Dual-channel Microwave Radiometer Research Articles

Supercooled Liquid Water In Winter Storms: A Preliminary Climatology from Remote Sensing Observations

Drawing from a remote sensing data base obtained from winter storm research programs in Colorado and Utah, the occurrence of supercooled liquid water over mountain barriers is examined. Combined polarization lidar and dual-channel microwave radiometer data reveal that liquid water was nearly always presenting the storms studied. Moreover, the highest frequency of occurrence and liquid water amounts were most often associated with relatively warm cloud base temperatures and Ku-band radar reflectivity factors between 0 and -10 dBZ at the liquid cloud base position. A preliminary climatology of super cooled liquid water in southern Utah reveals a bimodal distribution of liquid cloud base heights, representing of convective clouds (-3.0 km MSL) and generally prefrontal stratiform clouds (~4.5 km MSL). Although the liquid water associated with the efficient natural generation of precipitation may not always be detected by any single probe, the conditions which could be expected to yield a favorable seeding response can be identified through joint remote sensing observations.

Retrieval of effective radius and liquid water path from ground‐based instruments: A case study at Barrow, Alaska

Two methods for retrieving cloud droplet effective radius re from ground‐based near‐infrared spectral measurements of path‐integrated liquid water paths (PLWPs) are described. In one method the PLWP is compared with column measurements of liquid water path (LWP) from a dual channel microwave radiometer (MWR) to estimate the cloud path enhancement, which is then used to derive the cloud droplet effective radius. In the second method, PLWP is combined with absolutely calibrated zenith radiances at 500 nm to retrieve re and LWP simultaneously. Both techniques are used in a case study of marine stratocumulus at the Barrow, Alaska (71.32°N, 156.62°W) Atmospheric Radiation Measurement Program (ARM) site on 17 September 2004. The first method performed best for moderately thick clouds (LWP ≥ 100 g m−2), but the accuracy is limited by uncertainties in the MWR LWP on which it relies. The second method performed well over a wider range of values with 1σ retrieval errors of <4 g m−2 (∼4%) and ∼3 μm (∼7%) for 15 ≤ LWP ≤ 170 g m−2. The LWPs retrieved using the radiance‐PLWP method were highly correlated (r2 = 0.96) with LWPs from the MWR (with a bias subtracted) derived using the ARM statistical method. A limited comparison (LWP < 100 g m−2) to millimeter wave cloud radar showed that values of re retrieved using the radiance‐PLWP method were consistently higher (by ∼3 μm) than the LWC‐weighted mean re from the radar. Additional field studies are needed to resolve this discrepancy, although this first comparison is promising.

Measurements and Retrievals From a New 183-GHz Water-Vapor Radiometer in the Arctic

A new G-band (183.31-GHz) water-vapor radiometer, developed and built by ProSensing Inc., was deployed in Barrow, AK, in April 2005. The radiometer is part of a suite of instruments maintained by the Atmospheric Radiation Measurement program. The instrument measures brightness temperatures from four double-sideband channels centered at plusmn1, plusmn3, plusmn7, and plusmn14 GHz from the 183.31-GHz water-vapor line. Atmospheric emission in this spectral region is primarily due to water vapor, with some influence from liquid water. In this paper, data collected in November 2005, December 2005, and January 2006 are analyzed. Measurements are compared with simulations obtained by using a radiative transfer model. We show that the measurements agree well with model simulations. Precipitable water vapor (PWV) and liquid water path (LWP) are retrieved with a nonlinear physical algorithm, and results are compared with those from the colocated dual-channel microwave radiometer and radiosondes. Retrieval errors are estimated to be below 5% for PWV and of the order of 0.006 mm for LWP.

Climatology of Warm Boundary Layer Clouds at the ARM SGP Site and Their Comparison to Models

Abstract A 4-yr climatology (1997–2000) of warm boundary layer cloud properties is developed for the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site. Parameters in the climatology include cloud liquid water path, cloud-base height, and surface solar flux. These parameters are retrieved from measurements produced by a dual-channel microwave radiometer, a millimeter-wave cloud radar, a micropulse lidar, a Belfort ceilometer, shortwave radiometers, and atmospheric temperature profiles amalgamated from multiple sources, including radiosondes. While no significant interannual differences are observed in the datasets, there are diurnal variations with nighttime liquid water paths consistently higher than daytime values. The summer months of June, July, and August have the lowest liquid water paths and the highest cloud-base heights. Model outputs of cloud liquid water paths from the European Centre for Medium-Range Weather Forecasts (ECMWF) model and the Eta Model for 104 model output location time series (MOLTS) stations in the environs of the SGP central facility are compared to observations. The ECMWF and MOLTS median liquid water paths are greater than 3 times the observed values. The MOLTS data show lower liquid water paths in summer, which is consistent with observations, while the ECMWF data exhibit the opposite tendency. A parameterization of normalized cloud forcing that requires only cloud liquid water path and solar zenith angle is developed from the observations. The parameterization, which has a correlation coefficient of 0.81 with the observations, provides estimates of surface solar flux that are comparable to values obtained from explicit radiative transfer calculations based on plane-parallel theory. This parameterization is used to estimate the impact on the surface solar flux of differences in the liquid water paths between models and observations. Overall, there is a low bias of 50% in modeled normalized cloud forcing resulting from the excess liquid water paths in the two models. Splitting the liquid water path into two components, cloud thickness and liquid water content, shows that the higher liquid water paths in the model outputs are primarily a result of higher liquid water contents, although cloud thickness may a play a role, especially for the ECMWF model results.

Cloud Type and Macrophysical Property Retrieval Using Multiple Remote Sensors

A cloud detection algorithm based on ground-based remote sensors has been developed that can differentiate among various atmospheric targets such as ice and water clouds, virga, precipitation, and aerosol layers. Standard cloud type and macrophysical properties are identified by combining polarization lidar, millimeter-wave radar, infrared radiometer, and dual-channel microwave radiometer measurements. These algorithms are applied to measurements collected during 1998 from the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed site in north-central Oklahoma. The statistical properties of clouds for this year are presented, illustrating how extended-time remote sensing datasets can be converted to cloud properties of concern to climate research.

The Mesoscale Structure of a Nocturnal Dryline and of a Frontal–Dryline Merger

Abstract A horizontal gradient in moisture, termed the dryline, is often detected at the surface over the southern Great Plains of the United States during the spring and early summer. The dryline exhibits distinct diurnal variations in both its movement and structure. Recent research has focused on dryline structure during the afternoon and evening, particularly showing how strong (∼1–5 m s−1) ascent frequently creates an environment favorable to the initiation of convection, quite close (within ∼10 km) to the dryline interface. To date, however, there have been very few detailed analyses of the dryline interface at night, so that the nocturnal behavior of the interface predicted by theory and numerical studies is relatively poorly evaluated. In this study, special observations taken by a Doppler lidar, serial rawinsonde ascents, and a dual-channel microwave radiometer are utilized to describe the behavior of a nocturnal dryline observed on 12–13 May 1985. The analysis presented here reveals that the mes...

Continental Stratus Clouds: A Case Study Using Coordinated Remote Sensing and Aircraft Measurements

Abstract A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments and aircraft probes on 30 April 1994 from the Cloud and Radiation Testbed site in north-central Oklahoma. The boundary layer structure clearly resembled that of a cloud-topped mixed layer, and the cloud content is shown to be near adiabatic up to the cloud-top entrainment zone. A cloud retrieval algorithm using the radar reflectivity and cloud droplet concentration (either measured in situ or deduced using dual-channel microwave radiometer data) is applied to construct uniquely high-resolution cross sections of liquid water content and mean droplet radius. The combined evidence indicates that the 350–600 m deep, slightly supercooled (2.0° to −2.0°C) cloud, which failed to produce any detectable ice or drizzle particles, contained an average droplet concentration of 347 cm−3, and a maximum liquid water content of 0.8 g m−3 and mean droplet radius of 9 μm near cloud top. Lidar data indicate that the Ka...

Remote Sensing of Tropospheric Water Vapor and Cloud Liquid Water by Integrated Ground-Based Sensors

Abstract A technique is presented for deriving tropospheric water vapor and cloud liquid water, as well as temperature, from a suite of ground-based sensors. Included in the suite are a dual-channel microwave radiometer, a ceilometer, a radio acoustic sounding system (RASS), and conventional surface meteorological instruments. A linear statistical inversion algorithm, combined with a data classification technique, is applied to retrieve water vapor and cloud liquid water profiles. The linear statistical inversion algorithm is also applied to derive temperature profiles from RASS virtual temperature measurements and surface meteorological parameters. A physical retrieval algorithm is then applied to retrieve integrated water vapor and liquid water. Finally, these two algorithms are coupled in a two-step iteration process. The technique is evaluated by comparing retrieved quantities with radiosonde measurements and by comparing this technique with the traditional technique based solely on dual-channel micro...

Microwave radiometry of the atmosphere: an experiment from a sea-based platform during ERS-1 altimeter calibration

Abstract A steerable dual-channel microwave radiometer was installed on top of an oceanographic tower ofT shore from the Venice lagoon ( Italy) during the calibration experiment of the ERS-1 radar altimeter organized by the European Space Agency. Measurements of atmospheric brightness temperature were carried out over more than two months in the summer 1991. The main objective of the experiment was to obtain the tropospheric electromagnetic excess path length ( AW) required for the correction of the radar altimeter on board the satellite. The instrument calibration and the parameter retrieval algorithms were the main problems to be addressed. The retrieval of other meterological parameters, namely the precipitable water vapour ( K ) and the integrated cloud liquid ( L) was also assessed. An accuracy ( r.m.s.) of about 0-8cm in AH, 01cm in Vand 004mm in L is expected, resulting in a significant improvement when compared to those available from conventional meteorological data.

Relationship between Ice Water Path and Downward Longwave Radiation for Clouds Optically Thin in the Infrared: Observations and Model Calculations

A vertically pointing 3.2-cm radar is used to observe altostratus and cirrus clouds as they pass overhead. Radar reflectivities are used in combination with an empirical Z(sub i)-IWC (ice water content) relationship developed by Sassen (1987) to parameterize IWC, which is then integrated to obtain estimates of ice water path (IWP). The observed dataset is segregated into all-ice and mixed-phase periods using measurements of integrated liquid water paths (LWP) detected by a collocated, dual-channel microwave radiometer. The IWP values for the all ice periods are compared to measurements of infrared (IR) downward fluxes measured by a collocated narrowband (9.95-11.43 microns) IR radiometer, which results in scattergrams representing the observed dependence of IR fluxes on IWP. A two-stream model is used to calculate the infrared fluxes expected from ice clouds with boundary conditions specified by the actual clouds, and similar curves relating IWP and infrared fluxes are obtained. The model and observational results suggest that IWP is one of the primary controls on infrared thermal fluxes for ice clouds.

Study on clouds and marine atmospheric boundary layer

A set of remote sensing instruments of Peking University, which includes mainly a dual-channel(22.235GHz and 35.5GHz) microwave radiometer, a 8mm microwave and a 5mm microwave radiometer, has been developed for the Western North-Pacific Cloud-Radiation Experiment (WENPEX). The instruments were used to observe the cloud and marine atmospheric boundary-layer in the southwest sea area of Japan in winter time from 1989 to 1991. In the weather change process, the characteristics of the marine atmospheric boundary-layer and liquid water content in cloud of this area in winter time are studied from observation data. A one-dimensional mixed layer model is presented for the growth and evolution of a cloud-topped marine boundary-layer. The model is used to study in the WENPEX. The simulation results are in agreement with observation data, especially the integral water in cloud.

Field study of the potential for winter precipitation enhancement in the Australian Snowy Mountains

A research program was initiated in 1988 to investigate the potential for winter snowpack enhancement in the Snowy Mountains of Australia. Field studies were conducted during the austral winters of 1988 and 1989 in order to characterize the water and ice composition of clouds passing over these mountains. We utilized a dual-channel microwave radiometer and a Ka-band (8.6 mm wavelength) radar in a coordinated observational program to document the distribution of supercooled liquid water, ice phase precipitation and water vapor in the winter storm systems. The vertical temperature, humidity and wind profiles were obtained at high temporal resolution from local rawinsonde launches. Ground-based sampling of the snow crystal types and the stable isotopic composition of the snow provided information on the processes of crystal formation and riming during these storms. A precipitation trajectory model was used to simulate ice crystal growth for snow which precipitates within the study region. Analysis of these data indicate buildup of supercooled liquid water during certain storm periods, while during other periods the ice crystal growth processes efficiently remove cloud liquid. Several aspects of the cloud seeding potential are discussed in this paper and the overall conclusion is that winter precipitation enhancement is scientifically feasible, based upon the World Meteorological Organization criterion of supercooled liquid water availability.

An Algorithm for Operational Water Vapor Analyses Integrating GOES and Dual-Channel Microwave Radiometer Data on the Local Scale

Abstract An algorithm for the operational integration of satellite-derived and ground-based passive microwave precipitable-water measurements is presented. The technique merges the two data types by relying on the more. accurate ground-based data to correct the bias and scale the satellite field, thus providing an improved meso-β-scale analysis. The complete algorithm is described from the preparation of the derived satellite imagery, used here as a data source, to the optimization routine used to integrate the data. The technique is now used for a local-scale analysis of the Denver area. One case is presented illustrating the effectiveness of the technique to track a weak moisture gradient through eastern Colorado. Other cases demonstrate the performance of the analysis in varying topography.

Investigations of a Winter Mountain Storm in Utah. Part II: Mesoscale Structure, Supercooled Liquid Water Development, and Precipitation Processes

Abstract A comprehensive analysis of a deep winter storm system during its passage over the Tushar Mountains of southwestern Utah is reported. The case study, drawn from the 1985 Utah/NOAA cooperative weather modification experiment, is divided into descriptions of the synoptic and kinematic properties in Part I, and storm structure and composition here in Part II. In future parts of this series, the turbulence structure and indicated cloud seeding potential will be evaluated. The analysis presented here in Part II focuses on multiple remote sensor and surface microphysical observations collected from a midbarrier (2.57 km MSL) field site. The collocated remote sensors were a dual-channel microwave radiometer, a polarization lidar, and a Ka-band Doppler radar. These data are supplemented by upwind, valley-based C-band Doppler radar observations, which provided a considerably larger-scale view of the storm. In general, storm properties above the barrier were either dominated by barrier-level orographic clo...

The Characteristics and Evolution of Supercooled Water in Wintertime Storms over the Sierra Nevada: A Summary of Microwave Radiometric Measurements Taken during the Sierra Cooperative Pilot Project

A comprehensive examination of the evolution and vertical distribution of supercooled liquid water in Sierra Nevada winter storms was completed as part of the Sierra Cooperative Pilot Project. Measurements of supercooled water in 63 storms, which affected the area from the 1983/84 through the 1986/87 winter field seasons, were made with a dual-channel microwave radiometer located near the Sierra Nevada crestline. The analyses were carried out in two parts. In the first part, winter storms were grouped into two general categories according to the prevailing flow and resultant storm trajectory. The two storm types consisted of those propagating in zonal flow and those in meridional flow. A complete description of the storm typing is provided. Storms in zonal flow were either developing, occluding, or dissipating during the time they affected the Sierra Nevada. The stage of evolution of the storm dictated the characteristics of supercooled water observed during the storm's passage over the Sierra Nevada. The presence of supercooled water in meridional storms was influenced mainly by the trajectory of the storm. Results indicated that zonal storms had the most sustained periods of supecooled water in the postfrontal region, while sustained supercooled water measurements in meridional storms were usually greatest within the prefrontal portions of the storm. These periods of supercooled water were associated primarily with orographic clouds. Studies of the vertical supercooled water distribution showed that saturation often existed within 1 km of the local terrain when supercooled water was measured by the radiometer. Fifty percent of the rawinsondes launched through supercooled water-bearing clouds measured water-saturated layers deeper than 0.5 km. Saturated layers were found to exist most often between −8° and and −10°C, although they were often present over a much wider temperature range. Conditions favorable for ice crystal growth were common, which supports the pursuit of precipitation enhancement as a means of supplementing regional water supplies.

Field Evaluation of a Dual-Channel Microwave Radiometer Designed for Measurements of Integrated Water Vapor and Cloud Liquid Water in the Atmosphere

Abstract The dual-channel microwave radiometer is evaluated in regard to the measurement of integrated water vapor and supercooled liquid water. The study includes comparisons of integrated vapor content measured by the radiometer and rawinsondes as part of the Sierra Cooperative Pilot Project in the central Sierra Nevada. In addition, two radiometers with virtually identical characteristics were brought together on the Colorado Orographic Seeding Experiment at Steamboat Springs, Colorado, in order to study the stability and comparability of integrated vapor and liquid measurements. Comparison of vapor measurements by the radiometer and by rawinsonde yielded a correlation coefficient of 0.94, and a rms difference of 0.08 cm. There were no significant differences between results for paired data gathered in storms in the presence of supercooled liquid water and for paired data gathered on clear days under more ideal conditions. The collocated-radiometer experiment showed slightly closer agreement than did t...

Multiple Remote Sensor Observations of Supercooled Liquid Water in a Winter Storm at Beaver, Utah

Abstract The temporal and spatial distribution of cloud liquid water in a winter storm from the 1983 Utah/NOAA Cooperative Weather Modification Program is characterized using remote sensing observations. The remote sensors, located at a mountain-base site near Beaver, Utah, consisted of a dual-channel microwave radiometer operated in an azimuthal scanning mode, and a polarization lidar and Ku-band radar both operated in the vertically pointing mode. The cloud system was associated with the passage of a weak cold front and produced only light snowfall across the barrier network of precipitation gages. Although the amounts of supercooled water detected radiometrically varied considerably during the storm, liquid water depths were consistently enhanced in the direction of the barrier. The spatial distribution of liquid water was observed to undergo a transition from a primarily orographic distribution to a more area-wide pattern immediately behind the front, and then became convective as the storm dissipated...

Computer demodulation technique for a dual-channel microwave radiometer

The three stairstep output of the video amplifier has now been designed for the NOAA/WPL dual-channel radiometer in order to provide an analog input to one A/D converter instead of to two phase-detectors, and the synchronous detection for data and AGC is done in software. We put the new system into the dual-channel radiometer which was set in Boulder Atmospheric Observatory (BAO) to make experiments for three weeks and found the processor reliable and flexible.

The Characteristics and Distribution of Cloud Water over the Mountains of Northern Colorado during Wintertime Storms. Part I: Temporal Variations

Abstract The spatial and temporal evolution of supercooled water fields in ten wintertime storm systems occurring over the northern Colorado Rocky Mountain region have been examined using data collected by the recently developed scanning dual-channel microwave radiometer. These data were supported by several independent datasets including vertically pointing radar data, mountaintop liquid water content measurements, low and high altitude measurements of crystal rime characteristic rawinsonde data and precipitation intensity measurements. The ten case studies discussed in this paper represent various stages in the synoptic scale evolution of storms that affect the northern Colorado Rockies. Liquid water was found to occur in nearly all stages of most of these storms. The temporal variations in the magnitude of the liquid water content were significant. Three common features concerning the evolution of the liquid water field were observed in the prefrontal cloud systems: 1) an inverse relationship between p...

Deep Orographic Cloud Structure and Composition Derived from Comprehensive Remote Sensing Measurements

Abstract Coordinated polarization lidar, Ku-band radar and dual-channel microwave radiometer observations of a deep orographic cloud system were collected from a mountain-base site in northwestern Colorado as part of the Colorado Orographic Seeding Experiment (COSE) research effort. The remote sensing observations are presented for three distinct storm stages, corresponding to a pre-frontal altostratus cloud layer, a local orographically-induced cloud development, and a peak in storm activity accompanying the passage of a weak cold front. Supercooled liquid water in the form of thin but often dense liquid layers, and expansive, more weakly mixed-phase cloud regions were usually present even to temperatures approaching −40°C. The liquid water amounts present were often below the detection threshold of the vertically-pointing radiometer measurements, but during one brief interval a liquid water content as high as 0.5 g m−3 may have occurred. The lidar depolarization data also show the presence of a persiste...