ERTH 365 - Lecture 8 Notes

Yoon Chang Kimberly Le

Topics Discussed - Further processes for warming and cooling the atmosphere - Cold and warm core low pressure systems; - Hurricanes in depth - Hurricane Harvey review for Homework 2 (suppl. Notes)

Warming & Cooling the atmosphere - other processes ● Another way of WARMING air ○ Physicists tell us that when water vapor condenses, a certain amount of thermal energy is “given” to the atmosphere. In short, condensation is a warming process. ○ The amount of heating is proportional to the amount of water that condenses. At sea-level temperatures and pressures, the warming is about 590 calories for each gram of water that condenses. The 590 calories is known as “latent heat.” ○ Since air with 100% relative humidity at lower dewpoint temperatures holds less water vapor than air with 100% relative humidity at higher dewpoint temperatures, much more heat is liberated in cases for which cloud development is occurring in high dewpoint environments. ● Another way of COOLING air ○ Conductional Cooling ■ Air flows over a cold surface and is cooled by conduction. (this process is termed “advective cooling”) If the cooling is sufficient to take the air parcel’s temperature to the dew point, the resulting condensation will produce a layered cloud on the ground. ■ Air “sits” on a surface that gets cold overnight. (radiation cooling) if the cooling is sufficient to take the air parcel’s temperature to the

dew point, the resulting condensation will produce a layered cloud on the ground. ○ Expansion cooling ■ Air expands, molecules get “further apart”, do not strike each other as often, do not “vibrate” as much. Air expands when pressure around moving air parcels decreases markedly. Best example---air involved in vertical motion.

Warm and Cold Core Low Pressure ● Review: Two types of vertical motion: ○ BUOYANT (THERMAL) (WARM CORE) ■ Air rises because it’s less dense than its surroundings ○ FORCED (DYNAMIC) (COLD CORE) ■ Up over a hill ■ Warmer air over colder air ■ Horizontal convergence

COLD core low pressure ● Rising air in all levels of atmosphere ● Cover a relatively small synoptic scale area compared to high pressure systems. Their path can heavily influence precipitation totals ● Developed cold core lows tilt to the northwest with height ● Coldest temperatures are at center of low ● Air is cooled by adiabatic expansion and evaporational cooling of rain and or snow ● Most mid latitude cyclones are cold-core lows ● They cause widespread precipitation ● They are deep cored. Developed cold core lows will show at each mandatory level. They will have closed height contours in the low levels and shows as synoptic scale trough in the upper levels

WARM core low pressure Type 1: Thermal Low ● Thermal lows are shallow and most intense at surface ● Develop due to strong surface heating. Hot air due to intense thermal build up results in positive buoyancy at the surface ● Mid and upper levels are stable ● Most common in SW US during summer ● Develop over land (especially dry land with little vegetation)

Type 2: Tropical Low ● Deeper than thermal low, although they do weaken with height in upper levels ● Subsidence (sinking air) in center causes compressional warming ● Air rises rapidly around edges of eye (eye wall) ● Develop over water ● Warm core low is strongest when vertically stacked. Significant wind shear will weaken warm core low. This is the opposite situation of a cold core low; strong cold core lows tilt with height

WHY sinking air in the center: ● Centrifugal force operates outwards and cancels the effect of friction once speeds reach 74mph, so air can NOT CROSS the isobars right near the center ● A partial vacuum is caused, so air must be replaced and sinks from above, causing clear, calm conditions

WHY sinking air occurs in the center (eye) of a warm core low (hurricane)

Tropical Cycles aka Hurricanes

What is a Tropical Cyclone? ● Cyclonic system over tropical waters ● ~5 degrees to 25 degrees N/S latitude ○ But not at Equator because Coriolis Force (CF) = 0 ○ CCW in the N hemisphere (counterclockwise) ● Classification: ○ Tropical Disturbance ■ No circulation ○ Tropical Depression ■ Max sustained winds less than or equal to 38 mph ○ Tropical Storm ■ 39 to 73 mph ○ Tropical Cyclone (aka Hurricane) - Greater than or equal to 74 mph

Tropical vs. Midlatitude Storms

● Tropical are warm core systems while midlatitude storms are cold core systems

Tropical Cyclone ingredients - Warm Tropical Waters (5 degrees to 25 degrees N/S latitude) - Greater than or equal to 82 degrees Fahrenheit or 28 degrees Celsius) - Gigantic Heat Engine - Release of Latent Heat - Weak Upper Winds - Little wind shear - More wind shear during typical El Nino

Tropical Cyclone Formation

● Begin as tropical disturbances (easterly wave) ● 90% die out before becoming depression ● Small clusters of thunderstorms ● Cape Verde is a favored region

Hurricane Structure

● The most important part of the hurricane is the eye; strongest winds occur in the eye; the arrows raising up is the rising air; this area is called the eye wall

Hurricane Eye

● Average 15 mi diameter ● Range 3.5 to 60 mi ● Eye Wall ○ Outer perimeter of eye ○ Most intense storm activity ○ Strongest winds, thickest cloud cover, most intense precipitation ● Inside Eye ○ Blue skies, calm winds, no precipitation, fair weather cumulus ○ Shrinking eye = intensifying hurricane

○ Top Diagram = shows an intensifying tropical cyclone. As latent heat is released inside the clouds, the warming of the air aloft creates an area of high pressure, which induces air to move outward, away from the high. The warming of the air lowers the air density, which in turn lowers the surface air pressure. As surface

winds rush in toward the surface low, they extract sensible heat, latent heat, and moisture from the warm ocean. As the warm, moist air flows in toward the center of the storm, it’s swept upward into the clouds of the eyewall. As warming continues, surface pressure lowers even more, the storm intensifies, and the winds blow even faster. This situation increases the transfer of heat and moisture from the ocean surface. ○ Middle Diagram = illustrates how the air pressure drops rapidly as you approach the eye of the storm. ○ Lower Diagram = shows how surface winds normally reach maximum strength in the region of the eyewall.

Hurricane Naming

● Saint’s names in Latin America ● WWII Military names ● 1950-1952 Phonetic alphabet ● 1953-1978 Women’s names ● 1978-present ○ Mixed names by WMO ○ Male/Female ○ Anglo, Latin, French ● Retired names for deadly and infamous storms ○ Total: 77 storm names have been retired since 1954

Saffir-Simpson Scale

● Category 1*: 74-95 mph > 980 mb ○ Flooded coastal roads, minor pier damage ○ Surge 4-5 feet ● Category 2: 96-110 mph 965-970 mb ○ Flooding 24 hrs before eye, piers damaged ○ Surfe 6-8 feet ● Category 3 (Major)*: 111-130 mph 945-964 mb ○ Small bldgs destroyed, flooding 3-5 hrs prior ○ Surge 9-12 feet ● Category 4 (Major): 131-155 mph 920-944 mb ○ Major flooding, major damage ○ Surge 13-18 feet ● Category 5 (Major) >155 mph <920 mb ○ Flooding, extreme damage ○ Surge >18 feet

Tropical Cyclone Hazards

● Storm Surge: ocean can rise up to 25 feet ● High Winds: gusts up to 200 mph ● Flooding from rainfall: up to 40 inches of rain ● Tornadoes: spawned by the hurricane up to 1000 miles inland

Hurricanes are Right Handed

● Storm surge and winds are higher on right side

Hurricane Fatalities

U.S. Hurricane Deaths and Damage

Hurricane Watches & Warnings

● Watch: landfall predicted in more than 24 hours ● Warning: landfall predicted in less than 24 hours ● Unpredictability makes decision difficult: ○ Evacuation ○ People killed ○ Costs ●