize the rather complicated conceptual model presented, and possibly in the future enable operational forecasters to better assess the likelihood of fine-scale extreme snowfall. All numerical simulations are performed with the Non-hydrostatic Mesoscale Atmospheric Simulation System (NHMASS) model. The first chapter considers the development of two low-level jet/front systems. The continental (southwesterly) jet/front system is shown to result from a combination of diabatic and adiabatic processes, including (1) formation of a polar stream lee cyclone through adiabatic compression and surface sensible heating and the resulting horizontal circulation, and (2) secondary circulations formed within an unbalanced subtropical jet (STJ) exit region due to generation of mid-level mass perturbations forced by latent heating and adiabatic compression. The maritime (easterly) low-level jet/front system was found to develop as a result of a strong southward directed pressure gradient force between a strong anticyclone over southern Quebec and a deepening coastal trough (with convection and low-level latent heating-induced pressure falls) accelerating parcels exiting in the polar jet (PJ) right entrance region. The unbalanced (with respect to geostrophic and gradient wind balance) STJ exit region was shown to impact not only the formation of the continental low-level jet, but also the meso-α scale lift through generation of a region of strong upper-level divergence. The second chapter built on the first chapter by considering the impact of the twolow- level jet/front systems becoming superimposed over the mid-Atlantic U.S. The low- level jet structure was shown to be conducive to maintenance of apparent inertia-gravity wave activity generated within the unbalanced STJ exit region, with the wave activity acting to prolong lift over the region of interest. Two other mechanisms, frontal lifting and frontogenesis, produced narrow bands of strong lift over the region, through a process wherein confluent deformation produced locally steeper slopes of the continental front with the continental jet forced up these steep frontal inclines. Secondary circulations generated as a result of intense bands of diabatic frontogenesis reinforced the primary frontogenetical circulation produced through confluent deformation. The third chapter evaluated the finest scale circulations, each tied to the interactions of the continental and maritime low-level jets with the complex terrain within the region of interest. Pure upslope flow was considered and generally discarded as a contributing mechanism in this case due to the shallow layer of upslope and large vertical separation between upslope-induced ascent and deep lift forced by the larger-scale mechanisms. The primary mechanism proposed, termed the multi-ridge mountain wave mechanism, occurs wherein air parcels within the maritime low-level jet are directed normal to a series of 100-300 m deep terrain ridges and forced to pass through multiple hydrostatic mountain-wave positive phases, condensing out additional liquid water with the passing of each ridge in the already saturated lower-troposphere present in the region. The cloud condensate is then advected downstream by the continental low-level jet. A second mechanism, surface convergence banding in the lee of the terrain, was proposed but is considered to be of secondary importance.
Interactions of Gravity Waves and Moist Convection in the Troposphere and Stratosphere
(2006-12-15) Suffern, Paul Samuel; Matthew D. Parker, Committee Member; Yuh-Lang Lin, Committee Chair; Michael L. Kaplan, Committee Member
On December 12th and 13th, 2002 a deep large-amplitude tropospheric mesoscale gravity wave formed over Texas and propagated northeastward across several states. This study examines the role of the coupling of geostrophic adjustment, moist convection, and an area of shear instability from the meso- to the meso- scale as the mesoscale gravity wave is formed⁄maintained. Three main chapters comprise this thesis. Chapter 4 employs observations to describe the mesoscale gravity wave event. Numerical modeling of the mesoscale gravity event is explained in Chapter 5 to further understand the coupling of geostrophic adjustment, moist convection, and the area of shear instability. Chapter 6 analyzes the vertically propagating gravity waves in the lower stratosphere employing observations and numerical modeling. Observations of the mesoscale gravity wave system allow a detailed representation of the event. As an upper-level jet streak moved into central Texas, moist convection and a corresponding surface low-pressure system began to develop. The development of widespread convection lead to the downstream growth of a secondary jet streak, which in turn continued the imbalance of mass and momentum at the upper-levels across central Texas. Geostrophic adjustment and moist convection are closely correlated with the mesoscale gravity wave as the mesoscale gravity wave forms and moves northeastward all the way to Mississippi. Other observations also show dry air and large amounts of shear located directly behind the moist convection. The development of an area of possible shear instability behind the moist convection represents mixing and descending momentum in the troposphere and formation/maintenance of the mesoscale gravity wave. Chapter 5 confirms and enhances the observations of the coupling between geostrophic adjustment, moist convection, and the area of shear instability through numerical simulations. Mass and momentum perturbations from the moist convection are directed upward, upstream, and cross-stream into the jet streak momentum. Momentum in the moist convective process descends toward the stable layer, resulting in mass accumulation near the surface, as simultaneously, the updraft within the strengthening moist convection penetrates into the lower stratosphere. The mesoscale gravity wave(s) is(are) initiated/maintained as the negative buoyancy perturbation and descent of momentum interacts with the stable layer near the surface behind the moist convection. The impact of latent heating and its importance to the development of the coupling of mesoscale gravity wave system are discussed in this chapter employing model sensitivity studies. Chapter 6 analyzes the development of vertically propagating gravity waves in the lower stratosphere. Observations from the Advanced Microwave Sounding Unit-A (AMSU) polar orbiting satellite show a signal of possible vertically propagating gravity waves above the moist convection across Texas and the Gulf of Mexico. Numerical simulations were performed to diagnose the environment in which these gravity waves formed in the lower stratosphere as well as their realism to the observations.
Mesoscale Disturbances and Orographic Precipitation Distribution: Three Special Case Scenarios
(2007-04-05) Reeves, Heather Dawn; Yuh-Lang Lin, Committee Chair; Richard Rotunno, Committee Member; Gerald Janowitz, Committee Member; Fredrick Semazzi, Committee Member
Flow patterns and precipitation distribution in the vicinity of mountains are often attributed to whether the flow is in a blocked or unblocked regime, which, in turn, is dictated by the wind speed and static stability far upstream of the mountain and the mountain height. Herein, the notion that mesoscale disturbances immediately upstream of mountains and/or terrain inhomogeneities could alter the flow patterns significantly, resulting in a precipitation distribution that does not fit typical patterns for blocked vs. unblocked flows, was studied via consideration of specific case studies and idealized numerical experiments. The first scenario considered that of an orographically trapped stable layer upstream of a mountain. A series of quasi-idealized simulations suggests that flow patterns and precipitation distribution in the vicinity of a mountain are affected not only by the presence of such a layer, but by the strength of the capping inversion of this layer. Part two of this research investigated the effects of a pre-existing convective system upstream of a mountain for conditionally unstable flow, where a pre-existing convective system is defined as one that is triggered by some mechanism other than lifting by the mountain in question. Idealized numerical experiments with differing upstream wind speeds reveal that if the flow was in a blocked regime, the introduction of a pre-existing precipitation system had little effect on the precipitation distribution and flow patterns in the vicinity of the mountain. If the flow was in an unblocked regime, the introduction of a pre-existing precipitation system was associated with flow reversal upstream of the mountain and an upstream shift of the locus of maximum precipitation. The last part of the thesis considered the effects of terrain inhomogeneities on precipitation distribution via full-physics simulations of two heavy precipitation events along the western face of the Sierra Nevada mountains. Of the terrain features that were tested, the greatest sensitivity in the precipitation distribution was to directional changes in slope orientation along the upstream face of the range. In sensitivity tests where these undulations were removed, isolated maxima along the upslope that were present in the corresponding reference simulations were either reduced or non-existant. The overriding conclusion from this thesis is that although the wind speed and static stability far upstream of a mountain are important, mesoscale flow phenomena immediately upstream of a mountain can significantly change the flow and precipitation patterns.
Numerical Studies of Synoptic and Mesoscale Environments Conducive to Heavy Rainfall in Tropical and Extratropical Systems
(2003-11-13) Thurman, James Arnold; Michael L. Kaplan, Committee Member; Jerry M. Davis, Committee Member; Simon W. Chang, Committee Member; Yuh-Lang Lin, Committee Chair
The purpose of this research was to examine the environments conducive to heavy rainfall production, specifically a landfalling hurricane, Hurricane Floyd (September 1999) and an Alpine event, MAP IOP-2B (September 1999). In addition to studying the two events independently, a third study examined the link between Floyd's extratropical transition and IOP-2B given that the two events occurred a few days apart. Analysis of observations of both events led to the formation of the hypothesis that the coupling of transverse ageostrophic circulations over a pre-existing low-level confluence zone was a key precursor to heavy rainfall production. In both cases, a low-level confluence zone was found from the observations and simulations. For Floyd, the confluence zone developed as warm easterly winds ahead of the hurricane became juxtaposed with cooler northeast winds just inland over North Carolina and Virginia. In IOP-2B, the confluence zone developed as southerly winds from the Mediterranean became juxtaposed with easterly and southeasterly winds from eastern Italy. These easterlies and southeasterlies developed as southeast winds from the Adriatic Sea impinged upon the eastern Alps, and turned west in the form of a barrier jet. Also, in both cases, upper level diffluence, due to a split flow, became juxtaposed over the low-level confluence, enhancing the upward motion. MM5 simulations for both events revealed coupled thermally direct and thermally indirect circulations over the low-level confluence zone with their rising branches coupled over the zone, proving the hypothesis. Simulations of Floyd's extratropical transition showed a link existed between Floyd and IOP-2B. Parcels from Floyd's upper level circulation reached Italy around the time the heavy rainfall developed in IOP-2B. Simulations with and without latent heat release demonstrated the importance of latent heat release in maintaining the upper-level jets and split flow which in turn, aided in the maintenance of convection. Latent heat release was also found to be important in maintaining the strength of the transverse ageostrophic circulations
Synoptic and Mesoscale Environments for Orographic Rainfall Associated with MAP IOP-8
(2003-07-18) Chen, Shu-Yun; Yuh-Lang Lin, Committee Chair; Fredrick Semazzi, Committee Member; Larry D. Carey, Committee Member
In this study, we have adopted Penn State/NCAR Mesoscale Model version 5 (MM5) to simulate the synoptic and mesoscale environments conducive to orographic rainfall associated with Mesoscale Alpine Programme (MAP) Intensive Observation Period 8 (IOP-8). The model sensitivity tests on cumulus parameterization schemes, microphysical parameterization schemes, and terrain resolution were also included in this study. A deep trough system associated with low-level jet approached the Lago Maggiore target area at 0000UTC 20 October 1999. During the same time period, a high pressure system was located to the east of the trough system at the same time. The high-low pressure system then remained quasi-stationary through 1200 UTC 20 October and 1200UTC 21 October. The southerly flow advected conditionally unstable air, i.e. high [subscript e], up to the Po Valley and the southern Alpine slopes. The sounding upstream of the Ligurian Apennines appears to contain high convective available potential energy (CAPE). Meanwhile, an easterly flow penetrated the Po Valley along the foothill of the southern Alps. The easterly flow met the southerly flow near the northern coast of the Adriatic Sea and Ligurian Sea to help enhance the orographically induced low-level convergence. As a result, the low-level convergence near the Ligurian Apennines was stronger. The easterly flow was confined in the Po Valley between Alps and Apennines and kept moving toward the west. Eventually, it flowed out through the gap between Maritime Alps and Ligurian Apennines and formed a mesoscale vortex with the southerly flow around the western Po Valley. The relative cold and stable easterly flow then piled beneath to provide a stable environment. It was proved that the cold air serves as a cold dome to make the southerly flow easily ride on it. Therefore, the upward motion near the southern Alpine slopes was very weak and not able to produce convective rainfall. Only shallow clouds developed and stratiform precipitation was shown from both model results and observations. On the other hand, the southerly flow produced heavy orographic rainfall over the Ligurian Apennines. Along with the low-level convergence, which was enhanced by the confluence of easterly and southerly flow near the Ligurian Sea and Apennines, the upper-level divergence also played an important role in triggering and maintaining the convective systems near this region. The right entrance of jet streak was co-located with the Ligurian Apennines surrounding area through the model integration. As seen in the model simulation, the coupling of upper-level and lower-level forcing was essential for producing rainfall over Ligurian Apennines and Ligurian Sea during IOP-8. Based on model sensitivity tests on microphysical parameterization schemes, we found that the Reisner scheme tended to underpredict the precipitation over the southern Alpine slopes. The Goddard LFO scheme produced a reasonable amount of snow particles but overpredicted the amount of graupel, which may help explain the overprediction of rainfall over the southern Alpine slopes. As shown by the cumulus parameterization schemes sensitivity tests, the Grell scheme was not active enough to produce enough rainfall. Most of the rainfall was produced via microphysical parameterization scheme. In comparison, the Kain-Fritsch scheme did produce a fair amount of precipitation near the southern Alps and the ocean. However, it appears the Grell scheme is more suitable for the stable environment near the Lago Maggiore target area because it produced a reasonable amount of rainfall. For the sensitivity tests on Ligurian Apennines, we found that it has a significant impact on the rainfall distribution associated with MAP IOP-8. Without the Ligurian Apennines, the cold air was spread toward the ocean and the heavy rainfall was shifted toward the southern Alpine slopes.
Synoptic-scale Mesoscale Environments Conducive to Forest Fires During the October 2003 Extreme Fire Event in Southern California
(2007-04-25) Huang, Chenjie; Yuh-Lang Lin, Committee Chair; Gerald S. Janowitz, Committee Member; Michael L. Kaplan, Committee Member
This study has employed both observational data and numerical simulation results to diagnose the synoptic-scale and mesoscale environments conducive to forest fires during the October 2003 extreme fire event in southern California. A three-stage process is proposed to illustrate the coupling of the synoptic-scale forcing, such as the high pressure ridge and the upper-level jet streak, which leads to meso-α scale subsidence in its exit region, and the mesoscale forcing, such as wave breaking and turbulence and the wave-induced critical level, which leads to severe downslope (Santa Ana) winds. Two surges of dry air were found reaching the surface in southern California. The first dry air surge arrived as a result of moisture divergence and isallobaric adjustments behind a surface cold front. The second dry air surge reached southern California as the meso-α to meso-β scale subsidence and the wave-induced critical level over the coastal ranges phased together to transport the dry air from the upper-level jet streak exit region toward the surface and mix the dry air down to the planetary boundary layer on the lee side of the coastal ranges in southern California. The wave breaking region on the lee side acted as an internal boundary to reflect the mountain wave energy back to the ground and created severe downslope winds through partial resonance with the upward propagating mountain waves. The widely used Haines Index (HI) and the newly developed NCSU3 and NCSU4 fire indices were calculated and compared during this period using the numerical model data. The results show that all three indices predicted reasonably well the potential for large fire growth in southern California. The patterns predicted by the NCSU3 and NCSU4 indices were very similar until the phasing between the vertically increasing sinking motions, a deep well-mixed boundary layer and a strong vertical wind shear zone developed. A temperature inversion above a near surface well-mixed boundary layer in southern California provided an environment favorable for the development of plume-dominated fires, for which the HI was originally designated to predict the potential for large fire growth. This particular type of environment is part of the reason why the HI was successful in assessing the potential for large fire growth in southern California before the Santa Ana winds became the major controlling factor.