Question

#41 n Chapter 17, the book discusses windstorms associated with mountain wave situations. What are some of the key atmospheric features observed during these situations? Also, what are some locations in North America where you might expect these events to occur? (8 pts) Breaking waves Hydraulic jump Rotor Lee waves Chinooks? FS F6 F7 F8 F10 F1 F12 Fn

Answer 1

Mountain Waves can be defined as oscillations to the lee side (downwind) of high ground resulting from the disturbance in the horizontal air flow caused by the high ground, whereas the wind storms associated with these waves usually are strong enough to cause at least light damage to trees and buildings and may or may not be accompanied by precipitation. Wind speeds during a windstorm typically exceed 55 km (34 miles) per hour. And thus it can be said that Mountain waves are a form of internal gravity waves produced when stably stratified flow is forced over an obstacle this disturbance elevates air parcels above their level of neutral buoyancy. Buoyancy restoring forces therefore act to excite vertical oscillation of the perturbed air parcels at the Brunt-Vaisala frequency, which for the atmosphere is:

${\displaystyle N={\sqrt {{g \over \theta _{0}}{d\theta _{0} \over dz}}}}$

where   ${\displaystyle \theta _{0}(z)}$ is the vertical profile of potential temperature.

Oscillations tilted off the vertical axis at an angle of $\phi$ will occur at a lower frequency of   ${\displaystyle N\cos {\phi }}$ . These air parcel oscillations occur in concert, parallel to the wave fronts (lines of constant phase). These wave fronts represent extreme conditions  in the perturbed pressure field (i.e., lines of lowest and highest pressure), while the areas between wave fronts represent extreme conditions  in the perturbed buoyancy field (i.e., areas most rapidly gaining or losing buoyancy).

Energy is given along the wave fronts (parallel to air parcel oscillations), which is the direction of the wave group velocity. In contrast, the phase propagation (or phase speed) of the waves points perpendicular to energy transmission (or group velocity).

Mountain Waves usually increase the winds on the lee side of the mountains but typically not to severe levels. But in downslope wind cases they get very strong reaching severe levels mainly (>55 kts).

Causes and occurance:

Windstorms usually lasts for minutes when due to the outcome or downbursts from thunderstorms, or they may last for hours (and even several days) when they result from large-scale weather systems. A windstorm that travels in a straight line and is caused by the gust front (the boundary between descending cold air and warm air at the surface) of an approaching thunderstorm is called a derecho. Gustavus Hinrichs.

Longer-period windstorms have two main causes:

(1) large differences in atmospheric pressure across a region

(2) strong jet-stream winds overhead. Horizontal pressure differences may accelerate the surface winds substantially as air travels from a region of higher atmospheric pressure to one of lower. In addition, the vertical turbulent mixing of stronger jet-stream winds aloft can produce strong gusty winds at ground level.

Wind storms get get different names in different regions according to the atmospheric conditions.

PART 2 :

Breaking Waves

Breaking waves are usually found on the East Coast of the continent come out of the North Pacific Ocean, the East Coast of the Central American countries and southern United States get their waves from the Gulf of Mexico and finally the Caribbean and East Coast of North America get waves out of the North Atlantic.

Hawaii, known as the home of surfing is located on this Continent and deserves a special mention just for the amount of world class breaking waves. and also in winter's on the North Shore.

The countries in Central America are becoming more and more popular with surfers as the breaking waves here are a kind of adventurous thing.

California is another name on the list. Its numerous breaks and great lifestyle are attractive to the visiting surfer. There are so many quality breaks along this stretch of the North American Pacific coastline and even one of the worlds most notorious big waves spots if found here - Mavericks.

HYDRAULIC JUMPS

A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface. usually found on the Upper Spokane Falls, washington and Saint Anthony Falls on the Mississippi River, Cleveland Dam spillway at the head of the North Vancouver, British Columbia, Canada. are some examples of it.

Lee Waves

Mountain waves and rotors occurred over Owens Valley (eastern california, US) in a pre-cold-frontal environment. In this study, the evolution and structure of the observed and numerically simulated mountain waves and rotors during the event on 25 March 2004 , in which the horizontal circulation associated with the rotor was observed as an opposing, easterly flow by the meso-network of surface stations in Owens Valley.

Chinook

These are basically Fohn winds in the interior West of North America, where the Canadian Prairies and Great Plains meet various mountain ranges, although the original usage is in reference to wet, warm coastal winds in the Pacific Northwest.

Chinook is claimed by popular folk-etymology to mean 'ice-eater', however it is the name of the people in the region where the usage was first derived. The reference to a wind or weather system, simply 'a Chinook', originally meant a warming wind from the ocean into the interior regions of the Pacific Northwest of the USA (the Chinook people lived near the ocean, along the lower Columbia River).

A strong fohn wind can make snow one foot (30 cm) deep almost vanish in one day. The snow partly melts and partly sublimates in the dry wind. Chinook winds have been observed to raise winter temperature, often from below −20 °C (−4 °F) to as high as 10–20 °C (50–68 °F) for a few hours or days, then temperatures plummet to their base levels. The greatest recorded temperature change in 24 hours was caused by Chinook winds on 15 January 1972, in Loma, Montana; the temperature rose from −48 to 9 °C (−54 to 49 °F).