Oceanic Rain Research Articles

Combined Use of TRMM/PR and Disdrometer Data to Correct Reflectivity of Ground-Based Radars

A procedure to correct the reflectivity data from multiple radars is proposed. The TRMM/PR (Tropical Rainfall Measurement Mission satellite/Precipitation Radar) is used as the standard calibrator. The data from a disdrometer is also used to account for the effects of rain attenuation at short wavelengths. The procedure is evaluated in the case of oceanic rain observed in MISMO (Mirai Indian Ocean cruise for the Study of the MJO-convection Onset), in which two radars were deployed; a C-band shipborne radar and an X-band ground-based radar.On the C-band radar, the traditional corrections for the factors such as rain attenuation, second-trip echos, shadows by obstacles, and noise, result in reflectivity data which closely matches that of the TRMM/PR. For the X-band radar, iterative matching to the TRMM/PR worked effectively to correct the reflectivity, with the aid of the disdrometer data to reflect the attenuation by the rain in the vicinity of the radar. The corrected reflectivity from both radars well matches that of TRMM/PR with negligible biases and reduced deviations.

Bayesian Retrieval of Complete Posterior PDFs of Oceanic Rain Rate from Microwave Observations

Abstract A new Bayesian algorithm for retrieving surface rain rate from Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) over the ocean is presented, along with validations against estimates from the TRMM Precipitation Radar (PR). The Bayesian approach offers a rigorous basis for optimally combining multichannel observations with prior knowledge. While other rain-rate algorithms have been published that are based at least partly on Bayesian reasoning, this is believed to be the first self-contained algorithm that fully exploits Bayes’s theorem to yield not just a single rain rate, but rather a continuous posterior probability distribution of rain rate. To advance the understanding of theoretical benefits of the Bayesian approach, sensitivity analyses have been conducted based on two synthetic datasets for which the “true” conditional and prior distribution are known. Results demonstrate that even when the prior and conditional likelihoods are specified perfectly, biased retrievals may occur at high rain rates. This bias is not the result of a defect of the Bayesian formalism, but rather represents the expected outcome when the physical constraint imposed by the radiometric observations is weak owing to saturation effects. It is also suggested that both the choice of the estimators and the prior information are crucial to the retrieval. In addition, the performance of the Bayesian algorithm herein is found to be comparable to that of other benchmark algorithms in real-world applications, while having the additional advantage of providing a complete continuous posterior probability distribution of surface rain rate.

Oceanic rain rate estimates from the QuikSCAT Radiometer: A Global Precipitation Mission pathfinder

The SeaWinds scatterometer, launched onboard the QuikSCAT satellite in 1999, measures global ocean vector winds. In addition to measuring radar backscatter, SeaWinds simultaneously measures the microwave brightness temperature of the atmosphere/surface, and this passive microwave measurement capability is known as the QuikSCAT Radiometer (QRad). This paper presents a QRad retrieval algorithm used to infer instantaneous oceanic rain rates. This statistical algorithm is trained using near‐simultaneous observations of major rain events by QRad and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). Rain rate retrieval algorithm validation is presented through comparisons with independent rain measurements from the TMI 2A12 surface rain rates and the TRMM 3B42RT composite microwave and visible and infrared near‐real time data product. Results demonstrate that QRad rain rate measurements are in good agreement with these independent microwave rain observations and superior to the visible/infrared rain estimates. Thus the QRad rain measurement time series is a valuable addition to the oceanic precipitation climatology that can be used to improve the diurnal estimation of the global rainfall, which is a goal for the future Global Precipitation Mission program. Moreover, the availability of QRad data will provide GPM users early access to learn to use less‐precise rain measurements that will occur in the GPM era with the use of less‐capable constellation satellites. Finally, these QRad rain estimates will be available in the planned data reprocessing (FY 2006) of QuikSCAT winds to improve the rain flagging of rain‐contaminated oceanic wind vector retrievals.

Uncertainties in Oceanic Radar Rain Maps at Kwajalein and Implications for Satellite Validation

The Kwajalein, Marshall Islands, Tropical Rainfall Measuring Mission (TRMM) ground validation radar has provided a multiyear three-dimensional radar dataset at an oceanic site. Extensive rain gauge networks are not feasible over the ocean and, hence, are not available to aid in calibrating the radar or determining a conversion from reflectivity to rain rate. This paper describes methods used to ensure the calibration and allow the computation of quantitative rain maps from the radar data without the aid of rain gauges. Calibration adjustments are made by comparison with the TRMM satelliteborne precipitation radar. The additional steps required to convert the calibrated reflectivity to rain maps are the following: correction for the vertical profile of reflectivity below the lowest elevation angle using climatological convective and stratiform reflectivity profiles; conversion of reflectivity ( Z) to rain rate (R) with a relationship based on disdrometer data collected at Kwajalein, and a gap-filling estimate. The time series of rain maps computed by these procedures include low, best, and high estimates to frame the estimated overall uncertainty in the radar rain estimation. The greatest uncertainty of the rain maps lies in the calibration of the radar (630%). The estimation of the low-altitude vertical profile of reflectivity is also a major uncertainty ( 615%). The Z‐R and data-gap uncertainties are relatively minor (65% or less). These uncertainties help to prioritize the issues that need to be addressed to improve quantitative rainfall mapping over the ocean and provide useful bounds when comparing radar-derived rain estimates with other remotely sensed measures of oceanic rain (such as from satellite passive microwave sensors).

Simultaneous wind and rain retrieval using SeaWinds data

The SeaWinds scatterometers onboard the QuikSCAT and the Advanced Earth Observing Satellite 2 measure ocean winds on a global scale via the relationship between the normalized radar backscattering cross section of the ocean and the vector wind. The current wind retrieval method ignores scattering and attenuation of ocean rain, which alter backscatter measurements and corrupt retrieved winds. Using a simple rain backscatter and attenuation model, two methods of improving wind estimation in the presence of rain are evaluated. First, if no suitable prior knowledge of the rain rate is available, a maximum-likelihood estimation technique is used to simultaneously retrieve the wind velocity and rain rate. Second, when a suitable outside estimate of the rain rate is available, wind retrieval is performed by correcting the wind geophysical model function for the known rain via the rain backscatter model. The new retrieval techniques are evaluated via simulation and validation with data from the National Centers for Environmental Prediction and the Tropical Rainfall Measuring Mission Precipitation Radar. The simultaneous wind/rain estimation method yields most accurate winds in the sweet spot of SeaWinds' swath. On the outer-beam edges of the swath, simultaneous wind/rain estimation is not usable. Wind speeds from simultaneous wind/rain retrieval are nearly unbiased for all rain rates and wind speeds, while conventionally retrieved wind speeds become increasingly biased with rain rate. A synoptic example demonstrates that the new method is capable of reducing the rain-induced wind vector error while producing a consistent (yet noisy) estimate of the rain rate.

Rain retrievals using Tropical Rainfall Measuring Mission and Geostationary Meteorological Satellite 5 data obtained during the SCSMEX

An accurate technique for tracking heavy rains induced by mesoscale convective systems occurring over the northern South China Sea during the Mei-Yu season is urgently needed by nearby countries. By using data observed during the period of the South China Sea monsoon experiment, a simple technique was developed for combining passive microwave and infrared rainfall observations for retrieving oceanic rain rates. The equations for estimating oceanic rain rates were developed using the Tropical Rainfall Measuring Mission Microwave Imager data and the corresponding island rain rates with the linear regression method. By using the Visible and Infrared Scanner 11 w m infrared data, a calibrated infrared rain rate estimation technique (GOES Precipitation Index) used the microwave rain retrievals as the ground truth for estimating the best infrared rain threshold. At times when the Tropical Rainfall Measuring Mission's observations were not available, the following infrared rain retrievals were made using the 11 w m infrared data from the Geostationary Meteorological Satellite 5. The microwave/infrared technique presented the ability to track heavy rains induced by mesoscale convective systems. Though applications in other regions and different seasons have yet to be tested, the procedures followed in this study could be utilized for universal applications, especially for those oceanic regions lacking conventional stations.

Oceanic Rain Column Height Derived from SSM/I

Abstract The climatology of oceanic rain column height derived from 12 years (July 1987–June 1999) of Special Sensor Microwave Imager (SSM/I) data is presented. The estimation procedure is based on a technique developed by Wilheit et al. In the annual mean, the SSM/I-derived oceanic rain height shows a maximum of about 4.7 km in the Tropics and decreases toward the high latitudes to less than 3.5 km at 50°. Interannual variations exhibit seasonal dependency and show maxima of about 200–300 m in the oceanic dry zones and in the midlatitude storm track regions. The rain heights estimated from the morning passes of the SSM/I are lower than those computed from the afternoon passes by about 60 m in the Tropics but are higher north of 40°N. This small difference cannot change the conclusion about the morning maximum in rain rate. The nonsystematic error increases with decreasing rain column height and is estimated to be about 120 m for rain heights of 4–5 km and 200 m at 3.5 km. Comparison with the height of th...

The Comparison of Two Merged Rain Gauge–Satellite Precipitation Datasets

Abstract Two large–scale precipitation datasets, one produced by the Global Precipitation Climatology Project (GPCP) and the other by the Climate Prediction Center of the National Weather Service, and called Climate Prediction Center Merged Analysis of Precipitation (CMAP), were compared. Both datasets blend satellite and gauge estimates of precipitation. And while the latter has its heritage in the GPCP, different analysis procedures and some additional types of input data used by CMAP yielded different values. This study used the error characteristics of the data to assess the significance of the observed differences. Despite good spatial and temporal correlations between the two fields some of the observed differences were significant at the 95% level. These were traced to the use of some different input data such as the use by CMAP of atoll gauges in the tropical Pacific and gauges uncorrected for wetting evaporation and aerodynamic effects. The former impacts the tropical ocean rain amounts and the l...

Hurricanes Pauline and Nora rainwater chemical composition

Rainwater from hurricanes Pauline and Nora was sampled for chemical analysis at the Pacific Coast of Mexico. Rainwater sampling under extreme conditions presents a unique opportunity to study oceanic rain chemical composition. An excess sulphate ((SO2–4)xs) as low as 0% was measured near the centre of hurricane Pauline in Huatulco Bays. Another remarkable result was obtained in another rainwater sample of Pauline rain bands with a total SO2–4 concentration below the detection limit. Also, Na+ and Cl– concentrations were extremely low (0.02 and 0.025 mg L–1, respectively). The explanation of these results is also presented. Some light rains were also collected in Tapachula while Pauline was severely affecting Huatulco Bays. Only rainwater samples from hurricane Nora outer rain bands were sampled in Manzanillo, where it was interesting to evaluate the profound impact that a single power plant had on the chemical composition of hurricane Nora rains. Excess sulphate did not correlate with Mg2+ in Huatulco Bays and Manzanillo. However, it correlated with Mg2+ in Tapachula, even though this town is located 27 km from the coast. A further oxidation of organic sulphur containing compounds combined with a simultaneous transport of sea spray inland is proposed to explain this correlation.

Oceanic Rain Rate Retrievals Using TRMM Microwave Imager Multi-channel Brightness Temperatures during the 1998 SCSMEX

The purpose of this paper is to retrieve the oceanic rain rate (RR) over waters near Taiwan and Luzon from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) multi-channel brightness temperatures (Tbs). In addition to improving heavy rainfall forecasting by providing quantitative oceanic RR estimates and possible movement direction of Mesoscale Convective Systems (MCSs), another objective of this paper is to overcome the lack of oceanic conventional observations in heavy rainfall forecasting during the Mei-Yu season. Rainfall caused by MCSs during the 1998 South China Sea Monsoon Experiment (SCSMEX) was studied using TMI nine-channel Tbs, the hourly RR from ground rain gauges, and Geostationary Meteorological Satellite (GMS) infrared (IR) Tbs. In this paper, the domain ranges from 110°E to 130°DE and from 15°N to 30°N. Two oceanic RR retrievals taken on 3 June 1998 are shown and examined. The results show that a TMI nine-channel Tbs-RR equation can retrieve the amounts of heavy rainfall caused by the MCSs that often occur during the summer monsoon season in southeastern China, the northern part of the South China Sea, Luzon, and southern Taiwan. The RR retrievals were quantitatively consistent with the rainfall observations made on nearbyislands. The GMS IR Tb 240K isotherms included most of the TMI microwave (MW) Tbs retrieved rainfall region. The heaviest rainfall region wasmore or less included in the area where the IR Tb was colder than 210K. The region had a tendency to be concentrated in the forward part of the MCS in the direction of its movement. Moreover, analyses of the rainfall events that occurred in southern Taiwan during the SCSMEX period show that there was local rainfall enhancement caused by the orographic features of the island.

First results of the TRMM Microwave Imager (TMI) monthly oceanic rain rate: comparison with SSM/I

We evaluated the performance of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) at‐launch algorithm for monthly oceanic rain rate using six months (January–June 1998) of TMI data. Comparison with oceanic monthly rain rates derived from the Special Sensor Microwave Imager (SSM/I) data shows statistically significant differences. The TMI rain rates are lower than the SSM/I rain rates by about 10% overall, except for rain rates lower than 1.4 mm/day. The low TMI bias may be due to an overestimate of the columnar water vapor as indicated by the estimated rain layer thickness. The superior sampling by the TMI improves the algorithm statistics at the low rain rates. The averaged monthly rain rates over the latitudes between 40°S and 40°N are 2.78 and 3.17 mm/day, respectively for TMI and SSM/I, with RMS difference of 1.35 mm/day and correlation coefficient of 0.94.

Crossroads in the Caribbean: A site of encounter and exchange on Dominica

Dominica is the most mountainous of all the islands of the Lesser Antilles which forms the eastern arc of the Caribbean archipelago. It was the last island in the Caribbean to be colonized by Europeans. Its thick oceanic rain forests provided a refuge and ecological resources for the indigenous Carib people facing the Spanish, French, English and Dutch advance into the Caribbean during the sixteenth century. Dominica also provided a strategic location on which to cultivate and process raw materials for trading with the European ships en route to other parts of the Caribbean and American mainland. It is argued that the primary Carib product in this exchange was tobacco and that the traditionally established view of total resistance by ‘warlike Caribs’ to the European advance should be modified in the light of this trade. The recent discovery of a late Amerindian archaeological site at such an important maritime crossroad provides a unique opportunity to re‐evaluate the relationship between Caribs and Europeans in the years immediately following contact and the effects which this had on Carib society.

Examination of ?global atmospheric temperature monitoring with satellite microwave measurements?: 2. Analysis of satellite data

Using Microwave Sounding Unit (MSU) channel 2 (Ch. 2, 53.74 GHz) data, Spencer and Christy (1992a) determined that the earth exhibits no temperature trend in the period 1979–90, while other authors find a temperature increase of roughly 0.1 K. Based on a theoretical analysis Prabhakara et al. (1995) showed that the information about the global atmospheric temperature deduced from MSU Ch. 2 observations has a small contamination, δT 2, as a result of the attenuation due to hydrometeors in the atmosphere. A method is developed in this study, that utilizes coincident measurements made by MSU in Ch. 1 (50.3 GHz), to estimate this δT 2 over the global oceans. The magnitude of δT 2 is found to be about 1 K over significant parts of the tropical oceanic rain belts and about 0.25 K over minor portions of the mid-latitude oceanic storm tracks. Due to events such as El Nioo, there is variability from year to year in the rain areas and rain intensity leading to significant change in the patterns of δT 2. The patterns of δT 2 derived for March 82 and March 83 reveal such a change. When averaged over the global oceans, from 50° N to 50° S, δT 2 has a value of 0.25 and 0.29 K for March 1982 and 1983, respectively. Due to these reasons the interannual temperature change derived by Spencer and Christy from MSU Ch. 2 will contain a residual hydrometeor effect. Thus in evaluating decadal trend of the global mean temperature of the order of 0.1 K from MSU Ch. 2 data one has to take into account completely the contamination due to hydrometeors.

Diurnal Cycle of Oceanic Precipitation from SSM/I Data

Abstract Four and a half years of the global monthly oceanic rain rates derived from the DMSP (Defense Meteorological Satellite Program) F-8 SSM/I (Special Sensor Microwave/Imager) data are used to study the diurnal cycles. Annual mean rainfall maps based on the SSM/I morning and evening observations are presented, and their differences are examined using a paired t test. The morning estimates are larger than the afternoon estimates by about 20% over the oceanic region between 50°S and 50°N, with significant differences located mainly along the intertropical convergence zone region. Using the measurements from two satellites, either DMSP F-8 and F-10 or DMSP F-10 and F-11, amplitudes and phases of the 24-h harmonic are estimated. The diurnal cycle shows a nocturnal or early morning maximum in 35%–40% of the oceanic regions. Monte Carlo simulations show that the rms errors associated with the estimated amplitude and phase are about 100% and 2 h, respectively, mainly due to the large random errors (50%) ass...

Oceanic rainfall monitoring with the Microwave Sounding Units

Abstract The Microwave Sounding Units flying since late 1978 on the TIROS‐N series of NOAA satellites provide a fifteen year record of climate variations useful for a wide variety of studies. Although not possessing microwave frequencies ideally suited for monitoring rainfall, the MSU 50.3 GHz channel has a response to oceanic rain and cloud water that is similar to the 37 GHz channel of SSM/I. After correction for the influence of air temperature variations on this channel with two of the other MSU channels, any radiometric warming above a 15% cumulative frequency distribution threshold is assumed to be rain influenced and linearly related to footprint averaged rain amount. The monthly gridpoint averages of this rain index are calibrated with seven to ten years of monthly rain gauge data from 123 coastal and island locations scattered around the global oceans. The resulting rainfall climatology shows some interesting differences with existing rainfall climatologies. While it is not obvious which of these differences reflect real improvements over previous climatologies, they provide a valuable reference dataset which can be compared to new climatologies as they are developed from other satellite sensors such as the SSM/I. Many details of this method, including more extensive comparisons to rain gauge data, can be found in Spencer (1993).

High-Resolution Imaging of Rain Systems with the Advanced Microwave Precipitation Radiometer

An Advanced Microwave Precipitation Radiometer (AMPR) has been developed and flown in the NASA ER-2 high-altitude aircraft for imaging various atmospheric and surface processes, primarily the internal structure of rain clouds. The AMPR is a scanning four-frequency total power microwave radiometer that is externally calibrated with high-emissivity warm and cold loads. Separate antenna systems allow the sampling of the 10.7- and 19.35-GHz channels at the same spatial resolution, while the 37.1- and 85.5-GHz channels utilize the same multifrequency feedhorn as the 19.35-GHz channel. Spatial resolutions from an aircraft altitude of 20-km range from 0.6 km at 85.5 GHz to 2.8 km at 19.35 and 10.7 GHz. All channels are sampled every 0.6 km in both along-track and cross-track directions, leading to a contiguous sampling pattern ofthe 85.5-GHz 3-dB beamwidth footprints, 2.3 × oversampling of the 37.1-GHz data, and 4.4 × oversampling of the 19.35- and 10.7-GHz data. Radiometer temperature sensitivities range from 0.2° to 0.5°C. Details of the system are described, including two different calibration systems and their effect on the data collected. Examples of oceanic rain systems are presented from Florida and the tropical west Pacific that illustrate the wide variety of cloud water, rainwater, and precipitation-size ice combinations that are observable from aircraft altitudes.

Random Errors of Oceanic Monthly Rainfall Derived from SSM/I Using Probability Distribution Functions

Global averages and random errors associated with the monthly oceanic rain rates derived from the Special Sensor Microwave/Imager (SSM/I) data using the technique developed by Wilheit et al. are computed. Accounting for the beam-filling bias, a global annual average rain rate of 1.26 m is computed. The error estimation scheme is based on the existence of independent (morning and afternoon) estimates of the monthly mean. Calculations show overall random errors of about 50%–60% for each 5° × 5° box. The results are insensitive to different sampling strategy (odd and even days of the month). Comparison of the SSM/I estimates with raingage data collected at the Pacific atoll stations showed a low bias of about 8%, a correlation of 0.7, and an rms difference of 55%.

The sources and composition of mercury in Pacific Ocean rain

Here we report measurements of total Hg (HgT), reactive Hg (HgR), and methylmercury (MMHg) in precipitation from the equatorial Pacific Ocean, collected during a cruise in January and February 1990, and from a mid-continental location in the rural temperate lacustrine northcentral Wisconsin environs. The concentrations of HgT (14.4±6.5 pM), HgR (8.9±4.5 pM) and MMHg (<50 fM) found in equatorial Pacific rain were less than the average concentrations found in Wisconsin. In general, the results indicate that although particulate Hg is a small fraction of the total atmospheric Hg burden, it is the major contributor to Hg in precipitation. Furthermore, deposition could be an important source of HgR to the equatorial Pacific Ocean. In contrast, deposition is not a significant source of MMHg to either the equatorial Pacific Ocean or the remote seepage lakes of Wisconsin. This implies that methylated mercury is formedin situ in these systems.

A Satellite Passive 37-GHz Scattering-based Method for Measuring Oceanic Rain Rates

A combination of theory and measurement is used to develop a scattering-based method for quantitatively measuring rainfall over the ocean from Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) 37-GHz observations. This technique takes the observed scattering effects of precipitation on 37-GHz brightness temperatures and applies it to the oceanic environment. It requires an estimate of the effective radiating temperature of the cloudy portion of the atmosphere, and a brightness temperature measurement of the cloud-free ocean surface. These two measurements bound all possible combinations of clear and cloudy conditions within a footprint in terms of bipolarized brightness temperatures. Any satellite-observed brightness temperature T(B) lower than these values is assumed to reflect scattering, which at 37 GHz is only due to precipitation-size hydrometeors. Because the technique involves linear transformation between dual polarized brightness temperature and rain rate, there are no nonlinear 'footprint filling' effects and a unique footprint-averaged rain rate results. It is shown that these SMMR-derived rain rates for five cases of convection over the Gulf of Mexico are closely related to simultaneously derived radar rain rates, having a correlation of 0.90. This technique is then applied to a massive squall line over the Gulf of Mexico, and the resulting rain rate distribution reflects features found in cloud top heights and texture inferred from GOES infrared and visible imagery.

A satellite passive 37-GHz scattering-based method for measuring oceanic rain rates: Spencer, R.W., 1986. J. Climate appl. Met., 25(6):754–766. Universities Space Res. Assoc., NASA Marshall Space Flight Center, Huntsville, AL 35812 USA

A satellite passive 37-GHz scattering-based method for measuring oceanic rain rates: Spencer, R.W., 1986. J. Climate appl. Met., 25(6):754–766. Universities Space Res. Assoc., NASA Marshall Space Flight Center, Huntsville, AL 35812 USA