Dynamic Meteorology Reviewer Chapter 5 PDF

o The temperature a parcel of air Dynamic Meteorology Reviewer would have if adiabatically compressed to 1000 mb, accounting for...

o The temperature a parcel of air Dynamic Meteorology Reviewer would have if adiabatically compressed to 1000 mb, accounting for water vapor. Chapter 5 o Always higher in moist air than dry air at the same temperature due to Planetary Boundary Layer (PBL) moist air's lower density. The PBL is the lowest part of the atmosphere directly influenced by the Earth's surface, where surface friction slows down the wind and creates turbulence. This turbulence mixes air within the Sublayers of the PBL PBL, helping transport heat, moisture, and 1. Surface Layer: momentum vertically. o Closest to the ground (tens of meters deep), in direct contact with Characteristics: the Earth's surface, featuring strong o Height: Varies by time of day, wind shear, a superadiabatic lapse season, and location, ranging from rate during the day, and a hundreds of meters to several logarithmic wind-speed profile. kilometers. 2. Mixed Layer: o Role of Turbulence: Within the o Above the surface layer (hundreds PBL, turbulence controls vertical of meters deep), characterized by transport, while outside the PBL, well-mixed conditions with nearly molecular processes are negligible constant potential temperature and due to low viscosity. wind speed independent of height. 3. Entrainment Zone (EZ): Components of Air Flow o The layer at the top of the mixed layer, marked by a stable layer and 1. Mean Wind: The average wind speed and temperature inversion, where air direction over time, important for the from the mixed layer is entrained horizontal transport of air, momentum, into the free atmosphere. temperature, moisture, and trace constituents. 2. Waves: Periodic oscillations that transport Virtual Potential Temperature Profiles energy and momentum, balancing surface 1. Daytime: Surface heating creates a friction with the mean wind. superadiabatic lapse rate in the surface 3. Turbulence: Chaotic air motion that occurs layer, a nearly isothermal mixed layer, and when wind speed exceeds a critical value, an entrainment zone at the top. enabling effective momentum and heat 2. Nighttime: Surface cooling leads to a transfer, acting to smooth gradients in mean surface inversion and a stable boundary flow and dissipate the flow. layer, which deepens as the night progresses. 3. Well-Mixed Layer: On sunny days with strong winds, the virtual potential Theoretical Concepts temperature remains nearly constant with height. 1. Taylor’s Hypothesis: Assumes small, 4. Capping Inversion: A stable layer atop a short-lived turbulent eddies allow for the convective boundary layer restricts further replacement of advective terms with the vertical mixing. local time rate of change. Valid only when the local derivative dominates the advective term, applicable to steady, horizontally homogeneous flows, but limited for velocity in the presence of significant pressure gradient forces. 2. Virtual Potential Temperature: Plumes and Instability Turbulent Kinetic Energy (TKE): Measures turbulence intensity, defined as Plumes: Buoyant air parcels rising from the half the sum of squared velocity surface, visible as smoke or dust columns, fluctuations: influenced by atmospheric stability. Instability: o Convective Instability: Occurs when surface air is warmer than the air aloft. o Mechanical Instability: Results from increasing wind speed with Flux Richardson Number ($R_f$): height, creating wind shear. Indicates the balance between buoyancy production and shear production of turbulence: Important Concepts in Atmospheric Dynamics 1. Boussinesq Approximation: Simplifies the equations of motion, assuming density where B is the buoyancy flux and MP is the variations are negligible except in the shear flux. buoyancy term, allowing simpler equations to capture essential dynamics. 2. Reynolds Averaging: Separates mean flow from turbulent fluctuations, enabling study of Eddy Stress the mean flow and turbulent kinetic energy (TKE) in turbulent fluids. Definition: The force exerted by turbulent 3. Energy Cascade: Describes the transfer of eddies on the mean flow, responsible for energy from large-scale eddies to small- vertical momentum transport, given by: scale eddies, eventually dissipated by viscosity. Mathematical Tools in Turbulence ρ is the air density. Statistics: Used to analyze turbulence: o Mean: The average value over a period. Theories and Models of the PBL o Standard Deviation: The spread around the mean. 1. Similarity Theory: Assumes a similar PBL o Variance: The square of the structure at different heights. standard deviation. 2. K-Theory: Assumes turbulent fluxes o Covariance: How two variables vary proportional to mean variable gradients, together. applied in neutrally or stably stratified o Correlation: The linear relationship boundary layers. between two variables. 3. Ekman Theory: Describes the wind's balance between pressure gradient force, Coriolis force, and turbulent drag, leading to the Ekman spiral. Ekman Layer and Spiral 1. Ekman Layer: A theoretical layer where wind is affected by Coriolis force and friction, producing the Ekman spiral. 2. Modified Ekman Layer: Combines the logarithmic surface layer profile with the Ekman spiral, offering a more accurate wind profile representation. Importance of PBL Sub-Layers Surface Layer: Facilitates exchange of momentum, heat, and moisture with the surface. Upper Layer: Influences large-scale weather patterns. Communication within Boundary Layers 1. Entrainment: Transfer of air from the mixed layer to the free atmosphere at the top of the mixed layer. 2. Secondary Circulations: Vertical motions driven by horizontal temperature gradients, transporting heat and moisture globally. 3. Turbulent Mixing: Main mechanism for vertical transport of heat, moisture, and momentum. Key Takeaways The PBL’s height, structure, and turbulence are essential in atmospheric dynamics. Virtual potential temperature and turbulence help define stability and transport within the PBL. The Ekman layer and modified Ekman spiral offer theoretical insights into wind profiles. Air moves between boundary and upper layers via entrainment, secondary circulations, and turbulent mixing.