METR 415/715

METR 415/715

Monday March 11 2013

Outgoing Longwave Radiation at top of atmosphere

• Broadband IR radiation• The PSD Map Room• The utility and limitations of OLR information– Daily averagesand anomalies– Weekly averages and anomalies– Monthly averages and anomalies– Seasonal (3-month) averages and anomalies

OLR Maps as indication of convective cloudiness

• Thick cumulonimbus cloud clusters reach top of tropical troposphere

• You can assume they act as black body radiators• There is relatively little absorption of OLR

coming out of these cloud tops to space• You can use Stefan-Boltzmann’s Law to get a

crude estimate of the cloud top temperatures and from that estimate their heights

Estimating Surface Temperatures using OLR

• Even in cloud –free environments, and even assuming that the emissivity of the ground is close to 1, you will greatly underestimate the ground temperatures using OLR data and S-B Law

• This is because the clear atmosphere has relatively low transmissivity in the IR part of the spectrum, primarily due to the absorption by water vapor

The use of anomaly maps

• Now that there are more than 30 years worth of continuous worldwide satellite coverage, we can construct climatologies of OLR for every day of the year

• Looking at any particular day’s average OLR vs. the average OLR for that particular day over the period of satellite record

• The day’s OLR – the average OLR for that day gives us the anomaly OLR for that day

Average OLR for March 9 2013

7-Day Average OLR

Monthly Average OLR

90-Day Average OLR Anomaly