| 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 |
| 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 CO2 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 | |
| -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 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 | ||
| Analysis Phase (estimate present conditions) | ||
| Prediction Phase (computer modeling) | ||
| 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 |
| Surface Buoy Reports |
| Radiosonde Coverage |
| Aircraft Reports |
| Weather Satellites |
| 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 |
| Temperature from Polar Satellites |
| 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 Þ 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 |
| Model Grid Boxes |
| ÒA Lot Happens Inside a
Grid BoxÓ (Tom Hamill, CDC/NOAA) |
| Approximate Size of One Grid Box for NCEP Global Ensemble Model | |
| Note Variability in Elevation, Ground Cover, Land Use |
| 13 km Model Terrain |
| 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 |
| 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 |