NATS 101 Lecture 25 Weather Forecasting I Review

NATS 101 Lecture 25 Weather Forecasting I

Review: ET Cyclones Ingredients for Intensification deepening filling Ahrens, Meteorology Today, 5 th Ed. • Strong Temperature Contrast • Jet Stream Overhead • S/W Trough to West • UL Divergence over Surface Low • If UL Divergence exceeds LL Inflow, Cyclone Deepens • Similar Life Cycles

Reasons to Forecast Weather & Climate • Should I bring my umbrella to work today? • Should Miami be evacuated for a hurricane? • How much heating oil should a refinery process for the upcoming winter? • Will the average temperature change if CO 2 levels double during the next 100 years? • How much to charge for flood insurance? • How much water will be available for agriculture & population in 30 years These questions require weather-climate forecasts for today, a few days, months, years, decades

Forecasting Questions • How are weather forecasts made? • How accurate are current weather forecasts? • How accurate can weather forecasts be? We will emphasize mid-latitude forecasts out to 15 days where most progress has been made.

Types of Forecasts Persistence - forecast the future atmospheric state to be the same as current state -Raining today, so forecast rain tomorrow -Useful for few hours to couple days

Types of Forecasts Trend - add past change to current condition to obtain forecast for the future state 10 am 11 am 12 pm 59 F 63 F 67 F Past Now Future -Useful for few hours to couple days

Types of Forecasts Analog - find past state that is most similar to current state, then forecast same evolution -Difficulty is that no two states exactly alike -Useful forecasts up to one or two days

Types of Forecasts Climatology - forecast future state to be same as climatology or average of past weather for date -Forecast July 4 th MAX for Tucson to be 100 F -Most accurate for long forecast projections, forecasts longer that 30 days

Types of Forecasts Numerical Weather Prediction (NWP) - use mathematical models of physics principles to forecast future state from current conditions. Process involves three major phases 1. Analysis Phase (estimate present conditions) 2. Prediction Phase (computer modeling) 3. Post-Processing Phase (use of products) To justify NWP cost, it must beat forecasts of persistence, trend, analog and climatology

Analysis Phase • Purpose: Estimate the current weather conditions to use to initialize the weather forecast • Implementation: Because observations are always incomplete, the Analysis is accomplished by combining observations and the most recent forecast

Analysis Phase • Current weather conditions are observed around the global (surface data, radar, weather balloons, satellites, aircraft). • Millions of observations are transmitted via the Global Telecommunication System (GTS) to the various weather centers. • U. S. center is in D. C. and is named National Centers for Environmental Prediction (NCEP)

Analysis Phase • The operational weather centers sort, archive, and quality control the observations. • Computers then analyze the data and draw maps to help us interpret weather patterns. Procedure is called Objective Analysis. Final chart is referred to as an Analysis. • Computer models at weather centers make global or national weather forecast maps

Surface Data Sparse data over oceans and Southern Hemisphere Courtesy ECMWF

Surface Buoy Reports Some buoy data over Southern Hemisphere Courtesy ECMWF

Radiosonde Coverage Little data over oceans and Southern Hemisphere Courtesy ECMWF

Aircraft Reports Little data over oceans and Southern Hemisphere Courtesy ECMWF

Weather Satellites Geostationary Polar Orbit Ahrens, Figs. 9. 5 & 9. 6 Satellite observations fill data void regions Geostationary Satellites High temporal sampling Low spatial resolution Polar Orbiting Satellites Low temporal sampling High spatial resolution

Obs from Geostationary Satellites Courtesy ECMWF

Temperature from Polar Satellites Courtesy ECMWF

Atmospheric Models • Weather models are based on mathematical equations that retain the most important aspects of atmospheric behavior - Newton's 2 nd Law (density, press, wind) - Conservation of mass (density, wind) - Conservation of energy (temp, wind) - Equation of state (density, press, temp) • Governing equations relate time changes of fields to spatial distributions of the fields e. g. warm to south + southerly winds warming

Prediction Phase • Analysis of the current atmospheric state (wind, temp, press, moisture) are used to start the model equations running forward in time • Equations are solved for a short time period (~5 minutes) over a large number (107 to 108) of discrete locations called grid points • Grid spacing is 2 km to 50 km horizontally and 100 m to 500 m vertically

100 -500 m Model Grid Boxes 10 -20 km

“A Lot Happens Inside a Grid Box” (Tom Hamill, CDC/NOAA) Rocky Mountains Approximate Size of One Grid Box for NCEP Global Ensemble Model Note Variability in Elevation, Ground Cover, Land Use 50 km Denver Source: www. aaccessmaps. co

13 km Model Terrain Big mountain ranges, like the Sierra Nevada Range, are resolved. But isolated peaks, like the Catalinas, are not evident! 100 m contour

Post-Processing Phase • Computer draws maps of projected state to help humans interpret weather forecast • Observations, analyses and forecasts are disseminated to private and public agencies, such as the local NWS Forecast Office and UA • Forecasters use the computer maps, along with knowledge of local weather phenomena and model performance to issue regional forecasts • News media broadcast these forecasts to public

Suite of Official NWS Forecasts CPC Predictions Page

Summary: Key Concepts Forecasts are needed by many users There are several types of forecasts Numerical Weather Prediction (NWP) Use computer models to forecast weather -Analysis Phase -Prediction Phase -Post-Processing Phase Humans modify computer forecasts

Summary: Key Concepts National Center for Environment Prediction (NCEP) issues operational forecasts for El Nino tropical SST anomalies Seasonal outlooks 10 to 15 day weather forecasts 2 to 3 day fine scale forecasts

Assignment for Next Lecture • Topic - Weather Forecasting Part II • Reading - Ahrens pg 249 -254 • Problems - 9. 11, 9. 15, 9. 18