Low-Level Convergence Over the Palmer Divide Induced by Complex Terrain by Matthew Robert Steiner B.S., University of South Alab
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Low-Level Convergence over the Palmer Divide Induced by Complex Terrain by Matthew Robert Steiner B.S., University of South Alabama, Mobile, 2008 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Master of Science Department of Atmospheric and Oceanic Sciences 2017 This thesis entitled: Low-Level Convergence over the Palmer Divide Induced by Complex Terrain written by Matthew Robert Steiner has been approved by the Department of Atmospheric and Oceanic Sciences ___________________________________ Katja Friedrich ___________________________________ Edward Szoke Date_____________ The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. Matthew Robert Steiner, (M.S., Atmospheric and Oceanic Sciences) Low-Level Convergence over the Palmer Divide Induced by Complex Topography Thesis directed by Dr. Katja Friedrich The relationships between low-level convergence, vertical velocity, relative vorticity, and moderate-to-heavy precipitation indicate the significance of understanding both the meteorology and the topography of a given region. In this study, the three-dimensional winds, convergence, and relative vorticity associated with a terrain-induced low-level cyclonically-turning circulation (LLCC) are analyzed over the southern Colorado Palmer Divide on 9-10 October 2014 to diagnose the importance of complex topography in generating, enhancing, and distributing precipitation. Past studies show that the necessary ingredients for heavy precipitation over the Colorado Front Range (including the Palmer Divide) are: easterly upslope winds, a high water vapor environment, and an unstable or conditionally unstable atmosphere. Furthermore, many orographic precipitation mechanisms have been studied and identified across this region. However, few studies discuss the importance of the complex topography and its influence on both low-level circulations and precipitation over the Palmer Divide except for the Denver Cyclone. The results of this study indicate that the LLCC formed near Colorado Springs, CO, and contained high moisture content traced back to Tropical Storm Simon, in conjunction with an easterly flow over the Palmer Divide. The circulation enhanced convective activity due to latent heat release, which strengthened convergence that led to heavy precipitation over the southern Palmer Divide. As the LLCC dissipated, a larger-scale upper-level induced circulation developed and extended convergence to the east, which produced significant mesoscale lifting and iii widespread precipitation that mirrored the shape of the Palmer Divide. Reflectivities dissipated as the circulation moved east and southeastward off the higher terrain. This movement led to a developing northeasterly flow initiated by high pressure to the north that weakened convergence over much of the Palmer Divide, which ended the event. Acknowledgements I would like to give a special thanks to my Research Advisor Katja Friedrich of ATOC for all her assistance throughout this time. Additionally, I would like to thank her for supplying a research computer for my time at the University of Colorado at Boulder. I would also like to thank John Cassano of CIRES/ATOC, Edward Szoke of NOAA, and Matthias Steiner of NCAR for the questions and comments they presented during my entire research process. I would like to give a big thanks to Juanzhen Sun of NCAR for providing the VDRAS datasets that made the wind and thermodynamic analyses for this research possible. Finally, special thanks to ATOC Program assistants Laurie Conway, Sherry Yearsley, and Kelly Duong for their help with department requirements. iv Contents Section 1 Introduction 1 2 Data, Instruments, and Methods 7 2.1 Spatial Domain………………………………………………………… 7 2.2 Radar…………………………………………………………………... 7 2.3 VDRAS………………………………………………………………… 8 2.4 NCAR Stage IV Precipitation Data………………………………….. 9 2.5 Denver Soundings……………………………………………………... 9 2.6 Colorado Lightning Mapping Array………………………………… 10 3 Case Overview 10 4 Results 20 4.1 Role of Winds and Topography in Generating Convergence………. 20 4.2 Linking Convergence with Precipitation.............................................. 25 5 Concluding Remarks 26 References 30 Appendix 33 A 33 v Figures Figure 1. Accumulated precipitation from Stage IV Data and Rain Gauges 3 2. 500-mb Observations 11 3. Surface Observations 12 4. Denver Soundings 13 5. Colorado Lightning Mapping Array Observations 14 6. Reflectivity from the Denver Radar 15 7. Doppler Velocity from the Pueblo Radar 16 8. Doppler Velocity from the Denver Radar 17 9. VDRAS Convergence and Relative Vorticity 2D Distributions 18 10. NOAA Stage IV Accumulated Precipitation (PD1-PD5) 19 11. Denver Radar Reflectivity and VDRAS Wind Barb Vertical Cross Sections 21 12. VDRAS Convergence and Vertical Wind Velocity Vertical Cross Sections 22 13. Hovmöller Diagram 25 A1. VDRAS vs. KDNR Vertical Profiles 33 vi This page has been intentionally left blank. vii 1. Introduction Mountains strongly influence the meteorology for various regions across the globe, affecting the movement of wind and the distribution of precipitation. While understanding the ingredients necessary for orographic precipitation development has improved over the past couple of decades, it remains a challenge to forecast correctly both the spatial distribution of precipitation and the accumulated totals. This is due largely to the fact that current operational numerical models are incapable of resolving local topographic features and meteorological processes on the order of less than a few kilometers. Key physical reasons for forecasting problems are the complexities that mountains introduce. Pressure gradients are responsible for advecting air masses and thus generating winds. When considering intense precipitation from orographic dynamics, air masses with high water vapor content are the most significant, especially when considering air that is forced to move upslope against the topography. Additionally, the stability of the environment and wind dynamics near and within mountainous regions also affect precipitation intensity. A simplified method to understanding orographic precipitation on a basic level begins by analyzing three components: 1) water vapor content and the large-scale dynamics, 2) mesoscale uplift via large-scale dynamics, and 3) condensation and efficiency of precipitation production from turbulent winds or microphysical processes (Rotunno and Houze 2007). Panziera and Germann (2010) argue that air-mass stability only plays a minor role in precipitation intensity, whereas the wind speed and flow direction dominate precipitation patterns over the southern Alps. However, the differences in geography over the southern Alps and the Colorado Front Range (CFR) might affect the typical stability environment seen in the two regions. Many CFR orographic case studies consist of unstable atmospheres (possibly 1 conditionally unstable), creating convective activity which leads to larger precipitation rates (Friedrich et al. 2016a; Chen et al. 2001; Petersen et al. 1999; Caracena et al. 1979). Under the influence of strong easterly winds, the distribution of precipitation is dependent on the stability and the size of the mountain barrier. For example, Panziera and Germann (2010) showed that unblocked low-level flows greater than 10 m s-1 under neutrally stable or unstable conditions produced the heaviest precipitation over terrain features with reduced upstream precipitation enhancement. If the atmosphere is either stable or the winds are less than 10 m s-1, then a reduction of enhanced precipitation frequency is observed over higher terrain. Over the CFR, easterly flow is paramount for widespread precipitation (e.g., Friedrich et al. 2016b; Chen et al. 2001; Petersen et al. 1999; Caracena et al. 1979). Regardless, it is important to understand the different orographic precipitation mechanisms by understanding the factors that govern them. Types of orographic precipitation are discussed and illustrated in Figure 6 from Roe (2005) and include the following: 1) basic upslope ascending air, 2) partial blocking, 3) down-valley winds induced by evaporative cooling, 4) lee-side convergence zones, 5) convection on either the windward or leeward side of the mountain triggered by solar heating, 6) convection induced by initial orographically-lifted air rising to a layer where the atmosphere is unstable, and 7) seeder- feeder mechanism. The problem with classifying precipitation events according to these different mechanisms is not always straightforward, because often multiple orographic precipitation mechanisms may be at work simultaneously in the real atmosphere. This study analyzes the complex mechanisms producing orographic precipitation on 9-10 October 2014 over the Colorado Palmer Divide (PD) within the CFR (Fig. 1), and aims to describe the dynamically-driven convergence zones found in the complex topography of the east- 2 west oriented PD. In addition, we will analyze how mesoscale and small-scale flow features interacting with the terrain affect the distribution and intensity of precipitation. Figure 1. Color-shaded contours of accumulated precipitation (mm) derived from the Precipitation NCEP/EMC 4KM Gridded Stage IV Data for the Colorado Front Range and the Palmer Divide from 1600-0400 UTC 9-10 October 2014. Cities are