Air Masses, Fronts, and Weather Forecasting PDF Notes

Chap 8: Air Masses, Fronts, and Middle-Latitude Cyclones 2 and Chap 9: Weather Forecasting 1 Review Review 3  Review An air mass is an extremely large body of air whose properties of temperature and humidity ar...

Chap 8: Air Masses, Fronts, and Middle-Latitude Cyclones 2 and Chap 9: Weather Forecasting 1 Review Review 3  Review An air mass is an extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given altitude. Regions where air masses originate are known as source regions.  Review Classification of Air masses  Review Air masses that influence the continent of North America  Review Front: boundaries between different air masses A weather map showing surface pressure systems, air masses, fronts, and isobars (in millibars) as solid gray lines. Large arrows in color show airflow. (Green-shaded area represents rain; pink-shaded area represents freezing rain and sleet; white-shaded area represents snow.)  Review 1. Cold Front - cold air moving into warm air - fast (30 mph) - steep (the slop--ratio of vertical rise to horizontal distance--is 1:50) - short-lived heavy precipitation - As a cold front moves through, there are sharp temperature drops  Review 2. Warm Front - warm air moving into colder air - slower (12 mph) - shallower (the slop--ratio of vertical rise to horizontal distance--is 1:300) - long-lived light precipitation (can be sleet or freezing rain) - clouds are ahead of warm fronts  Review 3. Stationary Front - a boundary between 2 air masses which doesn’t move - notable temperature change or shift in wind direction  Review 4. Occluded Front - an active cold front which overtakes a warmer front - 3 air masses involved: cool, cold, and warm air masses - is often associated with a variety of precipitation intensities and duration - A developing low pressure system typically has a warm front preceding it and a fast moving cold front. The boundary that separates the cool air already in place north of the Cold front may catches up to and overtakes the warm front. warm front and the cold air behind the cold front is Occluded Front  Review Front: boundaries between different air masses  Overview Mid-Latitude Cyclone: polar front theory Upper-air support for a developing Mid- Latitude Cyclone Weather forecasting Cyclone – an area of low pressure around which the winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere  Fronts are part of mid-latitude cyclones A weather map showing surface pressure systems, air masses, fronts, and isobars (in millibars) as solid gray lines. Large arrows in color show airflow. (Green-shaded area represents rain; pink-shaded area represents freezing rain and sleet; white-shaded area represents snow.)  Questions Why are fronts connected to surface low-pressure systems but not to high-pressure systems? tophat.com join 845442  Questions Why are fronts connected to low-pressure systems but not to high-pressure systems? Fronts are regions with winds flowing together (i.e., convergence), which must be low pressure areas.  Polar front theory: How a mid-latitude cyclone progresses through the stages of birth, growth, and decay? Developed by Norwegian meteorologists during the first World War. It’s developed from 3-Cell theory At ~60 deg. North Semi-continuous global boundary Separate cold polar air from warm subtropical air Frontal wave forms with right conditions in a wavelike kink https://www.youtube.com/watch?v=i4qgaKh2Qz0&list=PLSQCLmKxpQOA6xrqjWl9t-Ci6wbPg0aXq&index=37 Polar front theory: How a mid-latitude cyclone progresses through the stages of birth, growth, and decay? Six Steps: (1) It starts as a Polar Front that supports Stationary Front.  Polar front theory: How a mid-latitude cyclone progresses through the stages of birth, growth, and decay? Six Steps: (2) Under the right conditions, a wavelike kink forms on the front. The wave is known as frontal wave. - Low pressure is strengthened - Rain starts to form  Polar front theory: How a mid-latitude cyclone progresses through the stages of birth, growth, and decay? Six Steps: (3) 12 – 24 hours later, the system gradually developed into an open wave. - More tightly packed isobars create a stronger cyclonic flow - Warm sector forms  Polar front theory: How a mid-latitude cyclone progresses through the stages of birth, growth, and decay? Six Steps: (4) The faster-moving cold front constantly inches closer to the warm front, squeezing the warm sector into a smaller area, and the wave quickly develops into a mature cyclone. The energy is provided by the rising warm, moist air. - Most intense weather because warm air is pushed up Polar front theory: How a mid-latitude cyclone progresses through the stages of birth, growth, and decay? Six Steps: (5): As the occlusion is advanced, the system gradually dissipates. - “Comma clouds” are often seen.  Polar front theory: How a mid-latitude cyclone progresses through the stages of birth, growth, and decay? Six Steps: (6): Without the supply of energy provided by the rising warm, moist air, the system dies out and gradually disappears.  Summary of Cyclone development stages Stationary front Frontal wave Open wave Life time of a typical cyclone is several days to a week Moves 1000’s of km during lifecycle occlusion dissipating Mature stage A low pressure system may develop into a cyclone if the low pressure system intensifies (pressure drops in the center of the low). What cause surface low pressure to intensify? A low pressure system may develop into a cyclone if the low pressure system intensifies (pressure drops in the center of the low). What cause surface low pressure to intensify? Existence of upper-air support !  Air going out of the column Air going into the surface low When upper air divergence is stronger than surface convergence (more air is taken out from the top than is brought in at the bottom), surface pressure drop, and the low intensifies, or deepens.  Air going out of the column For the surface low to develop into a major storm system, upper- level air must be diverging. Air going into the surface low When upper air divergence is stronger than surface convergence (more air is taken out from the top than is brought in at the bottom), surface pressure drop, and the low intensifies, or deepens.  Searching for the converging and diverging upper air Converging air aloft is most likely found to the left (west) of the upper trough, whereas diverging air is most likely found to the right (east) of the trough. A ridge is an elongated area of relatively high pressure extending from the center of a high-pressure region. A trough is an elongated area of relatively low pressure extending from the center of a region of low pressure.  500 mb height When upper-level Low divergence is stronger than lower-level Cyclogenesis region convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and Convergence Divergence High ahead of the low intensifies, or behind trough trough “deepens.” https://www.youtube.com/watch?v=deEAOl3ZVZQ  For the surface storm to intensify, the upper low pressure must be located to the left (or west) of the surface low Convergence, divergence, and vertical motions associated with surface pressure systems.  Summary of clouds, weather, vertical motions, and upper-air support associated with a developing mid-latitude cyclone. For a surface cyclonic storm to intensify, there must be an upper-level counterpart—a trough of low pressure—that lies to the west of the surface low. We say the surface storm has the necessary upper-air support. Dark green area represents precipitation.  Weather Forecasting Advanced Weather Interactive Processing System (AWIPS) has data communications, storage, processing, and display capabilities to better help the individual forecaster extract and assimilate information from available data. AWIPS gathers data from Doppler radar system, satellite imagery, and the automated surface observing system that are operational at selected airports and other sites throughout the United States. https://www.youtube.com/watch?v=V0Xx0E8cs7U  Weather Forecasting Methods There are several different methods that can be used to create a forecast. The method a forecaster chooses depends upon: (1) the experience of the forecaster, (2) the amount of information available to the forecaster, (3) the level of difficulty that the forecast situation presents, (4) the degree of accuracy or confidence needed in the forecast.  Weather Forecasting Methods Several methods are used to forecast weather Persistence Forecast: a prediction that future weather will be the same as present weather. Trend Forecast: Surface weather systems tend to move in the same direction and at approximately the same speed as they have been moving. Analogue Forecast: The future will be like weather that historically occurred when similar conditions were present. Statistical Forecast: Made routinely of weather elements based on the past performance of computer models. 37  Weather Forecasting Persistence Method -The simplest way of producing a forecast. -It assumes that the conditions at the time of the forecast will not change. - For example, if it is sunny and 87 degrees today, the persistence method predicts that it will be sunny and 87 degrees tomorrow. -The persistence method works well when weather patterns change very little and features on the weather maps move very slowly. However, if weather conditions change significantly from day to day, the persistence method usually breaks down and is not the best forecasting method to use.  Weather Forecasting Trend Method -The trends method involves determining the speed and direction of movement for fronts, high and low pressure centers, and areas of clouds and precipitation. Using this information, the forecaster can predict where he or she expects those features to be at some future time. - Below is an example of using the trends method to forecast the movement of a cold front. Initially, the cold front moved 800 miles during the first 24 hours, from the central Plains to the Great Lakes. Using the trends method, you would predict this weather system to move another 800 miles in the next 24 hours, reaching the East Coast of the United States. The trends method works well when systems continue to move at the same speed in the same direction for a long period of time. If they slow down, speed up, change intensity, or change direction, the trends forecast will probably not work as well.  Weather Forecasting Analog Method -The Analog Method involves examining today's forecast scenario and remembering a day in the past when the weather scenario looked very similar (an analog). The forecaster would predict that the weather in this forecast will behave the same as it did in the past. -The analog method is difficult to use because it is virtually impossible to find a perfect analog. Various weather features rarely align themselves in the same locations they were in the previous time. Even small differences between the current time and the analog can lead to very different results.  Weather Forecasting Numerical Weather Method (best) -Numerical Weather Prediction (NWP) uses the power of computers to make a forecast. - Complex computer programs, also known as forecast models, run on supercomputers and provide predictions on many atmospheric variables such as temperature, pressure, wind, and rainfall. - The NWP method is flawed in that the equations used by the models to simulate the atmosphere are not precise. This leads to some error in the predictions. In addition, the are many gaps in the initial data since we do not receive many weather observations from areas in the mountains or over the ocean. If the initial state is not completely known, the computer's prediction of how that initial state will evolve will not be entirely accurate.