A simulation technique for the determination of atmospheric water content with Bhaskara satellite microwave radiometer (SAMIR)

A simulation technique has been developed to estimate the integrated atmospheric water content over oceans using the 19·35 and 22·235 GHz brightness temperature data from satellite microwave radiometer (SAMIR) on board the Indian satellite Bhaskara. The results obtained have been compared with those from linear statistical regression and empirical methods as well as from the nearest radiosonde observations. Based on the simulation method, a map of total precipitable water for some of the Bhaskara passes in July 1980 over Bay of Bengal is given. The possible applications of such maps in the study of Indian summar monsoon and boundary layer characteristics have been pointed out.     

Remote sensing of atmospheric water vapour from Bhaskara IISAMIR data and its comparison withNOAA-7 water vapour data

Applying the method of ‘statistical linear regression’, atomspheric water vapour over oceanic areas has been estimated from the 19GHz and 22 GHz data of the satellite microwave radiometer (SAMIR) system onboard the Bhaskara II satellite. In the absence of any simultaneousin situ measurements on water vapour over ocean, theSAMIR-derived water vapour data have been compared with like data from theNOAA-7 satellite. It is suggested that a positive bias seen in theSAMIR data could be due to calibration errors in the basic data. In view of the observed bias, the original regression equation is modified and then used to obtain water vapour distributions over ocean for winter and south-west monsoon seasons usingSAMIR data of several orbits.     

青藏高原积雪深度和雪水当量的被动微波遥感反演

利用1993年1月份的SSM/I亮度温度数据反演了青藏高原的雪水当量,首先使用被动微波SSM/I数据19和37GHz的水平极化数据来反演雪深,根据积雪时间的函数来计算实时的雪密度,由雪的深度和密度计算出雪水当量.最后,利用SSM/I数据的19和37GHz的垂直极化亮度温度梯度对计算出的雪水当量进行回归分析,得到了利用SSM/I数据直接反演雪水当量的算法.     

Radiative transfer simulations for the MADRAS imager of Megha-Tropiques

This paper reports the radiative transfer simulations for the passive microwave radiometer onboard the proposed Indian climate research satellite Megha-Tropiques due to be launched in 2011. These simulations have been performed by employing an in-house polarized radiative transfer code for raining systems ranging from depression and tropical cyclones to the Indian monsoon. For the sake of validation and completeness, simulations have also been done for the Tropical Rainfall Measuring Mission (TRMM)’s Microwave Imager (TMI) of the highly successful TRMM mission of NASA and JAXA. The paper is essentially divided into two parts: (a) Radiometer response with specific focus on high frequency channels in both the radiometers is discussed in detail with a parametric study of the effect of four hydrometeors (cloud liquid water, cloud ice, precipitating water and precipitating ice) on the brightness temperatures. The results are compared with TMI measurements wherever possible. (b) Development of a neural network-based fast radiative transfer model is elucidated here. The goal is to speed up the computational time involved in the simulation of brightness temperatures, necessitated by the need for quick and online retrieval strategies. The neural network model uses hydrometeor profiles as inputs and simulates spectral microwave brightness temperature at multiple frequencies as output. A huge database is generated by executing the in-house radiative transfer code for seven different cyclones occurred in North Indian Ocean region during the period 2001–2006. A part of the dataset is used to train the network while the remainder is used for testing purposes. For the purpose of testing, a typical scene from the southwest monsoon rain is also considered. The results obtained are very encouraging and show that the neural network is able to mimic the underlying physics of the radiative transfer simulations with a correlation coefficient of over 99%.     

基于NoSREx实验的积雪演化及微波辐射分析研究

在被动微波雪水当量反演中,积雪物理参数随时间的变化特征影响着反演精度,为理解积雪随时间演化的特征及其对微波辐射亮温的影响,本研究选用2009—2013年北欧积雪实验（Nordic Snow Radar Experiment, NoSREx）积雪地面观测和微波辐射测量数据,通过雪深和温度把雪期分为积累期（10月—次年2月）、稳定期（2—4月）和消融期（4—5月）,发现各个雪期的积雪演化特征为：雪颗粒形状在积累期前期以融态颗粒（Melt Forms, MF）为主,积累期后期和稳定期以圆形颗粒、片状颗粒、深霜为主,消融期以MF为主;整个雪季底层雪粒径从小变大再变小的过程,粒径最大值出现在稳定期的2至3月,约为2.5~4.0 mm,均出现在近地表雪层,而表层粒径较小且较为稳定。通过雪深和微波亮度差（18~37 GHz）的关系分析,表明亮温差在不同雪期对于雪深的依赖关系不同,在积累期和稳定期,雪深变化与亮温差变化具有明显的正相关;在消融期由于积雪融化的影响,其相关性较差;基于多层积雪微波辐射模型（MEMLS）构建了一维微波辐射模拟环境,模拟表明MEMLS模型在3个雪期的垂直极化10.65 GHz和18.7 GHz模拟结果较37 GHz和90 GHz更好;10.65 GHz V极化在入射角为50°且稳定期时,微波亮温模拟均方根误差（RMSE）结果最小,为2.49 K。3个雪期90 GHz模拟结果水平极化优于垂直极化,由于受表层积雪变化影响,90 GHz模拟结果较不稳定,尤其是消融期时,RMSE最小也达到了42.7 K。本研究有助于理解积雪随时间演化的特征及其对微波辐射模拟的影响,表明在被动微波雪水当量反演算法中,针对不同积雪期需要考虑积雪演化动态过程。     

Fluxes of heat and momentum over sea surface during the passage of a depression in the north Bay of Bengal

Time variation of surface fluxes of heat, moisture and momentum over a sea station (20°N 89°E) in the north Bay of Bengal has been computed by profile method for the period 18th–25th August 1990 using meteorological data of MONTBLEX-90 from ORVSagarkanya. The fluxes showed synoptic and diurnal variations which are marked during depression (20th–21st August) compared to their variation prior to and after this period. Variations of heat and water vapour fluxes were in phase. Night time fluxes are relatively high compared to day time. Average momentum transfer during depression was two to three times large. Variations in Bowen ratio were relatively large during day time. During depression, it varied between 0·2 in day time and about 0·3 at night and in the undisturbed period between ?0·1 and 0·2 during day time and 0·2 and 0·25 at night. The study shows that the assumptionC _D=C_H=C_E of the exchange coefficients normally used in estimating the fluxes by the bulk aerodynamic method is not appropriate becauseC _H/C_D≈2,C _E/C_D≈1·5 andC _H/C_E≈1·4.     

Observations of mid-troposhheric circulation from water vapour radiances of polar orbiting satellites

Water vapour tracers can provide useful information on winds at ≈ 500mb by observing the 6·7μ radiances. This fills the data gap in the cloud motion winds provided by conventional meteorological geostationary satellites. There is no geostationary satellite at present over the Indian Ocean with 6·7μ imaging capability to provide mid-tropospheric winds. The potentials of 6·7μ radiances, available from polar orbiting satellites, for mid-tropospheric circulation features have been examined in this study. Tiros-N satellite data of May 1979 and ECMWF level-IIIb wind data were analysed to relate the radiances with the streamlines. We find that the radiances of 6·7μ from orbiting satellites agree well with the wind field.     

Structure of the class I methanol masers OMC-2 and NGC 2264

Results of interferometric observations of the class I methanol masers OMC-2 and NGC 2264 in the 7₀-6₁ A ⁺ and 8₀-7₁ A ⁺ lines at 44 and 95 GHz, respectively, are presented. The maser spots are distributed along the arcs bent toward infrared sources, which are young stellar objects. The distributions of the maser spots at 44 and 95 GHz are virtually identical, and the fluxes from the brightest spots are similar. The measured sizes of the maser spots at 44 GHz are, on average, about 50 AU. The brightness temperature of the strongest components at 44 GHz is 1.7 × 10⁷ K and 3.9 × 10⁷ K for OMC-2 and NGC 2264, respectively. A simple model for the excitation of Class I methanol masers is proposed; it yields an estimate of the limiting brightness temperature of the emission. The model is based solely on the properties of the methanol molecule without invoking the physical parameters of the medium. Using it, we showed that the emission opening angles for NGC 2264 and OMC-2 do not exceed 3° and 4.5°, respectively. The depth of the masing region is about 1000 AU. The emission directivity is naturally realized in the model of of maser consisting of a thermalized core and a thin inverted envelope, probably, with an enhanced methanol abundance. The maser emission has the greatest intensity in the direction tangential to the envelope. The size of the masing envelope estimated from the measured depth and spot extens is ～2 × 10⁴ AU, or 0.15 pc. This size is close to the sizes of the dense molecular cores surrounding the young stellar objects IRS 4 in OMC-2 and IRS 1 in NGC 2264.     

Seasonal evolution of the isotopic composition of atmospheric water vapour above a tropical lake: Deuterium excess and implication for water recycling

We present an analysis of the rainfall-evaporation-atmospheric moisture cycle in a semi-arid tropical zone (southwestern Madagascar) to quantify the recycling and mixing processes that occur above an endorheic lake system (Lake Ihotry) during an annual cycle. The study combines an isotope mass balance with a detailed field investigation of the lake system and a previously established daily time-step lake water balance model. The mass balance and Craig-Gordon equations are used to calculate the isotopic composition of the evaporative flux from the lake surface (δ_E) and to derive a daily time series of the ambient atmospheric water vapour composition above the lake (δ_AL) during a 8-month dry season. Calculated δ_AL results from a mixing between regional moisture (δ_AR) and locally evaporated water (δ_E), the latter representing 50% of δ_AL at the end of the dry season. The contribution of recycled moisture to on-lake precipitation during the wet season is estimated to ?16%. We show that, as expected, the deuterium excess is high in recycled precipitation and low in evaporated precipitation, but also that the recycled moisture in an endorheic system may have a low deuterium excess resulting from the low deuterium excess in regional precipitation. In case of a long evaporative season, the atmospheric moisture is not in isotopic equilibrium with the annual composition of precipitation because of the contribution of the recycled vapour to the local atmospheric pool. Our approach demonstrates the importance of water recycling on the atmospheric moisture cycle and precipitation in a tropical semi-arid system, and can be applied to other natural systems, enlarging the potential range of investigation of the atmospheric vapour cycle and rainfall sources in tropical lands. It may also represent a valuable complement to direct water vapour sampling, in yielding the long-term evolution of the atmospheric vapour composition with spatially averaged values and smoothed temporal variations.     

Western disturbances seen with AMSU-B and infrared sensors

Western disturbances (WD) of winter and pre-monsoon seasons are the important sources of rainfall in the Indo-Gangetic plains. WDs are troughs or circulations in the westerly winds modified by the Himalayas. Operationally, WDs are monitored using infrared (IR) and water vapour (WV) images. Advanced Microwave Sounding Unit-B (AMSU-B), flying onboard the NOAA satellites, also allows WDs to be monitored in five microwave frequencies. Two are in water vapour window (89, 150 GHz) and three are absorption channels (centred at 183.31 GHz). Unlike the top of cloud view in IR or WV, AMSU-B radiances show the effect of moisture and hydrometeors in different layers. Two cases of WD (17 April 2001 and 18–19 February 2003) are discussed using the microwave data from AMSU-B and the IR and WV data from Meteosat-5. The aim here is to demonstrate the skill of AMSU-B in delineating structure of WDs. In particular, the cold intrusion and the moist conveyor belts are examined. It was found that the multi-channel view of the AMSU-B permits a better understanding of the moist structures seen in the conveyor belts. The à trous wavelet transform is used to clearly bring out mesoscale features in WDs. AMSU-B brings out intense convection as a large depression of BTs (>50K) at 150/176 GHz, cirrus and moist bands at 180/182 GHz. Mesoscale convection lines within WDs that last short time are shown here for the first time only in the AMSU-B images. Large-scale cirrus features are separated using the à trous wavelet transform. Lastly, it is shown that there is a good likeness in the rain contours in the 3-h rain 3B42 (computed from TRMM and other data) to AMSU-B depressions in BT. Overall, AMSU-B shows better skill in delineating the structure of clouds and rain in WDs.     

Variability of the Blazar J1504+1029 on Timescales from Hours to Years

The results of observations of the blazar J1504+1029 (PKS 1502+106, OR 103), obtained in 2000–2018 on the RATAN-600 radio telescope of the Special Astrophysical Observatory at 2.3, 3.9 (4.7), 7.7 (8.2), 11.2, and 21.7 GHz and on the 32-m Zelenchuk and Badary radio telescopes of the Quasar-KVO complex of the Institute of Applied Astronomy of the Russian Academy of Sciences at 5.05 and 8.63 GHz are presented. The long-term variability is studied, as well as variability on time scales from several days to several weeks and intraday variability (IDV). The long-term light curves are correlated at all frequencies and show continuous activity, against which three flares with their maxima in 2002, 2009, and 2018 are distinguished. The time scale for variability of the flare in 2009 is τ_var ≈ 1 year. At 21.7 GHz, the linear size of the emitting region is R ≤ 0.3 pc, its angular size is θ ≤ 0.05 mas, its brightness temperature is T_b ≥ 2 × 1014 K, and the Doppler factor is δ ≥ 5.8. The flare with its maximum in 2018 has a long rising branch at 21.7 GHz: τ_var = 3.2 years, linear size R ≤ 1.1 pc, angular size θ ≤ 0.17 μas, brightness temperature T_b ≥ 2.2 × 10¹² K, and Doppler factor δ ≥ 2.8. Among eleven sets of daily observations of the source over 75–120 days in 2000–2017, variability was detected in eight data sets at two to four frequencies with characteristic time scales of 4–30 days. In seven data sets, the variability is due to one to three cyclic processes with characteristic time scales τ_acf = 4?30^d. The spectral indices of the variable components in different years vary from α_var = ?1.6 to +1.8. In at least four data sets, the variability is due to processes in the source itself. In this case, at 21.7 GHz, the apparent linear size of the emitting region is ≤4000 AU, the angular size is θ ≤ 3.5 μas, the brightness temperature is T_b ≥ 3 × 10¹⁴ K, and the Doppler factor is δ ≥ 14. In the 2004 data set, the variability has an “ anti-flare” form, with the flux density of the variable component falling at high frequencies. Thirty-six successful sessions were conducted on the 32-m telescopes at 8.63 GHz, and 16 at 5.05 GHz. IDV was detected in 17 sessions at 8.63 GHz and in three sessions at 5.05 GHz, with the IDV being detected mainly near flare maxima.

     

Evaluation of brightness temperature from a forward model of ground-based microwave radiometer

Ground-based microwave radiometers are getting great attention in recent years due to their capability to profile the temperature and humidity at high temporal and vertical resolution in the lower troposphere. The process of retrieving these parameters from the measurements of radiometric brightness temperature (T _B ) includes the inversion algorithm, which uses the back ground information from a forward model. In the present study, an algorithm development and evaluation of this forward model for a ground-based microwave radiometer, being developed by Society for Applied Microwave Electronics Engineering and Research (SAMEER) of India, is presented. Initially, the analysis of absorption coefficient and weighting function at different frequencies was made to select the channels. Further the range of variation of T _B for these selected channels for the year 2011, over the two stations Mumbai and Delhi is discussed. Finally the comparison between forward-model simulated T _B s and radiometer measured T _B s at Mahabaleshwar (73.66 ^°E and 17.93^°N) is done to evaluate the model. There is good agreement between model simulations and radiometer observations, which suggests that these forward model simulations can be used as background for inversion models for retrieving the temperature and humidity profiles.     

Aircraft measurements of SST using IR-radiometer and temperature corrections due to atmospheric effects

Using an airborne scanning IR-radiometer, measurements of sea surface temperature (SST) were made from nine different levels in the Sandheads region of the Bay of Bengal on 5 October 1978. To retrieve SST from the observed radiances a temperature correction scheme, which uses the radiosonde data in the vicinity of flight area, has been generated. Atmospheric effects which have been considered include absorption due to water vapour and carbon dioxide, and the re-emission from different atmospheric layers. The radiances observed at different altitudes when corrected by our scheme yield a fairly consistent value of SST. The special ship measurements of SST, at the same location, are found to have very good agreement with the SST retrieved from the observed radiances using our scheme. The temperature corrections turn out to be 0·3 and 3·3°C at 600 and 3000 meters respectively for the type of atmosphere which has been used in our study.     

Methanol maser groups and class I emission: Flux density distribution

A search for a relationship between class I and class II methanol maser flux densities has been carried out. A large sample of mixed-type sources has been studied, with each source in the sample radiating as a class I and class II maser simultaneously. In methanol maser groups for which the positions of prominent spectral features at different radial velocities coincide at different frequencies, the fluxes are anticorrelated, and are related as log S _6.7+12.2 = (?1.68 ± 0.38) × log S ₄₄ + (4.01 ± 0.60). For group I, which includes sources with preferred pumping for masers emitting at 6.7 GHz, the relationship between the 6.7 GHz masers and 44 GHz masers is less steep than for group II, which contains sources with normal pumping of class II masers. This implies that class I methanol masers that correspond to group I are suppressed more strongly.     

冻土遥感研究进展:被动微波遥感

多年冻土和季节冻土分别占北半球裸露地表的24%和55%.近地表土壤冻融的范围、冻结起始日期、持续时间及冻融深度对寒季/寒区植物生长、大气与土壤间能量、水分及温室气体交换都具有极其重要的影响.自20世纪70年代以来,应用卫星遥感结合地面观测资料研究局地到区域尺度的季节冻土和多年冻土已取得诸多成果,而遥感在冻土研究中的最直接应用是利用微波探测近地表土壤冻融状态.相对于主动sAR,星载被动微波传感器具有多通道观测且重访周期较高,空间分辨率很低的特点.重点评述了近几十年来被动微波辐射计在冻土研究中的算法发展及其应用前景,主要包括双指标算法、时间序列变化检测算法及判别树算法3类,其核心均是基于冻土的低温特征和"体散射变暗"效应.发展可靠实用的微波遥感土壤冻融状态判别算法,提供区域和全球尺度上的土壤冻融状态信息,对水文学、气象学以及农业科学、工程地质研究与应用都具有重要意义.     

Atmospheric temperature profile for a tropical region from TIROS-N microwave sounding unit

Atmospheric temperature profiles have been derived using 53·74 GHz, 54·96 GHz and 57·95 GHz channels data from the Microwave Sounding Unit onboard TIROS-N. For this purpose regression coefficients have been derived using MONEX-79 radiosonde data during the period May to July 1979. The temperature profiles derived at a few selected places were closer to radiosonde profiles than the profiles derived using standard regression coefficients for the tropical region supplied by NOAA.     

The solubility and speciation of molybdenum in water vapour at elevated temperatures and pressures: Implications for ore genesis

The solubility of molybdenum trioxide in liquid-undersaturated water vapour has been investigated experimentally at 300, 320, and 360 °C and 39-154 bars. Results of these experiments show that the solubility of MoO₃ in water vapour is between 1 and 29 ppm, which is 19-20 orders of magnitude higher than the vapour pressure of MoO₃(g). Molybdenum solubility increases exponentially with f_H2O, suggesting the formation of a gaseous hydrated complex of the type MoO₃·nH₂O by the reaction:

(A.1)     

Utility of DMSP-SSM/I for integrated water vapour over the Indian seas

Recent algorithms for Special Sensor Microwave/Imager (DMSP-SSM/I) satellite data are used for estimating integrated water vapour over the Indian seas. Integrated water vapour obtained from these algorithms is compared with that derived from radiosonde observations at Minicoy and Port Blair islands. Algorithm-3 of Schlussel and Emery (1990) performed best. On the basis of this algorithm, distribution of integrated water vapour is determined during the monsoon depression (22nd–27th July, 1992) that formed over the Bay of Bengal.     

非均匀月壤介质的被动微波辐射传输模拟

基于非均匀月壤物理模型和辐射传输方程,模拟月壤介质中的微波辐射传输特性,探讨频率、月壤厚度等与月表亮温的关系。结果表明:在低频段,月壤微波辐射亮温的动态变化范围较大,可探测的月壤厚度大,3 GHz时的最大可探测月壤厚度达12.4 m;在高频段对应的可探测月壤厚度较小,特别是从50GHz往后的频率段内,最大可探测月壤厚度均小于2 m。不同频率的亮温-厚度变化曲线没有交叉点,且频率越高,所能探测的月壤厚度越小。根据模拟结果,建立了月壤厚度与亮温的查找表。基于查找表,利用单个波段的亮温数据即可得到月壤厚度信息。     

The solubility of molybdenum dioxide and trioxide in HCl-bearing water vapour at 350 °C and pressures up to 160 bars

The solubility and speciation of the assemblage MoO₂-MoO₃ in water vapour were investigated in experiments conducted at 350 °C, P_total from 59 to 160 bar and fHCl from 0 to 3.4 bar (0-2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑fMo from 4.4 × 10⁻⁴ to 6.5 × 10⁻² bar). The concentration of Mo in the vapour at fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at fHCl below 0.1 bar, but increases with increasing fHCl above this pressure. The dominant Mo species at fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reaction

(A1)