Microwave Sounding Ben Kravitz October 29 2009 What is Microwave Sounding Passive sensor in the microwave to measure temperature and water vapor Technique was pioneered by Ed Westwater c 1978 Microwave Gamma X Rays Ultraviolet Visible Infrared Microwave and beyond Extremely High Frequency Super High Frequency Ultra High Frequency Very High Frequency High Frequency Medium Frequency Low Frequency Very Low Frequency Voice Frequency Super Low Frequency Extremely Low Frequency Main Purpose Measure total integrated water content in an atmospheric column both vapor and liquid Measure a coarse vertical profile of atmospheric water We will begin with ground based microwave radiometers Vocabulary Total column water vapor precipitable water Total column liquid water in a cloud liquid water path LWP Why 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 delays Why do we care about column liquid water Why do we care about column liquid water In a cloud the amount of liquid water is very important in determining optical depth Optical Depth note the wavelength dependence m I I 0e I 0 incident solar radiation at the top of the atmosphere I solar radiation that reaches the surface optical depth m 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 atmosphere Optical Depth Understanding optical depth is ESSENTIAL to understanding the radiation budget Until Westwater invented the technique of microwave sounding we were using radiosondes to determine atmospheric humidity Problems with Radiosondes Problems with Radiosondes Radiosondes tend to drift move with the wind Radiosondes are not released often enough Radiosonde measurements of humidity are sometimes suspect Passive Microwave Radiometers VERY accurate in determining column integrated water quantities Get complete measurements every 20 minutes Operate in nearly all conditions regardless of weather Passive Microwave Radiometers Can also be used to give vertical profiles of liquid water and water vapor albeit at very poor resolution Resonant Frequency of water Frequency at which vibration can be induced in water molecules 22 235 GHz Procedure 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 Merged Sounding 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 data Humidity Profiling Large peak at 22 235 GHz Radiation at 22 235 GHz Water molecule A Water molecule B Microwave Radiometer tuned to 22 235 GHz The microwave radiometer will not see much of molecule A This is not a good way to design a profiler Radiation at frequency higher than 22 235 GHz Water molecule A Water molecule B Microwave Radiometer tuned to different frequency The microwave radiometer at this frequency DOES see molecule A We do this at multiple frequencies and we can get a vertical profile Weighting Functions Each channel frequency has a function that tells it how much to weight each elevation Deriving these is as much an art as a science and requires a lot of experience doing this sort of thing Weighting Functions What frequencies are used 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 GHz Line Width All 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 Width 20 6 GHz frequency is relatively insensitive to line width If 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 GHz The 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 Resolution The 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 range Most microwave radiometers have 12 channels total There are 7 more channels in the 48 71 GHz range Oxygen Notice the reduction in absorption with height Absorption of microwave energy by oxygen is very dependent upon temperature We can use these 7 channels to profile temperature For 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 Calibration Microwave radiometers have a strong tendency to drift They need to be calibrated quickly and often Atmospheric Mass To a good approximation m sec zenith angle As zenith angle increases moves away from the vertical so does atmospheric mass Tipping Angle Distance to top of atmosphere increases with zenith angle so the amount of atmosphere between the radiometer and the sun grows with angle m increases 0 m 1 90 m Langley Plot log plot 0 1 best linear fit 0 01 0 001 1 2 3 m 4 5 Atmospheric Mass We define that for m 0 should equal 0 We can t actually measure anything for m 1 To get for m 0 we use the Langley plot and extrapolate backwards to m 0 Langley Plot 0 1 best linear fit 0 01 0 001 at m 0 1 2 3 m 4 5 Calibration Usually 0 at m 0 This tells us how much we need to correct our measurements This is how we calibrate the radiometer Space based microwave radiometers NASA has had microwave sounders in orbit since 1978 MSU which flew on TIROS N Microwave sounders have given us a very long satellitebased temperature record Resolution and number of channels has dramatically improved since then Using microwave sounders to measure temperature and humidity is an idea that is