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Sergey Y. Matrosov
,
Alexei V. Korolev
, and
Andrew J. Heymsfield
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Sergey Y. Matrosov
,
Taneil Uttal
, and
Duane A. Hazen

Abstract

The performance of radar reflectivity (Z e )–based relations for retrievals of marine stratiform cloud liquid water content (LWC) is evaluated by comparing liquid water path (LWP) estimates from microwave radiometers with vertically integrated LWC values retrieved from radar measurements. Based on a measurement dataset from a research vessel in the tropical eastern Pacific Ocean, it is shown that reflectivity thresholding allows minimizing of the influence of drizzle drops present in marine stratiform clouds to the extent that LWP estimates from a ground-/shipborne radar can have uncertainties that might be acceptable for different applications. The accuracies of Z e -based retrievals depend on the thresholding level Z et, and they are generally better than a factor of 2 for Z et ≲ −15 dBZ. These accuracies typically improve when Z et is lowered; however, the amount of cloud profiles that pass thresholding diminishes as Z et is decreased from about 50% for a −15-dbZ threshold to only about 10% for a −25-dBZ threshold. Different thresholding strategies are considered. Ancillary information on cloud-base heights can improve LWP estimates from reflectivities. The ship-based dataset was used to simulate measurements from prospective 94-GHz spaceborne cloud radar (CloudSat). CloudSat measurements would, on average, detect about 75% of warm marine stratiform clouds, though many clouds with negligible presence of drizzle will be missed. Because of sensitivity and resolution issues for the spaceborne radar, reflectivity-based estimates of LWP are generally biased toward high values and have higher uncertainties when compared with the ground-based radar, for the same Z et.

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Matthew D. Shupe
,
Taneil Uttal
, and
Sergey Y. Matrosov

Abstract

An operational suite of ground-based, remote sensing retrievals for producing cloud microphysical properties is described, assessed, and applied to 1 yr of observations in the Arctic. All measurements were made in support of the Surface Heat Budget of the Arctic (SHEBA) program and First International Satellite Cloud Climatology Project Regional Experiment (FIRE) Arctic Clouds Experiment (ACE) in 1997–98. Retrieval techniques and cloud-type classifications are based on measurements from a vertically pointing 35-GHz Doppler radar, microwave and infrared radiometers, and radiosondes. The retrieval methods are assessed using aircraft in situ measurements from a limited set of case studies and by intercomparison of multiple retrievals for the same parameters. In all-liquid clouds, retrieved droplet effective radii Re have an uncertainty of up to 32% and liquid water contents (LWC) have an uncertainty of 49%–72%. In all-ice clouds, ice particle mean sizes D mean can be retrieved with an uncertainty of 26%–46% while retrieved ice water contents (IWC) have an uncertainty of 62%–100%. In general, radar-only, regionally tuned empirical power-law retrievals were best suited among the tested retrieval algorithms for operational cloud monitoring at SHEBA because of their wide applicability, ease of use, and reasonable statistical accuracy. More complex multisensor techniques provided a moderate improvement in accuracy for specific case studies and were useful for deriving location-specific coefficients for the empirical retrievals but were not as frequently applicable as the single sensor methods because of various limitations. During the yearlong SHEBA program, all-liquid clouds were identified 19% of the time and were characterized by an annual average droplet Re of 6.5 μm, LWC of 0.10 g m−3, and liquid water path of 45 g m−2. All-ice clouds were identified 38% of the time with an annual average particle D mean of 73 μm, IWC of 0.014 g m−3, and ice water path of 30 g m−2.

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Andrew J. Heymsfield
,
Sergey Y. Matrosov
, and
Norman B. Wood

Abstract

Microphysical data and radar reflectivities (Z e , −15 < Z e < 10 dB) measured from flights during the NASA Tropical Clouds, Convection, Chemistry and Climate field program are used to relate Z e at X and W band to measured ice water content (IWC). Because nearly collocated Z e and IWC were each directly measured, Z e IWC relationships could be developed directly. Using the particle size distributions and ice particle masses evaluated based on the direct IWC measurements, reflectivity–snowfall rate (Z e S) relationships were also derived. For −15 < Z e < 10 dB, the relationships herein yield larger IWC and S than given by the retrievals and earlier relationships. The sensitivity of radar reflectivity to particle size distribution and size-dependent mass, shape, and orientation introduces significant uncertainties in retrieved quantities since these factors vary substantially globally. To partially circumvent these uncertainties, a W-band Z e S relationship is developed by relating four years of global CloudSat reflectivity observations measured immediately above the melting layer to retrieved rain rates at the base of the melting layer. The supporting assumptions are that the water mass flux is constant through the melting layer, that the air temperature is nearly 0°C, and that the retrieved rain rates are well constrained. Where Z e > 10 dB, this Z e S relationship conforms well to earlier relationships, but for Z e < 10 dB it yields higher IWC and S. Because not all retrieval algorithms estimate either or both IWC and S, the authors use a large aircraft-derived dataset to relate IWC and S. The IWC can then be estimated from S and vice versa.

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Sergey Y. Matrosov
,
Patrick C. Kennedy
, and
Robert Cifelli

Abstract

Correcting observed polarimetric radar variables for attenuation and differential attenuation effects in rain is important for meteorological applications involving measurements at attenuating frequencies such as those at X band. The results of estimating the coefficients in the correction-scheme relations from dual-wavelength polarimetric radar measurements of rainfall involving attenuating and nonattenuating frequencies are described. Such coefficients found directly from measurements are essentially free from different assumptions about drop shapes, drop size distributions, and/or relations between different radar variables that are typically used in many attenuation and differential attenuation correction schemes. Experimentally based estimates derived using dual-wavelength radar measurements conducted during a project in northern Colorado indicate values of the coefficients in the attenuation–differential phase quasi-linear relations at X band in the approximate range of 0.20–0.31 dB deg−1. The corresponding coefficients in the differential attenuation–differential phase relations are in the range of 0.052–0.065 dB deg−1.

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Sergey Y. Matrosov
,
Peter T. May
, and
Matthew D. Shupe

Abstract

An attenuation-based method to retrieve vertical profiles of rainfall rate from vertically pointing Ka-band radar measurements has been refined and adjusted for use with the U.S. Department of Energy’s cloud radars deployed at multiple Atmospheric Radiation Program (ARM) test bed sites. This method takes advantage of the linear relationship between the rainfall rate and the attenuation coefficient, and can account for a priori information about the vertical profile of nonattenuated reflectivity. The retrieval method is applied to a wide variety of rainfall events observed at different ARM sites ranging from stratiform events with low-to-moderate rainfall rates (∼5 mm h−1) to heavy convective rains with rainfall rates approaching 100 mm h−1. The Ka-band attenuation-based retrieval results expressed in both instantaneous rainfall rates and in rainfall accumulations are compared to available surface data and measurements of a scanning C-band precipitation polarimetric radar located near the Darwin, Australia, ARM test bed site. The Ka-band retrievals are found to be in good agreement with the C-band radar estimates, which are based both on conventional radar reflectivity approaches and on polarimetric differential phase shift measurements. Typically, the C-band–Ka-band radar estimate differences are within the expected retrieval uncertainties. The magnitude of the Ka-band rainfall-rate estimate error depends on the retrieval resolution, rain intensity, and uncertainties in the profiles of nonattenuated reflectivity. It is shown that reasonable retrieval accuracies (∼15%–40%) can be achieved for a large dynamic range of observed rainfall rates (4–100 mm h−1) and the effective vertical resolution of about 1 km. The potential enhancements of the Ka-band attenuation-based method by including a priori information on vertical profiles of nonattenuated reflectivity and increasing the height range of the retrievals by using Ka-band polarization measurements are also discussed. The addition of the precipitation products to the suite of ARM hydrometeor retrievals can enhance the overall characterization of the vertical atmospheric column.

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Sergey Y. Matrosov
,
Alexei V. Korolev
, and
Andrew J. Heymsfield

Abstract

A remote sensing method is proposed for the retrievals of vertical profiles of ice cloud microphysical parameters from ground-based measurements of radar reflectivity and Doppler velocity with a vertically pointed cloud radar. This method relates time-averaged Doppler velocities (which are used as a proxy for the reflectivity-weighted particle fall velocities) to particle characteristic sizes such as median or mean. With estimated profiles of particle characteristic size, profiles of cloud ice water content (IWC) are then calculated using reflectivity measurements. The method accounts for the intrinsic correlation between particle sizes and parameters of the fall velocity–size relations. It also accounts for changes of particle bulk density with size. The range of applicability of this method encompasses ice-phase clouds and also mixed-phase clouds that contain liquid drops, which are small compared to ice particles, so the radar signals are dominated by these larger particles. It is, however, limited to the observational situations without strong up- and downdrafts, so the residual of mean vertical air motions is small enough compared to the reflectivity-weighted cloud particle fall velocities. The Doppler-velocity reflectivity method was applied to the data obtained with an 8.6-mm wavelength radar when observing Arctic clouds. Typical retrieval uncertainties are about 35%–40% for particle characteristic size and 60%–70% for IWC, though in some cases IWC uncertainties can be as high as factor of 2 (i.e., −50%, +100%). Comparisons with in situ data for one observational case yielded 25% and 55% differences in retrieved and in situ estimates of characteristic size and IWC, respectively. The results of the microphysical retrievals obtained from the remote sensing method developed here were compared with data obtained from the multisensor technique that utilizes combined radar–IR radiometer measurements. For pure ice-phase layers unobstructed by liquid clouds (i.e., conditions where the multisensor approach is applicable), the relative standard deviations between the results of both remote sensing approaches were about 27% for mean particle size and 38% for IWC, with relative biases of only 5% and 20%, respectively.

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Sergey Y. Matrosov
,
Kurt A. Clark
, and
David E. Kingsmill

Abstract

This article describes polarimetric X-band radar-based quantitative precipitation estimations (QPE) under conditions of low freezing levels when, even at the lowest possible elevation angles, radar resolution volumes at longer ranges are in melting-layer or snow regions while it rains at the ground. A specifically adjusted vertical-profile-of-reflectivity (VPR) approach is introduced. The mean VPR is constructed based on the range–height indicator scans, and the effects of smoothing of brightband (BB) features with range are accounted for. A principal feature of the suggested QPE approach is the determination of the reflectivity BB boundaries and freezing-level heights on a beam-by-beam basis using the copolar correlation coefficient ρ hv, which is routinely available from the X-band radar measurements. It is shown that this coefficient provides a robust discrimination among the regions of rain, melting hydrometeors, and snow. The freezing-level estimates made using ρ hv were within 100–200 m from the corresponding estimates of the 0° isotherm heights from radiosonde soundings. The suggested VPR approach with the polarimetric determination of the reflectivity BB boundaries was used for QPE during the wintertime deployment of the NOAA X-band radar as part of the 2006 Hydrometeorological Test Bed (HMT-06) field experiment in the California Sierra Nevada foothills. It is shown that this approach noticeably improves radar-rainfall accumulation estimates. The use of the HMT-06 mean X-band reflectivity–rain-rate (Z ehR) relation resulted in an approximately 65% relative standard deviation of radar estimates from the surface rain gauges if no VPR adjustments were made. Applying the VPR approach with polarimetric detection of the melting layer resulted in reduction of the corresponding relative standard deviation by about a factor of 2.

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Sergey Y. Matrosov
,
Matthew D. Shupe
, and
Irina V. Djalalova

Abstract

It is demonstrated that millimeter-wavelength radars that are designed primarily for cloud studies can be also used effectively for snowfall retrievals. Radar reflectivity–liquid equivalent snowfall rate (Ze–S) relations specifically tuned for Ka- and W-band radar frequencies are applied to measurements taken by vertically pointing ground-based 8-mm cloud radars (MMCR) that are designed for the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Program and by the nadir-pointing spaceborne 94-GHz CloudSat radar. Comparisons of the MMCR-based snowfall accumulations estimated during experimental events with no significant snowflake riming and controlled gauge measurements indicated an 87% standard deviation between radar and gauge data that is consistent with the uncertainties in the coefficients of the Ze–S relations resulting from variability in snowflake microphysical properties. Comparisons of CloudSat-based snowfall-rate retrievals in heavy snowfall were consistent with estimates from surface S-band precipitation surveillance radars made using algorithms that were specifically designed for use with these radars. A typical difference between the CloudSat and the S-band precipitation radar estimates of snowfall rate for approximately collocated resolution pixels was within a factor of 2, which is of the order of the uncertainty of each estimate. The results of this study suggest that the ground-based and satellite-borne radars operating at Ka and W bands can provide valuable retrieval information on vertical profiles of snowfall, which is an important component of the global water cycle. This information is particularly important in Arctic regions where precipitation information from other sources is scarce.

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Sergey Y. Matrosov
,
David E. Kingsmill
,
Brooks E. Martner
, and
F. Martin Ralph

Abstract

The utility of X-band polarimetric radar for quantitative retrievals of rainfall parameters is analyzed using observations collected along the U.S. west coast near the mouth of the Russian River during the Hydrometeorological Testbed project conducted by NOAA’s Environmental Technology and National Severe Storms Laboratories in December 2003 through March 2004. It is demonstrated that the rain attenuation effects in measurements of reflectivity (Z e) and differential attenuation effects in measurements of differential reflectivity (Z DR) can be efficiently corrected in near–real time using differential phase shift data. A scheme for correcting gaseous attenuation effects that are important at longer ranges is introduced. The use of polarimetric rainfall estimators that utilize specific differential phase and differential reflectivity data often provides results that are superior to estimators that use fixed reflectivity-based relations, even if these relations were derived from the ensemble of drop size distributions collected in a given geographical region. Comparisons of polarimetrically derived rainfall accumulations with data from the high-resolution rain gauges located along the coast indicated deviation between radar and gauge estimates of about 25%. The Z DR measurements corrected for differential attenuation were also used to retrieve median raindrop sizes, D 0. Because of uncertainties in differential reflectivity measurements, these retrievals are typically performed only for D 0 > 0.75 mm. The D 0 estimates from an impact disdrometer located at 25 km from the radar were in good agreement with the radar retrievals. The experience of operating the transportable polarimetric X-band radar in the coastal area that does not have good coverage by the National Weather Service radar network showed the value of such radar in filling the gaps in the network coverage. The NOAA X-band radar was effective in covering an area up to 40–50 km in radius offshore adjacent to a region that is prone to flooding during wintertime landfalling Pacific storms.

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