NATS 101 Lecture 25 Weather Forecasting I

Review: ET Cyclones Ingredients for Intensification 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 Ahrens, Meteorology Today, 5th Ed. filling deepening

Reasons to Forecast Weather 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? 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 -Useful for few hours to couple days 10 am11 am12 pm 59  F63  F67  F PastNowFuture

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 for 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 4th 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 (most expensive piece) 2.Prediction Phase (modeling, computing) 3.Post-Processing Phase (use of products) To justify NWP cost, it must beat forecasts of persistence, trend, analog and climatology

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

Courtesy ECMWF Sparse data over oceans and Southern Hemisphere Surface Data

Courtesy ECMWF Some buoy data over Southern Hemisphere Surface Buoy Reports

Courtesy ECMWF Little data over oceans and Southern Hemisphere Radiosonde Coverage

Aircraft Reports Courtesy ECMWF Little data over oceans and Southern Hemisphere

Weather Satellites Geostationary Polar Orbit Satellite observations fill data void regions Geostationary Satellites High temporal sampling Low spatial resolution Polar Orbiting Satellites Low temporal sampling High spatial resolution Ahrens, Figs. 9.5 & 9.6

Courtesy ECMWF Obs from Geostationary Satellites

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 2nd 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 fed into the model equations Equations are solved for a short time period (~5 minutes) over a large number (10 7 to 10 8 ) of discrete locations called grid points Grid spacing is 5 km to 50 km horizontally and 100 m to 500 m vertically

Model Grid Boxes km m

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

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

40 km Model Grid and Terrain

NWP Forecasts Next lecture, we will show some analyses and forecasts from the current suite of NCEP forecast products

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 Problems , 9.15, 9.18