NATS 101

Lecture 25Weather Forecasting I

Review: ET CyclonesIngredients 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 CyclesAhrens, Meteorology Today, 5th Ed.

filling

deepening

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 yearsThese 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 am 11 am 12 pm

59 F 63 F 67 F

Past Now Future

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 (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

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 SatellitesHigh temporal samplingLow spatial resolutionPolar Orbiting SatellitesLow temporal samplingHigh 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 fieldse.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

Model Grid Boxes

10-20 km

100-

500

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: www.aaccessmaps.co

Rocky Mountains

Denver50 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

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 usersThere are several types of forecastsNumerical 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