(MWR) Handbook
Contents
1. ARM TR 016 Microwave Radiometer MWR Handbook August 2006 V R Morris Work supported by the U S Department of Energy Office of Science Office of Biological and Environmental Research NYDN E GO H ra VD Gi JA Ch LP LO H ra S ec ba August 2006 ARM TR 016 Contents General Overview ere ee er 1 ENT EE 1 Deployment Locations and Historie 1 Ne ar Real Time Data Plots ege ee ENEE REESEN 2 Data Description and Bxamples inng nnensnsn ni E N N E T R 2 Data Quality EE 11 Instrument Detall S es tee ee ee ENER 12 Figures Radiometer receiver block diagram 00 0 eenia oaoa EE E A EEEE EE 13 Tables C rrent Statusiand Lea Etgen ele ee n sects ete a teeta e a o ENEE 1 Primary Variables ierni ee raa ih ate Bical Aiea ee en ee 2 Meastirement Uncertamtiess ed ee BE shbaaddeissucdestacsedebel Aerer tes eabebbueslelblastetee 3 secondary V arlables sisi 22 iste ei eege AE EE RENE LEE cede 3 Diagnostic Variables errno ient ee iee A E de Measla E aa a EE Seale o ocaeek RE E RES 3 Data Quality EE 4 Data Quality Thresholds TEE 4 Tame Quality Flags ssc isc Siege Eeer ee Ee eelere Ee 5 L nits for re E 6 Dimension Variables seod n reae aea aea Ee 6 Iistr ment Specifications eege Ale AER ea aa aae aa Ae eara eieaa 13 iii August 2006 ARM TR 016 1 General Overview The Microwave Radiometer MWR provides time series measurements of column integrated amounts of water vapor and liquid water The2. with the retrieval coefficients The truth is that Te varies diurnally enough to cause the zero LWP to vary in a most annoying fashion within the uncertainty bounds of the retrieval How can we correct calibration problems that affect the values of liquid water path The retrieval of PWV and LWP is based on a weighted difference of the optical thicknesses of the two channels The weights are the retrieval coefficients Because the retrieval coefficients are determined by linear regression over a climatological range of likely conditions they implicitly assume the mean conditions e g mean radiating temperature mean cloud liquid water temperature in the case of LWP The farther the actual conditions are from the mean due to diurnal and synoptic variations that the retrieval does not account for the more error in the retrieval Clear sky conditions are far from the mean LWP and so they have a larger uncertainty than cloudy conditions that may be very close to the average Calibration issues aside the clear sky LWP may be wrong but the cloudy sky LWP may be very close to correct so subtracting the clear sky offset from all values of LWP is not an optimal solution With respect to calibration errors the problem is that the calibration value the noise injection temperature affects the slope of the calibration K count so that an error in the calibration really affects the scale rather than the offset of the brightness temperatures but
3. Global Positioning System and Radiosondes during the Cloudiness Inter Comparison Experiment IEEE Transactions of the Geosciences Remote Sensing in press Westwater E Y Han M Shupe and S Matrosov 2001 Analysis of integrated cloud liquid and precipitable water vapor retrievals from microwave radiometers during the Surface Heat Budget of the Arctic Ocean project Journal of Geophysical Research 106 32 019 32 030 Westwater E B Stankov D Cimini Y Han J Shaw B Lesht and C Long 2003 Radiosonde Humidity Soundings and Microwave Radiometers during Nauru99 Journal of Atmospheric and Oceanic Technology 20 953 971 20
4. ass Since the absorption should be zero for m 0 no atmosphere the intercept represents the error in the current calibration If the correct brightness temperatures had been used the intercept would pass though the origin Thus the true zenith optical thickness t 1 is equal to the slope of the regression line Note that the quality of the fit indicates the degree to which the atmosphere is horizontally homogeneous In the presence of clouds for example the tip curve calibration method is not valid because the absorption is not linearly related to air mass For ARM the regression must account for at least 99 8 of the observed variance R 0 998 to be considered valid Angles on both sides of the zenith corresponding to the same air mass are used to assess horizontal homogeneity 17 August 2006 ARM TR 016 5 The true zenith optical thickness tl is now used to compute the true zenith brightness temperature Tee Te e Tur 1 ei 6 Now the gain can be computed G V zenith Wret TBenithy Tre Cryogenic Calibration The tip curves will determine the gain and offset of the radiometer and transfer this to a highly stable noise diode gain reference Therefore the MWR does not require cryogenic calibration However an external cold target is occasionally used to verify the calibration of the internal noise diode The target consists of blackbody foam immersed in about 30 liters of liquid nitrogen contained in a polystyre
5. because the retrieved values result from the difference of the two channels which are weighted differently it can look like there is an offset in LWP for clear sky conditions The only good way to fix calibration errors is to reprocess the data to fix the brightness temperatures then reapply the retrievals What is a reasonable maximum liquid water path Suppose the cloud averages 1 g m of liquid water and is 1 km thick Then the LWP would be 10 6 g cm x 10 5 cm 0 1 g em 0 1 cm 1 mm Thus values above say 3 mm would be rare such high values would probably be accompanied by rain and thus not measured anyway August 2006 ARM TR 016 Why do we see occasional spikes way over 3 mm in the liquid water path data Two events cause the LWP to exceed 1 mm or 3 mm The first is rain or melting snow The second is condensation dew A rule of thumb Ed Westwater NOAA ETL uses is brightness temperatures over 100 K aren t generally reliable i e the optical depth can t get that large without precipitation or condensation on the Teflon window This rule is used to set the upper limit for brightness temperatures i e when the brightness temperatures exceed 100 K a flag is set in the netCDF data file Are the data from the MWRs independent of the radiosondes No not entirely The retrievals are based on NWS radiosonde data from 1994 1999 Are the tuning functions still used No Use of the so called tuning functions wa
6. cks the qc_time field is also supplied The purpose of the qc_time field is to help detect duplicate samples missing samples or other sample time problems The qc_time field contains a value for each sample time Refer to the table below for details Table 8 Time Quality Flags Value Description Dt is within specified range Dt is 0 duplicate sample Dt is less than specified lower limit AJIN Dt is greater than specified upper limit August 2006 ARM TR 016 Finally the table below specifies the qce_time limits used for the MWR datastream Table 9 Limits for Time Lower Upper SE iiai eat mwrlos 20 39 5 1 5 Dimension Variables Table 10 Dimension Variables Variable Name Quantity Measured Unit base_time Base time in Epoch seconds lat north latitude Degrees lon east longitude degrees alt altitude meters above Mean Sea Level 5 2 Annotated Examples This section is not applicable to this instrument 5 3 User Notes and Known Problems Positive spikes are produced in the measurements during preventative maintenance due to water used to clean the teflon window BA Frequently Asked Questions How should we use the QC flags qcmin qcmax qcdelta Do we disregard suspicious data Precipitable water vapor PWV and liquid water path LWP that exceed the maximum should be eliminated these usually indicate rain PWV be
7. e large and the attenuation e great when either z r or k is large Put another way if r and k are large enough that a very small value of z will still cause e 1 then the region is said to be optically thick one cannot see very far into it On the other hand if r and k are sufficiently small a very large value of z will be required to produce e 1 and the region is said to be optically thin one can see a large distance at this frequency The second term in Eq 1 represents the sum of the contributions from the atmosphere along the line of sight i e the path B T z is the Planck function which describes the blackbody emission from the molecules at height z which are at a temperature T z The product rk is the amount of blackbody radiation that is emitted i e not re absorbed by the molecules in the layer The factor e accounts for the attenuation by the atmosphere between the source molecules and the microwave radiometer antenna In the microwave region the Planck function may be expressed as B T 2KTc I 3 where K is Boltzmann s constant c is the speed of light and is the wavelength of the radiation We can rearrange this expression to define the equivalent blackbody brightness temperature Tp ke If Eq 1 is divided through by 2Kc I then Tp T ele J 0 00 T z r z k z e dz 4 14 August 2006 ARM TR 016 where T 2 75 kelvins To actually calculate Tg the atmo
8. h is useful because it is nearly insensitive to changes in ambient temperature even though V a and G are strong functions of temperature Because the instantaneous output of the noise diode represents a random process having a Gaussian distribution the results of many tip curves gt 500 are used to compute G and Vna and thus Tna Once calibrated the system gain can be determined using the noise diode by viewing the blackbody target and measuring the change in the radiometer output due to switching on the noise diode G V ef na Vret Tha Temperature Dependence of the Calibration The gain is very sensitive to the temperature of the radiometer components i e the feed horn waveguides mixer local oscillators etc As a result the stability of the gain is directly related to the thermal stability of the instrument The microwave hardware in the ARM radiometers is mounted on a thick aluminum plate in an insulated enclosure and thermally stabilized to 0 25 K Even so the slight variations in the gain which is the slope of the calibration curve that arise from these slight temperature variations must be accounted for because the sky brightness temperatures typically range from 10 80 K whereas the blackbody reference temperature is at ambient 300 K so small errors in the gain will result in significant errors in the brightness temperature Consequently the tip curve data are used to derive a linear relationship between the noise injec
9. he MWR that are available from the ARM Archive e mwrlos water liquid amp vapor along line of sight LOS path e mwrtip brightness temperatures along tipping TIP curve airmasses 5 1 1 Primary Variables and Expected Uncertainty The MWR receives microwave radiation from the sky at 23 8 GHz and 31 4 GHz These two frequencies allow simultaneous determination of water vapor and liquid water burdens along a selected path Atmospheric water vapor observations are made at the hinge point of the emission line where the vapor emission does not change with altitude pressure Cloud liquid in the atmosphere emits in a continuum that increases with frequency dominating the 31 4 GHz observation whereas water vapor dominates the 23 8 GHz channel The water vapor and liquid water signals can therefore be separated by observing at these two frequencies Table 2 Primary Variables Variable Name Quantity Measured Unit tbsky23 23 8 GHz sky brightness temperature K tbsky31 31 4 GHz sky brightness temperature K vap Total water vapor along LOS path cm liq Total liquid water along LOS path cm sky_ir_temp IR Brightness Temperature K tbsky23tip 23 8 GHz sky brightness temperature derived from tip curve K tbsky3 tip 31 4 GHz sky brightness temperature derived from tip curve K Total water vapor along zenith path using tip derived brightness vaptip temperatures cm Total liquid water along zenith path using tip der
10. instrument itself is essentially a sensitive microwave receiver That is it is tuned to measure the microwave emissions of the vapor and liquid water molecules in the atmosphere at specific frequencies 2 Contacts 2 1 Mentor Marie Cadeddu Argonne National Laboratory 9700 S Cass Avenue Argonne Illinois 60439 Phone 630 252 7408 mcadeddu anl gov 2 2 Instrument Developer Radiometrics Corporation 2840 Wilderness Place Unit G Boulder Colorado 80301 5414 Phone 303 449 9192 Fax 303 786 9343 Website www radiometrics com 3 Deployment Locations and History Table 1 Current Status and Locations Serial Property Installation Number Number Location Date Status 4 WD06605 SGP EF14 2004 06 24 operational 10 WD11023 SGP CF1 1993 12 13 operational 11 WD14131 SGP BF1 1993 12 13 operational 12 WD14132 SGP BF5 1993 12 13 operational 15 WD14869 TWP CF2 1998 11 12 operational 16 WD13409 TWP CF1 1996 09 23 operational 17 WD13410 TWP CF3 2002 02 27 operational 18 WD12906 SGP BF6 1995 06 27 operational 20 WD24769 NSA CF1 1997 02 28 operational 21 WD24770 NSA CF2 1999 05 20 operational 33 WD23179 AMF 2004 09 02 operational 38 WD30750 SGP BF4 2004 09 02 operational August 2006 ARM TR 016 4 Near Real Time Data Plots MWR Data Plots 5 Data Description and Examples Available data plots and other data products 5 1 Data File Contents Datastreams produced by t
11. ived brightness liqtip temperatures cm 5 1 1 1 August 2006 ARM TR 016 Definition of Uncertainty Table 3 Measurement Uncertainties Measurement Uncertainty Sky 0 018 K Blackbody 0 12K Blackbody noise 0 15 K Gain reference 0 02 K Receiver gain 0 09 K Receiver offset 0 035 K 5 1 2 Secondary Underlying Variables Table 4 Secondary Variables Variable Name Quantity Measured Unit time Time offset from midnight seconds tnd23 Noise injection temp at 23 8 GHz adjusted to tkbb K bb23 23 8 GHz Blackbody signal count bbn23 23 8 GHz blackbody noise injection signal count sky23 23 8 GHz sky signal count tnd31 Noise injection temp at 31 4 GHz adjusted to tkbb K bb31 31 4 GHz Blackbody signal count bbn31 31 4 GHz blackbody noise injection signal count sky31 31 4 GHz sky signal count actaz Actual Azimuth deg actel Actual elevation angle deg tipsky23 23 8 GHz sky signal count tipsky31 31 4 GHz sky signal Count tnd231 Noise injection temp at 23 8 GHz derived from this tip K tnd3 11 Noise injection temp at 31 4 GHz derived from this tip K 5 1 3 Diagnostic Variables Table 5 Diagnostic Variables Variable Name Quantity Measured Unit time_offset Time offset from base_time seconds tknd Noise diode mount temperature K tkxc Mixer kinetic physical temperature K tkbb Blackbody kinetic temperature K tkair Ambient temperatu
12. lculations How were the tuning functions determined After each sonde launch the model which computes the integrated vapor from the sonde as well as the microwave brightness temperatures is run automatically by the data system Jim Liljegren collected all of these modeled and measured brightness temperatures between Oct 92 and Dec 93 selected those for which the sky was clear that is for which the RMS variation in the liquid sensing channel brightness temperature was less than 0 4 K and calculated a regression for each channel August 2006 ARM TR 016 What changes were made to the data ingest in October 1998 The MWR software was revised to provide additional functionality as described below 1 Faster sampling rate Standard line of sight LOS observations can now be acquired at 15 second intervals vs 20 second intervals previously The standard LOS cycle is comprised of one sky sample per blackbody sample and gain update 2 More flexible sampling strategy Multiple sky observations can be acquired during a LOS cycle up to 1024 per gain update This permits sky samples to be acquired at intervals of 2 67 seconds for improved temporal resolution of cloud liquid water variations and better coordination with the millimeter cloud radar during IOPs 3 Separation of zenith LOS observations from tipping curve TIP data When the radiometer is in TIP mode the zenith LOS observations are now extracted the PWV and LWP computed and
13. le failed delta and missing data checks 10 Sample failed minimum and delta checks 11 Sample failed minimum delta and missing value checks 12 Sample failed maximum and delta checks 14 Sample failed minimum maximum and delta checks 15 Sample failed minimum maximum delta and missing value checks The minimum and maximum thresholds are currently defined as follows Table 7 Data Quality Thresholds Field Name Units Min Max Delta tknd K 303 333 N A tkxc K 303 333 0 5 tkbb K 250 320 1 tkair K 253 323 N A tnd23 K 163 353 N A August 2006 ARM TR 016 Table 7 cont d Field Name Units Min Max Delta bb23 counts 0 N A N A bbn23 counts 0 N A N A sky23 counts 0 N A N A tbsky23 K 2 73 100 0 01 tnd31 K 163 353 N A bb31 counts 0 N A N A bbn31 counts 0 N A N A sky31 counts 0 N A N A tbsky31 K 2 73 100 0 01 vap cm 0 N A N A liq cm 3 rms see note 1 N A N A sky_ir_temp K 213 313 50 wet_window unitless see note 2 N A N A tnd_nom23 K 163 353 80 tnd_nom31 K 163 353 80 tc23 K K N A N A N A tc31 K K N A N A N A Note 1 rms is liquid_retrieval_rms_accuracy Note 2 A value of 1 for the wet_window field means that the heater was ON at the time the sample was taken Additional information may be found at MWR Data Object Design for ARM netCDF file header descriptions In addition to the above data quality che
14. low zero is unphysical and arises during rain because of the opposite signs of the retrieval coefficients Negative LWP is OK as long as it is within or close to the RMS uncertainty in the retrieval The RMS uncertainties in the PWV and LWP are included in the meta data e vapor_retrieval_rms_accuracy cm 0 057881 lt for December e liquid_retrieval_rms_accuracy cm 0 003083 lt for December August 2006 ARM TR 016 Why are qcmax flags not completely coincident with the weather log reports of rain The qcmax flag for liquid is raised when the retrieved liquid water exceeds 1 cm Such a value is not possible it indicates a serious failure of the retrieval most likely due to water standing on the instrument There are two reasons why this may not be completely coincident with weather log reports of rain First the operators are only onsite from 8 am to 5 pm local time at the central facilities if it rains when the operators aren t there no log entry is made Second the problem can arise from standing water not just rainfall there is some time between the end of the rainfall and the evaporation of the water from the teflon window which covers the mirror on the instrument So the operators could report that the rain has stopped but the microwave radiometer window is still wet and thus still reporting invalid data Can we use the data when there are long periods of qcmin flags for liquid Yes yo
15. nd azimuth gt 90 second elevation Field of view 5 9 at 23 GHz 4 5 at 31 4 GHz full width at half maximum 13 August 2006 ARM TR 016 7 2 Theory of Operation The instrument itself is essentially a sensitive microwave receiver That is it is tuned to measure the microwave emissions of the vapor and liquid water molecules in the atmosphere at specific frequencies For a specific frequency n the amount of microwave radiation observed by a radiometer at the earth s surface looking directly upward can be expressed as I ec J 0 0 B T z rke dz 1 The first term represents the amount of cosmic i e extraterrestrial radiation entering at the top of the atmosphere L that reaches the radiometer The exponential decay factor accounts for attenuation of the cosmic radiation by the intervening atmosphere t is the optical thickness 0 2 J 0 z r z k z dz 2 where r is the density mass per volume or number per volume and k is the extinction coefficient area per mass or area per number It is highly dependent on frequency Note that extinction is the sum of absorption plus scattering however because scattering is negligible in the microwave region of the electro magnetic spectrum except during heavy rain k can be taken as the absorption coefficient alone The physical significance of t is that it represents an effective thickness of the atmosphere for a particular frequency t will b
16. ne foam cooler This provides a reference point in the vicinity of 77 K that can be known to about 0 3 K by a simple barometric pressure measurement 7 3 3 History The ARM MWRSs are now able to calibrate themselves during clear sky periods as described in the following book chapter Liljegren J C 1999 Automatic self calibration of ARM microwave radiometers Microwave Radiometry and Remote Sensing of the Earth s Surface and Atmosphere eds P Pampaloni and S Paloscia pp 433 443 VSP Press The calibration coefficients are found as time dependent variables in the netCDF files They are the noise injection temperatures at the nominal temperature usually 290 K tnd23 and tnd31 and the temperature correction coefficients tc23 and tc31 7 4 Operation and Maintenance 7 4 1 User Manual This section is not applicable to this instrument 7 4 2 Routine and Corrective Maintenance Documentation SGP Preventative Maintenance Procedure TWP Operating Procedure NSA Preventative Maintenance Procedure Manual 18 August 2006 ARM TR 016 7 4 3 Software Documentation Installation Operation and Troubleshooting Guide for MWR EXE Software for ARM Microwave Water Radiometers ARM netCDF file header descriptions may be found at MWR Data Object Design Changes 7 4 4 Additional Documentation See Routine and Corrective Maintenance Documentation Section 7 4 2 7 5 Glossary See the ARM Glossary at http www arm gov about gl
17. of the heater ON OFF is indicated in the netCDF files by the wet_window variable This system seems to work quite well but the sensitivity of the heater needs to be maintained Because it is a resistive element it is somewhat temperature dependent so it periodically triggers unnecessarily on cold nights Although this doesn t affect the PWV or LWP measurements it does cause some confusion when using the wet_window data as an indication of rain or dew or fog The MWRs have been retrofitted with new blowers that have greater air flow and heater circuitry that is less sensitive to ambient temperature August 2006 ARM TR 016 When should we call the liquid water path zero i e what is the noise level Why do we see significant 30 g m 0 03 mm positive negative values of the liquid water path when the sky is clear according to the ceilometer The noise level is very low 0 003 mm 0 0003 cm RMS The problem is in the retrieval uncertainty Statistical retrieval is essentially a multiple linear regression Any regression will have a residual error In the LWP retrieval the residual error or theoretical accuracy is 0 03 mm RMS 10 times the sensitivity or noise limit So a value of LWP that is 0 03 mm of zero could be clear sky The real problem here is that the mean radiating temperature Lo of the atmosphere which is determined at the time the retrieval coefficients are computed is assumed to only vary monthly
18. ossary stm 7 6 Acronyms See the ARM Acronyms at http www arm gov about glossary stm 7 7 Citable References Braun J C Rocken and J Liljegren 2003 Comparisons of Line of Sight Water Vapor Observations Using the Global Positioning System and a Pointing Microwave Radiometer Journal of Atmospheric and Oceanic Technology 20 606 612 Cimini D E Westwater Y Han and S Keihm 2003 Accuracy of Ground Based Microwave Radiometer and Balloon Borne Measurements During the WVIOP2000 Field Experiment IEEE Transactions of the Geosciences Remote Sensing 41 2605 2615 Han Y and E Westwater 2000 Analysis and Improvement of Tipping Calibration for Ground Based Microwave Radiometers IEEE Transactions of the Geosciences Remote Sensing 38 1260 1276 Liljegren J C 1994 Two channel microwave radiometer for observations of total column precipitable water vapor and cloud liquid water path Fifth Symposium on Global Change Studies pp 262 269 January 23 28 1994 American Meteorological Society Nashville Tennessee Liljegren J C and B M Lesht 1996 Measurements of integrated water vapor and cloud liquid water from microwave radiometers at the DOE ARM Cloud and Radiation Testbed in the U S Southern Great Plains Presented at the JEEE International Geosciences and Remote Sensing Symposium IGARSS May 21 26 1996 Lincoln Nebraska Liljegren J C 1999a Automatic self calibration of ARM microwave
19. power cable for heater Serial communications cable 7 1 2 System Configuration and Measurement Methods The water vapor radiometer receiver is composed of a gaussian optical antenna a noise diode injection device a dual junction isolator a balanced mixer an IF amplifier a detector video amplifier and two Gunn diode oscillators Figure 1 The receiver accepts input power from the antenna and supplies a voltage proportional to antenna temperature plus antenna noise via a square law detector to the radiometer voltage to frequency converter on the microprocessor digital board Receiver frequency selection is accomplished by alternately powering the 23 8 and 31 4 GHz Gunn diode local oscillators Brightness temperature calibration is provided by a noise source injected at the input added to antenna temperature The Gunn diode oscillators and noise source are powered by the radiometer analog board and controlled by the radiometer digital board The MWR uses low noise low power IF amplifiers The receiver is linear with antenna power over a range of the sky and calibration observables The receiver is thermally stabilized to ensure stability of the mixer and the noise diode and Gunn diode output and frequency The sky brightness temperature is measured in the following manner The small angle receiving cone of the gaussian lensed microwave antenna is steered with a rotating flat mirror Both the 23 8 GHz and 31 4 GHz waveband signals are t
20. r the total liquid water L 7 3 Calibration 7 3 1 Theory When brightness temperature measurements are taken the calibrated noise diode is automatically used to inject a known temperature into the antenna wave guide to determine gain and offset is determined by observing the internal blackbody with the radiometer antenna This eliminates error due to any drift in the microwave receiver 15 August 2006 ARM TR 016 Background The electrical output of the radiometer V in volts or digital counts is linearly related to the equivalent microwave brightness temperature TB Tp Tief V R Vief G Here Tyer is the reference temperature of the internal blackbody target and Vef is the corresponding radiometer signal G is the gain of the system in units of volts or digital counts per kelvin One way to determine the gain G is through the use of tip curves The ARM radiometers use a reverse biased diode which injects broadband microwave energy noise directly into the waveguide when it is switched on causing the signal output of the radiometer to increase by Vna The output of the noise diode is determined during the tip curve procedure by pointing the elevation mirror at the reference blackbody target and measuring the radiometer output with the noise diode on and off Na V ref nd Vref This noise diode output is calibrated from the tip curve derived gain to yield the noise injection temperature Tha Nal G whic
21. radiometers Microwave Radiometry and Remote Sensing of the Earth s Surface and Atmosphere eds P Pampaloni and S Paloscia pp 433 443 VSP Press 19 August 2006 ARM TR 016 Liljegren J C 1999b Observations of integrated water vapor and cloud liquid water at the SHEBA ice station Microwave Radiometry and Remote Sensing of the Earth s Surface and Atmosphere eds P Pampaloni and S Paloscia pp 155 163 VSP Press Liljegren J C E Clothiaux G Mace S Kato and X Dong 2001 A new retrieval for cloud liquid water path using a ground based microwave radiometer and measurements of cloud temperature Journal of Geophysical Research 106 14 485 14 500 Lin B P Minnis A Fan J Curry and H Gerber 2001 Comparison of cloud liquid water paths derived from in situ and microwave radiometer data taken during the SHEBA FIREFACE Geophysical Research Letters 28 975 978 Liou Y A Y T Teng J Liljegren and T Van Hove 2001 Comparison of precipitable water observations in the near tropics by GPS radiometer and radiosondes Journal of Applied Meteorology 4 5 15 MacFarland S and F Evans 2002 A Bayesian Algorithm for the Retrieval of Liquid Water Cloud Properties from Microwave Radiometer and Millimeter Radar Data Journal of Geophysical Research accepted Mattioli V E Westwater S Gutman and V Morris 2004 Forward Model Studies of Water Vapor using Scanning Microwave Radiometers
22. ransmitted through a single waveguide into an isolator and into the 12 August 2006 ARM TR 016 mixer section Output from one of two Gunn diodes is injected into the local oscillator port of the mixer The resultant IF signal is amplified filtered to yield a 400 MHz wide dual sideband signal detected amplified again and converted by a voltage to frequency converter Zero crossings of this signal are counted yielding the raw data in counts Counts are then converted to brightness temperature through algorithms in the FORTRAN program Water vapor liquid water and phase path delay are calculated using site specific retrieval coefficients read from the configuration file IF OUT 22 33GHz WR34 MIXER SMA PREAMP POSTAMP cy TUNNEL OP AMP DET ISOLATOR VIDEO OUT 0 1V FOR 0 500 K 31 4GHz 23 8GHz GUNN GUNN OSCILLATOR OSCILLATOR Figure 1 Radiometer receiver block diagram 7 1 3 Specifications Table 11 Instrument Specifications Parameter Value Sample Time User Selectable Nominally 20 s in LOS Mode 58 s in TIP Mode Accuracy 0 3K Resolution 0 25 K Radiometric range 0 to 700 K Operating range 20 to 50 C Power requirements 120W maximum Voltage requirements 90 to 130 or 180 to 260 VAC 50 to 440 Hz Output ASCII data files to laptop computer via RS 232 at 9600 baud Dimensions 50 x 28 x 76 cm Weight 17 kg Angular coverage all sky Pointing slew rate 3 seco
23. re K wet_window Water on Teflon window 1 WET 0 DRY unitless tnd_nom23 Noise injection temp at nominal temperature at 23 8 GHz K tnd_nom31 Noise injection temp at nominal temperature at 31 4 GHz K August 2006 ARM TR 016 Table 5 cont d Variable Name Quantity Measured Unit tc23 Temperature correction coefficient at 23 8 GHz K K tc31 Temperature correction coefficient at 31 4 GHz K K r23 23 8 GHz goodness of fit coefficient unitless r31 31 4 GHz goodness of fit coefficient Unitless 5 1 4 Data Quality Flags Most fields contain a corresponding sample by sample automated quality check field in the b1 level datastreams These flags are named qc_ lt fieldname gt For example the tknd field also has a companion qc_tknd field Possible values for each sample of the qe_ lt fieldname gt are shown in the table below Table 6 Data Quality Flags Value Definition 0 All QC checks passed 1 Sample contained missing data value 2 Sample was less than prescribed minimum value 3 Sample failed both missing data and minimum value checks 4 Sample greater than prescribed maximum value 5 Sample failed both minimum and maximum value checks highly unlikely 7 Sample failed minimum maximum and missing value checks highly unlikely 8 Sample failed delta check change between this sample and previous sample exceeds a prescribed value 9 Samp
24. reported separately in the output file This eliminates the periods of missing LOS data during calibration checks updates 4 Automatic self calibration The software now permits the calibration to be updated at specified intervals or continuously To ingest the new format of the raw data significant changes were made to the MWR Data Object Design Why are there anomalies in the data near local solar noon These anomalies are due to the sun in the field of view of the radiometer They occur near the equinoxes at TWP around noon in both TIP and LOS modes and at SGP near sunrise and sunset in east west tip curve data Why are there spikes in the data from November 1999 to July 2002 The intermittent spikes in the LWP and PWV data were caused by the occurrence of blackbody signals in counts that were half of those expected yielding negative sky brightness temperatures This problem was due to some component of the Windows98 configuration that conflicted with the DOS based MWR program or affected the serial port or the contents of the serial port buffer It was finally corrected by upgrading the MWR software with a new Windows compatible program 10 August 2006 ARM TR 016 6 Data Quality 6 1 Data Quality Health and Status The following links go to current data quality health and status results e DQ Hands Data Quality Health and Status e NCVweb for interactive data plotting using The tables and graphs shown contain the
25. s discontinued on 6 April 1996 The tuning functions were removed from the SGP CF MWR data that were collected between 950101 960409 Data collected before this date have already had these removed by Jim Liljegren while data after this time window never had the tuning functions applied At the time it was commonly held that the sondes represented ground truth and that the tuning functions i e regressions of model calculated vs measured brightness temperatures accounted for errors in the microwave absorption model upon which the retrievals were based Jim Liljegren and Barry Lesht have since determined that the variation in the sonde calibration explains the differences between the model calculations and the microwave radiometric measurements By removing the tuning functions the PWV and LWP retrieved from the microwave radiometer are independent of the radiosondes What were the tuning functions The tuning functions linearly relate model calculated microwave brightness temperatures using radiosonde data to brightness temperatures measured with a microwave radiometer These were needed to account for imperfections in the microwave absorption model used to develop the retrievals which relate precipitable water vapor and liquid water path to the microwave brightness temperatures The tuning functions should be independent of the instrument and of the location they should depend only on the microwave absorption model used in the ca
26. sphere is divided into a number of layers N which are considered isothermal Ts Tee YGN THz 1 Kz e dz 5 If N 1 i e the entire atmosphere is taken to be isothermal then Tg T eo Tur 1 0 00 r z k z e dz T et Tyrell ef 6 where t t 0 00 the total zenith absorption and Tyr is the mean radiating temperature of the atmosphere at this frequency In general there is a different Tyr for each frequency Equation 6 is the brightness temperature equation it is used to relate the observed emission Tg to the absorption t In TMR s T TMR Ts V and L are then derived by relating them to the microwave absorption t t tary tvap tig tay ky V k L tary is due to the approximately constant emission of O molecules ky and ky are calculated from climatology with sufficient accuracy to determine t Observations of Te at a vapor sensitive frequency 23 8 GHz and a liquid sensitive frequency 31 4 GHz yield two linear equations which can be solved for the two unknowns V and L Note that the vapor sensitive frequency is chosen such that ky is not dependent on pressure and thus not dependent on height The unknowns V and L can be represented in terms of the optical depths V a a t a2t where the variables in italics are climatological mean values and the a are determined through a linear regression i e they are the regression coefficients A similar expression can be written fo
27. techniques used by ARM s data quality analysts instrument mentors and site scientists to monitor and diagnose data quality 6 2 Data Reviews by Instrument Mentor Data quality control procedures for this system are mature On a weekly basis the instrument mentor produces and inspects plots of the precipitable water vapor PWV and liquid water path LWP versus time The base level of LWP is evaluated for clear sky episodes and the PWV estimates are compared to those from the BBSS DQRs are submitted when needed and a summary report of data quality is sent monthly to the SGP site scientist team 6 3 Data Assessments by Site Scientist Data Quality Office All DQ Office and most Site Scientist techniques for checking have been incorporated within DQ HandS and can be viewed there 6 4 Value Added Procedures and Quality Measurement Experiments Many of the scientific needs of the ARM Program are met through the analysis and processing of existing data products into value added products or VAPs Despite extensive instrumentation deployed at the ARM CART sites there will always be quantities of interest that are either impractical or impossible to measure directly or routinely Physical models using ARM instrument data as inputs are implemented as VAPs and can help fill some of the unmet measurement needs of the program Conversely ARM produces some VAPs not in order to fill unmet measurement needs but instead to improve the quality of exis
28. ting measurements In addition when more than one measurement is available ARM also produces best estimate VAPs A special class of VAP called a Quality Measurement Experiment QME does not output geophysical parameters of scientific interest Rather a QME adds value to the input datastreams by providing for continuous assessment of the quality of the input data based on internal consistency checks comparisons between independent similar measurements or comparisons between measurement with modeled results and so forth For more information see see the VAPs and QMEs web page and specifically e MWR PROF Retrievals of water vapor liquid water and temperature profiles from a suite of ground based instruments e LSSONDE Radiosonde profiles where the moisture profile is scaled to match the MWR s total precipitable water vapor 11 August 2006 ARM TR 016 e QME MWR PROF Comparisons of retrieved water vapor and temperature profiles from MWR PROF with radiosonde profiles e QME MWR LBL Comparisons of observed versus calculated microwave radiance at two frequencies e QME MWR COL Comparisons of the MWR with an instrument performance model 7 Instrument Details 7 1 Detailed Description 7 1 1 List of Components Radiometrics WVR 1100 Radiometer Radiometrics dew blower heater rain sensor assembly Radiometrics tripod or quadrapod and tribrach leveling base Small form factor computer AC power cable for radiometer AC
29. tion temperature and the ambient temperature represented by the temperature of the blackbody target Tal 16 August 2006 ARM TR 016 Tha Tha 290 K a Trer 290 where a is the temperature coefficient which typically ranges from 0 08 to 0 08 K K 7 3 2 Procedures Tip Curves To perform a tip curve calibration measurements of optical thickness t are required along paths at various elevation angles a If the atmosphere can be assumed to be horizontally homogeneous then the optical thickness along a path at an angle a above the horizon is directly proportional to the optical thickness at the zenith t m mt 1 where m 1 sin a is the air mass e g a line of sight path inclined at an angle of 30 degrees above the horizon or m 2 traverses twice the mass of air as at 90 degrees or m 1 The procedure is as follows 1 Use the existing calibration as an approximation Use the approximate calibration and measure brightness temperatures at elevation angles corresponding to several different air masses For ARM the elevation angles are 19 5 23 6 30 0 41 8 90 0 138 2 150 0 156 4 160 5 and 90 0 which corresponds to m 3 0 2 5 2 0 1 5 1 0 1 5 2 0 2 5 3 0 and 1 0 Angles on both sides of zenith are used to ensure horizontal homogeneity Relate these brightness temperatures to optical thickness t m In Tr TM we Tee Fit a straight line to the optical thickness as a function of air m
30. u can use the data Actually liquid values that are negative but within the RMS accuracy of the retrieval of zero may be considered equal to zero which is the physically plausible lower bound When the weather log reports fog is it possible that the microwave window gets covered with dew and leads to bad data even though no rain is reported Dew has a fairly distinct signature a smooth hump in the retrieved vapor and liquid as well as in the underlying brightness temperatures This is often seen just at or before dawn however it usually goes away before the operators would have reported to the site This assessment is borne out by the surface station data which indicated that the dew point was within the sensor accuracy of the ambient temperature at about the time when the hump first appeared For example on one occasion the operators reported heavy ground fog at 1400 GMT 8 am local time and the instrument showed 85 90 microns of liquid Pd say the two were consistent The instruments are equiped with an anti dew system that is comprised of a continuous fan and a 500 750 W heater controlled by a moisture sensor mounted on top of the instrument Normally the fan blows ambient air over the Teflon window to keep it clear of dust During condensing or precipitating conditions the heater turns on to prevent the formation of dew or the settling of fog on the window as well as to promote the evaporation of rain and snow The condition
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