1. In the Northern Hemisphere, the geostrophic wind blows ( parallel / perpendicular ) to _______________ with _______________ pressure to the _______________.
1. In the Northern Hemisphere, the geostrophic wind blows ( parallel / perpendicular ) to isobars with _______________ pressure to the _______________.
1. In the Northern Hemisphere, the geostrophic wind blows ( parallel / perpendicular ) to isobars with low pressure to the _______________.
1. In the Northern Hemisphere, the geostrophic wind blows ( parallel / perpendicular ) to isobars with low pressure to the left. 2. A scale analysis of the horizontal momentum equations reveals what two forces being most important on the synoptic scale? What is the balance between these two forces called? Pressure Gradient Force Coriolis Geostrophic Balance
3. Briefly discuss the difference between vertical motion expressed in Cartesian coordinates (i.e., w) and vertical motion expressed in pressure coordinates (i.e., ω). As part of your answer, be sure to indicate the correct sign for positive vertical motion in both coordinate systems. w: vertical motion in Cartesian coordinates; w = dz/dt change in vertical distance with time; w > 0 is positive vertical motion ω: vertical motion in pressure coordinates; ω = dp/dt change in pressure with time; ω < 0 is positive vertical motion The signs are different because pressure decreases as you increase in altitude.
4. In the Northern Hemisphere, positive vertical vorticity is associated with ( cyclonic / anti-cyclonic ) rotation, which indicates the winds are blowing ( clockwise / counter-clockwise ).
5. Briefy discuss why we’re so interested in identifying areas of low- to mid-level temperature advection. Low- to mid-level temperature advection is a proxy for vertical motion, which is ultimately what we are trying to diagnose in this class. WAA lift/rising air CAA sinking air
Quiz #1 (out of 17 points): Mean – 13(76.5) Max – 15 Min – 10 Quiz #2 (out of 15 points): Mean – 9.4(63) Max – 11 Min – 7
Shear vs. Curvature Vorticity –Be able to identify and explain Shear vorticity is caused by a change in wind speed over some distance –Typically found near jet streaks Curvature vorticity is caused by a change in wind direction over some distance –Typically maximized in trough/ridge axes
The Vorticity Equation (e) Friction (f) Horizontal advection most important on synoptic scale!
Thermodynamic Review What is an adiabatic process? One in which there is no heat transfer; for example, no heat exchanged between a parcel and its environment would be considered an adiabatic process dq = 0 Lapse rate – the rate of decrease with height for an atmospheric variable, typically temperature The dry adiabatic lapse rate (DALR) is the rate of temperature decrease with height for a parcel of dry or unsaturated air rising under adiabatic conditions No heat exchanged between the parcel and the environment Pressure decreases as the parcel rises, causing it to expand As the parcel expands, it pushes on the air around it, causing it to lose internal energy, so the temperature of the parcel decreases ≈ 10 K km -1
Potential Temperature, θ Poisson’s Equation: where p 0 is some reference pressure level, usually 1000 mb What is the physical interpretation of potential temperature? The temperature that an air parcel would have if it were dry adiabatically compressed (or expanded) to a reference pressure level What are some potential uses of potential temperature? Used to determine static stability An examination of the horizontal θ distribution is useful in locating frontal boundaries -- especially over uneven terrain where surface temperature differences may be caused by the terrain. When diabatic processes are neglible, air parcels are typically constrained to travel along surfaces of constant θ, known as isentropic surfaces. This can help identify areas of lift. (Chapter 3).
Static Stability The link between potential temperature and stability…
Below is an idealized vertical cross section through a well-mixed planetary boundary layer (PBL) beneath a capping inversion layer. On the right, the corresponding profile of potential temperature as a function of height is given. In this layer, air parcels undergo quasi-random ascent and descent – turbulent mixing The potential temperature profile is a result of the “mixing” of the random motions of the air parcels. Surface based mixed-layers usually = PBL Notice the potential profile slightly decreases with height near the surface – this is an absolutely unstable situation and is due to strong solar heating at the ground. This is known as a superadiabatic situation
Being able to identify/forecast the mixed-layer has important considerations for surface temperature and wind conditions Surface Temperature & Mixed Layer: The development of a deep mixed layer often means warmer surface temperatures because compression of air with warmer potential temperature from aloft.
Skew-T diagram, 00Z 21 August 2007 – Greensboro, NC What were the surface conditions like in central NC based on this sounding? Warm and dry T ~ 35°C = 95°F What do you see on this sounding that supports the extremely warm & dry weather? Unusually deep mixed layer extending to near 700-mb What’s happening, synoptically, to cause the deep mixed layer? High pressure is causing the air to sink and warm adiabatically at the DALR… note how the temperature parallels the dry adiabat
An inversion is a departure from the usual decrease or increase with altitude of the value of an atmospheric property. Usually in meteorology, an inversion refers to an increase in temperature with height. Temperature inversions occur when air above a certain level is warmer than the air below. 2 Main Types: Subsidence Inversion – an increase in temperature with height that develops aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression. Often enhanced by vertical mixing in the air below the inversion. Radiation Inversion – formed when the lowest levels of the atmosphere are cooled by contact with the earth’s surface, which cools by emitting radiation Factors that help form a radiation inversion include: calm windsdry air clear skieslong nights surface wetnesssurface type vegetation An Aside - Inversions
Being able to identify/forecast the mixed-layer has important considerations for surface temperature and wind conditions Surface Temperature & Mixed Layer The development of a deep mixed layer often means warmer surface temperatures because compression of air with warmer potential temperature from aloft. Surface Winds & Mixed Layer: Strong winds aloft and the presence of a deep mixed layer often allows for efficient vertical momentum transport strong winds can be brought down to the surface
Skew-T diagram, 00Z 21 August 2007 – Greensboro, NC Imagine the winds were much faster (~50 knots) just above the subsidence inversion. The air sinking and warming along the lines of constant θ can efficiently bring the higher momentum (faster) air to the surface.
Back to Stability….
The slope of the temperature profile relative to the dry adiabats on an atmospheric sounding can help you determine stability. Below is a sounding from Forth Worth, TX. Where are the most and least stable layers? Where is the PBL? Mixed layer of neutral stability extending to 875- mb Strong, stable inversion layer that extends to 825-mb Atmosphere exhibits weak stability between 825 and 175-mb Above 175-mb, very strong stability corresponding to the stratosphere.
What is a diabatic process? Process that involves heat transfer (addition or loss of heat to the surroundings) dq ≠ 0 What are some examples of a diabatic process? Warming due to the sun Evaporational cooling Condensational warming What happens to a moist parcel of air as it rises? As the parcel rises, it cools at the DALR (as described before) but now, moisture begins to condense, which releases latent heat. The latent heat release partially offsets some of the cooling; however, the DALR is greater than the rate of warming due to latent heat release, so the parcel still cools as it rises. The saturated parcel cools at a lesser rate than an unsaturated parcel… moist adiabatic lapse rate
Equivalent Potential Temperature, θ e (theta-e) Similar to potential temperature, but now includes all potential heating due to condensation What is the physical interpretation of theta-e? The temperature that results after all latent heat is released in a parcel of air and brought adiabatically to the 1000-mb level. Theta-e increases as dewpoint (moisture) and/or temperature increases Theta-e has important operational significance: Instability – a region with a relatively high theta-e is often the region with the most instability. Warmer low-level temperatures and higher low-level dewpoints increase instability. Low-level Jet – a low-level jet from a moisture source (e.g. Gulf of Mexico) will often bring in higher theta-e values and thus decrease stability Theta-e Ridge – these are regions with higher theta-e. They are often the burst point for convective activity
Similar to potential temperature, theta-e is related to atmospheric stability. What does it mean if theta-e decreases with increasing height? The atmosphere is convectively (or potentially) unstable Physically, this means that if the layer of air in which theta-e decreases with height were lifted to saturation, then an unstable lapse rate would occur. The lifting is typically done dynamically (WAA, low-level convergence, upper-level divergence). Convective instability typically occurs when the mid-levels of the troposphere are fairly dry and the PBL has high dewpoints. When lifting takes place, the lower portion of the layer (moist) reaches saturation first and cools at the lesser moist adiabatic lapse rate while the upper layer (dry) still cools at the DALR…. This will increase the environmental lapse rate, causing an instability
Assessing Convective Instability This morning (12Z) sounding displays convective instability. No SBCAPE (surface based convective available potential energy). A forecaster should expect daytime heating to increase SBCAPE. However, SBCAPE will increase even more if this location was in an environment supportive of dynamic lifting The lifting will cool the mid-levels at a rate greater than the low levels.
Example RAX (Raleigh, NC) base-level reflectivity, 902 UTC 8/6/2011 These storms formed very early, before sunrise. Why did this happen? – examine nearby soundings for clues!
12 UTC 8/6/11 GSO (Greensboro, NC) sounding upstream from RAX What’s good about this sounding for heavy rain? Very moist low- levels Wind profile veers with height WAA! lift! Weakly unstable Note the drier mid- levels convective instability! This likely caused the environment to become more unstable.
Check further upstream for moisture, instability RUC Analysis Winston-Salem, NC, 4 UTC 8/6/11 You can use BUFKIT to plot these types of model analysis soundings. ols/BUFKIT/
Theta-e analysis from the previous RUC Sounding Theta-e decreases with height in a layer that is nearly saturated in the lower levels – convective instability
Below: 312K isentropic surface (theta surface). **Everywhere in the image, the potential temperature is 312K Contour lines represent pressure… so this map shows where the 312K surface is located. Since air tends to rise and fall at the DALR, or along surfaces of constant theta… this type of analysis can show where upward motion is occurring. In central NC, the air along the 312K surface is moving from 730mb to 710mb. Remember that pressure decreases with height… so this is an area lift. This is known as isentropic upglide
.DISCUSSION... /ISSUED 307 AM CST WED FEB / A SPLIT UPPER FLOW PATTERN EXISTS ACROSS THE CENTRAL AND EASTERN U.S. TODAY AS A SHORTWAVE APPROACHES THE SOUTHERN/CENTRAL PLAINS. IN ADDITION, A WEAK CONFLUENCE REGION OVER CENTRAL LA WITHIN A WAA REGIME IS BRINGING THUNDERSTORM ACTIVITY OVER LA/MS. THE WEAK CONVERGENCE AND ISENTROPIC LIFT ACROSS THE TN VALLEY IS BRINGING SCATTERED SHOWER ACTIVITY THAT WILL BECOME MORE WIDESPREAD THROUGHOUT THE DAY. Example from last year What’s going on here?
Key Concepts When theta-e decreases with height: if air lifted to saturation, will become absolutely unstable BUT, only if saturation is reached If layer lifted, top cools more than bottom due to more latent heat release at bottom of layer Parcels at bottom of layer rise along a warmer moist adiabat than those higher up, denoting instability
Remember that theta-e includes both temperature and moisture! This makes it an even better tool at helping to recognize surface frontal locations! Look for tight gradients in theta-e Areas with low values of theta-e often are experiencing cold, dry air at the surface Areas with high values of theta-e are often experiencing warm, humid conditions But be aware of it’s limitations!
RUC Theta-E Analysis for 0200 UTC 1/30/12 Where are the gradients in theta-e?
Corresponding surface analysis Frontal locations generally analyzed in areas with theta-e gradients Except the coastal locations… what’s going on here?