Analysis of upper tropospheric humidity measurements by microwave sounders and radiosondes

This thesis describes results of several analyses of humidity measurements by microwave humidity sounders and radiosondes. The goal of this work is to pave the way for fully utilizing these measurements for climatological applications. High resolution radiosonde data are used to examine the variability of the clear-sky outgoing longwave radiation (OLR). The global variability of OLR is found to be 33 Wm-2, of which a large part can be attributed to temperature variations. The variability after filtering the temperature part is associated with the humidity variability in the horizontal and the vertical. The impact of the vertical structures on the OLR calculations is also investigated in detail. It is observed that smoothed profiles in relative humidity are sufficient to obtain the mean value of OLR, even though the variability cannot be exactly reproduced. AMSU-B Channel 18 brightness temperatures are sensitive to upper tropospheric humidity (UTH). A simple method is developed to transform the brightness temperatures to UTH. This method is validated with high quality radiosonde data. An initial attempt to make a UTH climatology and the usefulness of a robust estimator such as the median in climatological studies are discussed. Finally, a robust method was developed to compare the humidity measurements from satellite humidity sounders and radiosondes. The method is developed and tested using the high quality radiosonde data from the Lindenberg radiosonde station. A case study using different versions of the data shows that the method is sensitive to humidity differences in the different versions. The main result from the case study is that the corrected radiosonde data still have a slight dry bias in the upper troposphere. The method is then applied to assess the performance of different radiosonde sensors and stations. It isfound to be useful for monitoring the global radiosonde network, using the microwave satellite data as a benchmark

Upper tropospheric humidity from SAPHIR on-board Megha-Tropiques

Upper tropospheric humidity (UTH) has been derived using a ‘brightness temperature (Tb) transformation’ method from the humidity sounder channels of SAPHIR payload on - board Megha - Tropiques (MT). These channels are very close to the water vapour absorption peak at 183.31 GHz. The channel at 183.31 0.2 GHz enables retrieval of humidity up to the hig h est altitude possible wit h the present nadir - looking microwave humidity sounders. Megha - Tropiques satllite has an equatorially inclined orbit, which e n sures frequent spatial and temporal coverage of the global tropical belt. Transformation coeff i cients for the first three channels for all the incidence angles have been derived and are used to convert brightness temperatures to weighted average upper tropospheric humidity having weighting function peaks at different pressure levels. The methodology has been validated by comparing the SAPHIR - derived UTH with that derived from radiosonde observations. Inter - comparison of the derived UTH has been done with layer averaged humidity product from SAPHIR measurements and with UTH product using infrared measurements from Kalpana satellite ( MOSDAC). UTH over the tropical belt for six months has been studied taking the advantage of the humidity product with high spatial and temporal resolution. The transformation coefficients and methodology to identify the cloud - free pixels to derive UTH from the three channels for all the possible incidence angles are presented here, so that the users can directly derive UTH from the brightness temperature data

Observing CMB polarisation through ice

Ice crystal clouds in the upper troposphere can generate polarisation signals at the uK level. This signal can seriously affect very sensitive ground based searches for E- and B-mode of Cosmic Microwave Background polarisation. In this paper we estimate this effect within the ClOVER experiment observing bands (97, 150 and 220 GHz) for the selected observing site (Llano de Chajnantor, Atacama desert, Chile). The results show that the polarisation signal from the clouds can be of the order of or even bigger than the CMB expected polarisation. Climatological data suggest that this signal is fairly constant over the whole year in Antarctica. On the other hand the stronger seasonal variability in Atacama allows for a 50% of clean observations during the dry season.Comment: 7 Pages, 4 figure

Feuchtemessungen in der oberen Troposphäre mit Mikrowellen-Satellitensensoren und Radiosonden

This thesis describes results of several analyses of humidity measurements by microwave humidity sounders and radiosondes. The goal of this work is to pave the way for fully utilizing these measurements for climatological applications. High resolution radiosonde data are used to examine the variability of the clear-sky outgoing longwave radiation (OLR). The global variability of OLR is found to be 33 Wm-2, of which a large part can be attributed to temperature variations. The variability after filtering the temperature part is associated with the humidity variability in the horizontal and the vertical. The impact of the vertical structures on the OLR calculations is also investigated in detail. It is observed that smoothed profiles in relative humidity are sufficient to obtain the mean value of OLR, even though the variability cannot be exactly reproduced. AMSU-B Channel 18 brightness temperatures are sensitive to upper tropospheric humidity (UTH). A simple method is developed to transform the brightness temperatures to UTH. This method is validated with high quality radiosonde data. An initial attempt to make a UTH climatology and the usefulness of a robust estimator such as the median in climatological studies are discussed. Finally, a robust method was developed to compare the humidity measurements from satellite humidity sounders and radiosondes. The method is developed and tested using the high quality radiosonde data from the Lindenberg radiosonde station. A case study using different versions of the data shows that the method is sensitive to humidity differences in the different versions. The main result from the case study is that the corrected radiosonde data still have a slight dry bias in the upper troposphere. The method is then applied to assess the performance of different radiosonde sensors and stations. It isfound to be useful for monitoring the global radiosonde network, using the microwave satellite data as a benchmark

Radiative transfer calculations for a passive microwave satellite sensor : comparing a fast model and a line-by-line model

A comparison between the fast radiative transfer model Radiative Transfer for the TIROS Operational Vertical Sounder (RTTOV-7) and the physical radiative transfer model Atmospheric Radiative Transfer Simulator ( ARTS) was carried out. Radiances were simulated for the sounding channels of the Advanced Microwave Sounding Unit B (AMSU-B) for the whole globe for a single time of a single day ( 1 January 2000, 0000 UT). Temperature, pressure, and specific humidity profiles from the reanalysis data set ERA-40 of the European Centre for Medium-Range Weather Forecasts (ECMWF) were used as input for both models; geopotential height profiles were also used but only as input for ARTS. The simulations were made for two different surface emissivities, 0.60 and 0.95. The low surface emissivity case exhibits the larger radiance differences. Although the global values of the mean difference and standard deviation are small ( for example, the global mean difference for channel 18 is 0.014 K and the standard deviation is 0.232 K), the examination of the geographical distribution of the differences shows that large positive or negative values are observed over dry regions of high northern and southern latitudes and over dry elevated regions. The origin of these differences was found to be due to errors introduced by the transmittance parametrization used in RTTOV.Validerad; 2006; 20070427 (pafi)</p

A cautionary note on the use of Gaussian statistics in satellite-based UTH climatologies

This letter presents a cautionary note on the assumption of Gaussian behavior for upper tropospheric humidity (UTH) derived from satellite data in climatological studies, which can introduce a wet bias in the climatology. An example study using European Centre for Medium-Range Weather Forecasts reanalysis data shows that this wet bias can reach up to 6 %RH, which is significant for climatological applications. A simple Monte Carlo approach demonstrates that these differences and their link to the variability of brightness temperatures are due to a log-normal distribution of the UTH. This problem can be solved by using robust estimators such as the median instead of the arithmetic mean.Validerad; 2006; 20070427 (pafi)</p

Recent developments in the line-by-line modeling of outgoing longwave radiation

High frequency resolution radiative transfer model calculations with the Atmospheric Radiative Transfer Simulator (ARTS) were used to simulate the clear-sky outgoing longwave radiative flux (OLR) at the top of the atmosphere. Compared to earlier calculations by Clough and coworkers the model used a spherical atmosphere instead of a plane parallel atmosphere, updated spectroscopic parameters from HITRAN, and updated continuum parameterizations from Mlawer and coworkers. These modifications lead to a reduction in simulated OLR by approximately 4.1%, the largest part, approximately 2.5%, being due to the absence of the plane parallel approximation. As a simple application of the new model, the sensitivity of OLR to changes in humidity, carbon dioxide concentration, and temperature were investigated for different cloud-free atmospheric scenarios. It was found that for the tropical scenario a 20% change in humidity has a larger impact than a doubling of the carbon dioxide concentration. The sensitive altitude region for temperature and humidity changes is the entire free troposphere, including the upper troposphere where humidity data quality is poor.Upprättat; 2006; 20070427 (pafi)</p

A cloud filtering method for microwave upper tropospheric humidity measurements

The paper presents a cloud filtering method for upper tropospheric humidity (UTH) measurements at 183.31±1.00 GHz. The method uses two criteria: a viewing angle dependent threshold on the brightness temperature at 183.31±1.00 GHz, and a threshold on the brightness temperature difference between another channel and 183.31±1.00 GHz. Two different alternatives, using 183.31±3.00 GHz or 183.31±7.00 GHz as the other channel, are studied. The robustness of this cloud filtering method is demonstrated by a mid-latitudes winter case study. The paper then studies different biases on UTH climatologies. Clouds are associated with high humidity, therefore the possible dry bias introduced by cloud filtering is discussed and compared to the wet biases introduced by the clouds radiative effect if no filtering is done. This is done by means of a case study, and by means of a stochastic cloud database with representative statistics for midlatitude conditions. Both studied filter alternatives perform nearly equally well, but the alternative using 183.31±3.00 GHz as other channel is preferable, because that channel is less likely to see the Earth's surface than the one at 183.31±7.00 GHz. The consistent result of all case studies and for both filter alternatives is that both cloud wet bias and cloud filtering dry bias are modest for microwave data. The recommended strategy is to use the cloud filtered data as an estimate for the true all-sky UTH value, but retain the unfiltered data to have an estimate of the cloud induced uncertainty. The focus of the paper is on midlatitude data, since atmospheric data to test the filter for that case were readily available. The filter is expected to be applicable also to subtropical and tropical data, but should be further validated with case studies similar to the one presented here for those cases.Validerad; 2007; 20071109 (sbuehler)</p

Radiance Simulations in Support of Climate Services

International audienceClimate services are largely supported by climate reanalyses and by satellite Fundamental (Climate) Data Records (F(C)DRs). This paper demonstrates how the development and the uptake of F(C)DR benefit from radiance simulations, using reanalyses and radiative transfer models. We identify three classes of applications, with examples for each application class. The first application is to validate assumptions during F(C)DR development. Hereto we show the value of applying advanced quality controls to geostationary European (Meteosat) images. We also show the value of a cloud mask to study the spatio‐temporal coherence of the impact of the Mount Pinatubo volcanic eruption between Advanced Very High Resolution Radiometer (AVHRR) and the High‐resolution Infrared Radiation Sounder (HIRS) data. The second application is to assess the coherence between reanalyses and observations. Hereto we show the capability of reanalyses to reconstruct spectra observed by the Spektrometer Interferometer (SI‐1) flown on a Soviet satellite in 1979. We also present a first attempt to estimate the random uncertainties from this instrument. Finally, we investigate how advanced bias correction can help to improve the coherence between reanalysis and Nimbus‐3 Medium‐Resolution Infrared Radiometer (MRIR) in 1969. The third application is to inform F(C)DR users about particular quality aspects. We show how simulations can help to make a better‐informed use of the corresponding F(C)DR, taking as examples the Nimbus‐7 Scanning Multichannel Microwave Radiometer (SMMR), the Meteosat Second Generation (MSG) imager, and the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Water Vapor Profiler (SSM/T‐2)