Ben KravitzOctober 29, 2009Microwave SoundingWhat is Microwave Sounding?Passive sensor in the microwave to measure temperature and water vaporTechnique was pioneered by Ed Westwater (c. 1978)MicrowaveGammaX-RaysUltravioletVisibleInfraredMicrowave (and beyond):“Extremely” High Frequency“Super” High Frequency“Ultra” High Frequency“Very” High FrequencyHigh FrequencyMedium FrequencyLow Frequency“Very” Low FrequencyVoice Frequency“Super” Low Frequency“Extremely” Low Frequency{{•Measure total integrated water content in an atmospheric column (both vapor and liquid)•Measure a (coarse) vertical profile of atmospheric waterMain PurposeWe will begin with ground-based microwave radiometers•Total column water vapor = precipitable water•Total column liquid water (in a cloud) = liquid water path (LWP)VocabularyWhy do we care about column water vapor?Why do we care about column water vapor?•Water vapor is the most abundant greenhouse gas in the atmosphere•Essential for weather forecasting models•Atmospheric propagation delaysWhy do we care about column liquid water?•In a cloud, the amount of liquid water is very important in determining optical depthWhy do we care about column liquid water?Optical DepthIλ=Iλ,0e-τλmIλ,0 = incident solar radiation (at the top of the atmosphere)Iλ = solar radiation that reaches the surfaceτλ = optical depthm = atmospheric mass (how much of the atmosphere the radiation is passing through)Optical depth describes the attenuation of solar radiation as it passes through the atmospherenote the wavelength dependence!Optical DepthUnderstanding optical depth is ESSENTIAL to understanding the radiation budgetUntil Westwater invented the technique of microwave sounding, we were using radiosondes to determine atmospheric humidityProblems with Radiosondes•Radiosondes tend to drift (move with the wind)•Radiosondes are not released often enough•Radiosonde measurements of humidity are sometimes suspectProblems with Radiosondes•VERY accurate in determining column integrated water quantities•Get complete measurements every 20 minutes•Operate in nearly all conditions, regardless of weatherPassive Microwave Radiometers•Can also be used to give vertical profiles of liquid water and water vapor, albeit at very poor resolutionPassive Microwave RadiometersResonant Frequency(of water)•Frequency at which vibration can be induced in water molecules•22.235 GHzProcedure•Point the radiometer in a given direction and calculate atmospheric mass•Measure the amount of radiation at the frequency to which you tune the radiometer•Plug it into a simple radiative transfer equation which gives you total optical depth in the column•Pioneered by Dr. Miller (and colleagues)•Take the column profiles and feed them into the weather forecasting model run by the European Center for Medium Range Weather Forecasting (ECMWF)•Use this to correct radiosonde dataMerged SoundingHumidity ProfilingLarge peak at 22.235 GHzMicrowave Radiometer tuned to 22.235 GHzWater molecule AWater molecule BRadiation at 22.235 GHzThe microwave radiometer will not see much of molecule AThis is not a good way to design a profilerMicrowave Radiometer tuned to different frequencyWater molecule AWater molecule BRadiation at frequency higher than 22.235 GHzThe microwave radiometer at this frequency DOES see molecule AWe do this at multiple frequencies, and we can get a vertical profileWeighting FunctionsEach channel (frequency) has a function that tells it how much to weight each elevationDeriving these is as much an art as a science and requires a lot of experience doing this sort of thingWeightingFunctions•Most microwave radiometers have 5 channels devoted to water•Each group has their own preference as to which frequencies they use•Most important ones: 20.6 GHz, 22.235 GHz, and 31.65 GHzWhat frequencies are used?Line WidthAll instruments are imperfect and have an aperture of some kind. If you want to measure, say, 22.235 GHz, you cannot measure exactly that frequency and no others. You might actually measure something like 22.235±0.5 GHz. This 0.5 GHz is called the line width. The line width affects the shape of the absorption curve.Line Width20.6 GHz frequency is relatively insensitive to line widthIf we measure at 20.6 GHz, we can be sure that the line width is not introducing a source of error into our measurements.There is a similar feature at 24.4 GHz.relative minimum at 31.65 GHzThe 31.65 GHz frequency shows a drop-off of absorption. This reduction of absorption is greater for water vapor than for liquid water. By measuring at this frequency, we can differentiate between water vapor and liquid water.ResolutionThe resolution of microwave humidity profilers is about 1 km. This is not at all useful for a large majority of clouds.We’ve discussed the 5 channels in the 20-32 GHz rangeMost microwave radiometers have 12 channels totalThere are 7 more channels in the 48-71 GHz rangeOxygenNotice the reduction in absorption with heightAbsorption of microwave energy by oxygen is very dependent upon temperatureWe can use these 7 channels to profile temperatureFor very dry conditions, the 31.65 GHz channel is not very good at distinguishing water vapor from liquid water. For this reason, some radiometers also take measurements at 183 GHz. This frequency is very sensitive to liquid water.•Microwave radiometers have a strong tendency to drift•They need to be calibrated quickly and oftenCalibrationAtmospheric MassTo a good approximation, m≈sec(θ) θ = zenith angleAs zenith angle increases (moves away from the vertical), so does atmospheric massTipping AngleDistance to top of atmosphere increases with zenith angle, so the amount of atmosphere between the radiometer and the sun grows with angleθ=0° ⇒ m=1θ=90° ⇒ m=∞m increasesLangley Plotmτ1 2 3 4 50.10.010.001log plotbest linear fit•We define that for m=0, τ should equal 0•We can’t actually measure anything for m<1•To get τ for m=0, we use the Langley plot and extrapolate backwards to m=0Atmospheric MassLangley Plotmτ1 2 3 4 50.10.010.001best linear fitτ at m=0•Usually, τ≠0 at m=0•This tells us how much we need to correct our measurements•This is how we calibrate the radiometerCalibrationSpace-based microwave radiometersNASA has had microwave sounders in orbit since 1978 (MSU, which flew on