Maximum Rain Rate Research Articles

Improved Stepped Frequency Microwave Radiometer Tropical Cyclone Surface Winds in Heavy Precipitation

AbstractSurface wind speeds retrieved from airborne stepped frequency microwave radiometer (SFMR) brightness temperature measurements are important for estimating hurricane intensity. The SFMR performance is highly reliable at hurricane-force wind speeds, but accuracy is found to degrade at weaker wind speeds, particularly in heavy precipitation. Specifically, a significant overestimation of surface wind speeds is found in these conditions, suggesting inaccurate accounting for the impact of rain on the measured microwave brightness temperature. In this study, the wind speed bias is quantified over a broad range of operationally computed wind speeds and rain rates, based on a large sample of collocated SFMR wind retrievals and global positioning system dropwindsonde surface-adjusted wind speeds. The retrieval bias is addressed by developing a new SFMR C-band relationship between microwave absorption and rain rate (κ−R) from National Oceanic and Atmospheric Administration WP-3D aircraft tail Doppler radar reflectivity and in situ Droplet Measurement Technologies Precipitation Imaging Probe measurements to more accurately model precipitation impacts. Absorption is found to be a factor of 2 weaker than is estimated by the currently operational algorithm. With this new κ–R relationship, surface wind retrieval bias is significantly reduced in the presence of rain at wind speeds weaker than hurricane force. At wind speeds greater than hurricane force where little bias exists, no significant change is found. Furthermore, maximum rain rates computed using the revised algorithm are around 50% greater than operational measurements, which is more consistent with maximum reflectivity-estimated rain rates in hurricanes.

Impact of MJO on the diurnal cycle of rainfall over the western Maritime Continent in the austral summer

This paper investigates the impact of the Madden-Julian Oscillation (MJO) on the diurnal cycle of rainfall over the western Maritime Continent during the austral summer. For this purpose, cyclostationary empirical orthogonal function analysis is applied to the tropical rainfall measuring mission rain rate and the Japanese Reanalysis-25 data for the period 1998–2008. The real-time multivariate MJO index by Wheeler and Hendon (Mon Wea Rev 132:1917–1932, 2004) is adopted to define the intensity and the phase of MJO. It is demonstrated that the hourly maximum rain rate over the domain tends to increase when convectively active phase of MJO approaches the Maritime Continent. In contrast, the hourly maximum rain rate tends to decrease when convectively suppressed phase of MJO resides over the region. The changes in the rain rate due to MJO differ over the ocean and the land. This difference is the greatest when the MJO is in the mature stage. Throughout the day during this stage, terrestrial rain rates show minimum values while diurnally varying oceanic rain rates record maximum values. Thus, precipitation becomes more intense in the morning over the Java Sea and is weakened in the evening over Borneo and Sumatra during the mature stage of MJO. During the decaying stage of MJO over the Maritime Continent, the diurnal cycle of precipitation weakens significantly over the ocean but only weakly over land. Analyses suggest that the anomalous lower level winds accompanied by MJO interact with the monsoonal flow over the Maritime Continent. Westerlies induced by MJO convection in the mature stage are superimposed on the monsoonal westerlies over the equator and increase wind speed mainly over the Java Sea due to the blocking effect of orography. Mountainous islands induce flow bifurcation, causing near-surface winds to converge mainly over the oceanic channels between two islands. As a result, heat flux release from the ocean to the atmosphere is enhanced by the increased surface wind resulting in instability as described in the wind-induced surface heat exchange mechanism. This may contribute to heavy rainfall over the Java Sea in the morning during the mature stage. On the other hand, convergence and vertical velocity over the islands, which play important roles in inducing nighttime rainfall, tend to be weak in the evening during the mature stage of MJO. Strong westerlies arising from MJO and the seasonal flow during the mature stage tend to interrupt convergence over islands. This interruption of convergence by MJO gives rise to decreased rain rates over the land regions.

Mechanisms Affecting the Transition from Shallow to Deep Convection over Land: Inferences from Observations of the Diurnal Cycle Collected at the ARM Southern Great Plains Site

AbstractSummertime observations for 11 yr from the Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains (SGP) site are used to investigate mechanisms controlling the transition from shallow to deep convection over land. It is found that a more humid environment immediately above the boundary layer is present before the start of late afternoon heavy precipitation events. The higher moisture content is brought by wind from the south. Greater boundary layer inhomogeneity in moist static energy, temperature, moisture, and horizontal wind before precipitation begins is correlated to larger rain rates at the initial stage of precipitation. In an examination of afternoon rain statistics, higher relative humidity above the boundary layer is correlated to an earlier onset and longer duration of afternoon precipitation events, whereas greater boundary layer inhomogeneity and atmospheric instability in the 2–4-km layer above the surface are positively correlated to the total rain amount and the maximum rain rate. Although other interpretations may be possible, these observations are consistent with theories for the transition from shallow to deep convection that emphasize the role of a moist lower free troposphere and boundary layer inhomogeneity.

Spatial Variability of Mercury Wet Deposition in Eastern Ohio: Summertime Meteorological Case Study Analysis of Local Source Influences

Extensive exploration of event precipitation data in the Ohio River Valley indicates that coal combustion emissions play an important role in mercury (Hg) wet deposition. During July-September 2006, an intensive study was undertaken to discern the degree of local source influence. Source-receptor relationships were explored by establishing a set of wet deposition sites in and around Steubenville, Ohio. For the three month period of study, volume-weighted mean Hg concentrations observed at the eight sites ranged from 10.2 to 223 ng L(-1), but this range increased drastically on an event basis with a maximum concentration of 89.4 ng L(-1) and a minimum concentration of 4.1 ng L(-1). A subset of events was explored in depth, andthe degree of variability in Hg concentrations between sites was linked to the degree of local source enhancement. Samples collected at sites less than 1 km from coal-fired utility stacks (near-field) exhibited up to 72% enhancement in Hg concentrations over regionally representative samples on an event basis. Air mass transport and precipitating cell histories were traced in order to evaluate relationships between local point sources and receptor sites. It was found that the interaction of several dynamic atmospheric parameters combined to favor local Hg concentration enhancement over the more regional contribution. When significant meteorological factors (wind speed at time of maximum rain rate, wind speed 24 h prior to precipitation, mixing height, and observed ceiling) were explored, it was estimated that during summertime precipitation, 42% of Hg concentration in near-field samples could be attributed to the adjacent coal-fired utility source.

Diurnal phase of late-night against late-afternoon of stratiform and convective precipitation in summer southern contiguous China

Using the tropical rainfall measuring mission (TRMM) Precipitation Radar (PR) observations combined with the surface rain gauge data during 1998-2006, the robust diurnal features of summer stratiform and convective precipitation over the southern contiguous China are revealed by exploring the diurnal variations of rain rate and precipitation profile. The precipitation over the southern contiguous China exhibits two distinguishing diurnal phases: late-night (2200-0600 LST) and late-afternoon (1400-2200 LST), dependent on the location, precipitation type and duration time. Generally, the maximum rain rate and the highest profile of stratiform precipitation occur in the late-afternoon (late-night) over the southeastern (southwestern) China, while most of the stratiform short-duration rain rate tends to present late-afternoon peaks over the southern China. For convective precipitation, the maximum rain rate and the highest profile occur in the late-afternoon over most of the southern contiguous China, while the convective long-duration rain rate exhibits late-night peaks over the southwestern China. Without regional dependence, the convective precipitation exhibits much larger amplitude of diurnal variations in both near surface rain rate and vertical extension compared with stratiform precipitation and the convective rain top rises most rapidly between noon and afternoon. However, there are two distinctive sub-regions. The diurnal phases of precipitation there are very weakly dependent on precipitation type and duration time. Over the eastern periphery of the Tibetan Plateau, the maximum rain rate and the highest profile of either convective or stratiform precipitation occur in the late-night. Over the southeastern coastal regions, both the near surface rain rate and rain top of convective and stratiform precipitation peak in the late-afternoon.

Comparison of Diurnal Variations in Precipitation Systems Observed by TRMM PR, TMI, and VIRS

Abstract Tropical Rainfall Measuring Mission (TRMM) data during June–August 1998–2003 are used to investigate diurnal variations of rain and cloud systems over the tropics and midlatitudes. The peak time of the coldest minimum brightness temperature derived from the Visible and Infrared Scanner (VIRS) and the maximum rain rate derived from the Precipitation Radar (PR) and the TRMM Microwave Imager (TMI) are compared. Time distributions are generally consistent with previous studies. However, it is found that systematic shifts in peak time relative to each sensor appeared over land, notably over western North America, the Tibetan Plateau, and oceanic regions such as the Gulf of Mexico. The peak time shift among PR, TMI, and VIRS is a few hours. The relationships among the amplitude of diurnal variation, convective frequency, storm height, and rain amount are further investigated and compared to the systematic peak time shifts. The regions where the systematic shift appears correspond to large amplitude of diurnal variation, high convective frequency, and high storm height. Over land and over ocean near the coast, the relationships are rather clear, but not over open ocean. The sensors likely detect different stages in the evolution of convective precipitation, which would explain the time shift. The PR directly detects near-surface rain. The TMI observes deep convection and solid hydrometeors, sensing heavy rain during the mature stage. VIRS detects deep convective clouds in mature and decaying stages. The shift in peak time particularly between PR (TMI) and VIRS varies by region.

Rain event properties in nature and in rainfall simulation experiments: a comparative review with recommendations for increasingly systematic study and reporting

AbstractIn hydrology and geomorphology, less attention has been paid to rain event properties such as duration, mean and peak rain rate than to rain properties such as drop size or kinetic energy. A literature review shows a lack of correspondence between natural and simulated rain events. For example, 26 studies that report event statistics from substantial records of natural rain reveal a mean rain rate of just 3·47 mm h−1 (s.d. 2·38 mm h−1). In 17 comparable studies dealing with extreme rain rates including events in cyclonic, tropical convective, and typhoon conditions, a mean maximum rain rate (either hourly or mean event rain rate) of 86·3 mm h−1 (s.d. 57·7 mm h−1) is demonstrated. However, 49 studies using rainfall simulation involve a mean maximum rain rate of 103·1 mm h−1 (s.d. 81·3 mm h−1), often sustained for > 1 h, exceeding even than of extreme rain events, and nearly 30 times the mean rain rate in ordinary, non‐exceptional, rain events. Thus rainfall simulation is often biased toward high rain rates, and many of the rates employed (in several instances exceeding 150 mm h−1) appear to have limited relevance to ordinary field conditions. Generally, simulations should resemble natural rain events in each study region. Attention is also drawn to the raindrop arrival rate at the surface. In natural rain, this is known to vary from < 100 m−2 s−1 to > 5000 m−2 s−1. Arrival rate may need to be added to the list of parameters that must be reproduced realistically in rainfall simulation studies. Copyright © 2008 John Wiley & Sons, Ltd.

Small-Scale Cloud Activity over the Maritime Continent and the Western Pacific as Revealed by Satellite Data

Abstract Cloud systems over the Maritime Continent and the tropical western Pacific defined by the Geostationary Meteorological Satellite (GMS) were tracked, and their evolution was compared with cloud parameters [e.g., minimum blackbody brightness temperature (TBB), cloud area size, TBB gradient at cloud edges]. In addition, cloud systems observed by the Tropical Rainfall Measuring Mission (TRMM) were examined, and the relationship with precipitation was investigated. Analysis areas were divided into four regions: open ocean, coastal sea, coasts, and land. Cloud systems that did not split from or merge with other systems (28% of a total of 290 717 cloud systems) showed common features on cloud parameters in spite of different lifetimes or their locations. While the minimum TBB appeared in the beginning of their lifetimes, the cloud area was still expanding. At the time of first detection, the TBB gradient at the edge of the cloud system was the maximum and decreased with time. The rain rate was maximized when the TBB was at a minimum or earlier. For example, a system with the lifetime of 5 h over the ocean has a minimum TBB 2 h after the occurrence, a maximum area at 3 h, a maximum TBB gradient at 1 h, and a maximum rain rate at 1 h. Vertical development was significant in coasts, while remarkable horizontal expansion appeared over land. In particular, precipitation ice and storm height profiles showed differences among regions.

Comparison between Pludix and impact/optical disdrometers during rainfall measurement campaigns

The performances of two couples of disdrometers based on different measuring principles are compared: a classical Joss–Waldvogel disdrometer and a recently developed device, called the Pludix tested in Ferrara, Italy, and Pludix and the two-dimensional video disdrometer (2DVD) tested in Cabauw, The Netherlands. First, the measuring principles of the different instruments are presented and compared. Secondly, the performances of the two pairs of disdrometers are analysed by comparing their rain amounts with nearby tipping bucket rain gauges and the inferred drop size distributions. The most important rainfall integral parameters (e.g. rain rate and radar reflectivity) and drop size distribution parameters are also analysed and compared. The data set for Ferrara comprises 13 rainfall events, with a total of 20 mm of rainfall and a maximum rain rate of 4 mm h − 1 . The data set for Cabauw consists of 9 events, with 25–50 mm of rainfall and a maximum rain rate of 20–40 mm h − 1 . The Pludix tends to underestimate slightly the bulk rainfall variables in less intense events, whereas it tends to overestimate with respect to the other instruments in heavier events. The correspondence of the inferred drop size distributions with those measured by the other disdrometers is reasonable, particularly with the Joss–Waldvogel disdrometer. Considering that the Pludix is still in a calibration and testing phase, the reported results are encouraging. A new signal inversion algorithm, which will allow the detection of rain drops throughout the entire diameter interval between 0.3 and 7.0 mm, is under development.

Drop Size Distributions Measured by a 2D Video Disdrometer: Comparison with Dual-Polarization Radar Data

An analysis of drop size distributions (DSDs) measured in four very different precipitation regimes is presented and is compared with polarimetric radar measurements. The DSDs are measured by a 2D video disdrometer, which is designed to measure drop size, shape, and fall speed with unprecedented accuracy. The observations indicate that significant DSD variability exists not only from one event to the next, but also within each system. Also, despite having vastly different storm structures and maximum rain rates, large raindrops with diameters greater than 5 mm occurred with each system. By comparing the occurrence of large drops with rainfall intensity, the authors find that the largest median diameters are not always associated with the heaviest rainfall, but are sometimes located either in advance of convective cores or, occasionally, in stratiform regions where rainfall rates are relatively low. Disdrometer and polarimetric radar measurements of radar reflectivity Z, differential reflectivity ZDR, specific differential phase KDP, and R(Z) and R(KDP) rain-rate estimators are compared in detail. Overall agreement is good, but it is found that both R(Z) and R(KDP) underestimate rain rate when the DSD is dominated by small drops and overestimate rain rate when the DSD is dominated by large drops. The results indicate that a classification of different rain types (associated with different DSDs) should be an essential part of polarimetric rainfall estimation. Furthermore, observations suggest that ZDR is a key parameter for making such a distinction. Last, the authors compute and compare maximum and average of gamma shape, slope, and intercept parameters for all four precipitation events. Potential measurement errors with the 2D video disdrometer are also discussed.

Palaeoecology of the biserial planktonic foraminifer Heterohelix moremani (Cushman) in the late Albian to middle Turonian Circum-North Atlantic

The Cretaceous Heterohelix moremani (Cushman) was the only biserial planktonic foraminiferal species from its first appearance in the late Albian up to the Cenomanian/Turonian boundary. Within that time, it increased gradually in abundance relative to other planktonic foraminifera in five Circum-North Atlantic sections. It is generally rare in upper Albian sediments, common in most of the Cenomanian and very abundant in sediments representing the latest Cenomanian Oceanic Anoxic Event. Short-term variations on the overall abundance trend correlate with positive excursions in the bulk carbonate δ 13C record. Maximum rain rates of H. moremani during OAE2 show that this species was an opportunist that did well in extreme conditions, but its overall distribution indicates that it is not necessarily a marker for very high palaeoproductivity environments. Stable oxygen and carbon isotope measurements on foraminiferal species indicate that H. moremani was a surface water dweller at least in part of its geographic range, but incorporated 13C out of equilibrium with ambient seawater. It is depleted in δ 13C relative to other planktonic foraminifera, which is attributed to vital effects related to its opportunistic character.

Precipitation rate of tropical maritime convective clouds as related to the coldest cloud top temperatures

The maximum rain rates produced at ground level in the island of Guadeloupe (French West Indies) by 75 cloud systems during 1994 have been studied in relation to the coldest cloud surface temperatures obtained by using METEOSAT 3 infrared satellite imagery. The analysis reveals that the cloud systems in which warm rain production mechanisms (collision and coalescence) are dominant may produce non negligible rain rates and should therefore be taken into account in the algorithms that relate the precipitation rates to temperature of the coldest cloud surfaces. Further, the orographic precipitation enhancement due to the mountainous part of the island has been estimated in terms of a rain rate amplification coefficient. It appears that high amplification coefficient values correspond to low rain rate values over the flat part of the island and vice versa. Two algorithms that take into account the above analysis conclusions and allow the estimation of the maximum and mean rain rates over the flat part of the island have been developed.

Results of WetNet PIP-2 Project

The second WetNet Precipitation Intercomparison Project (PIP-2) evaluates the performance of 20 satellite precipitation retrieval algorithms, implemented for application with Special Sensor Microwave/Imager (SSM/I) passive microwave (PMW) measurements and run for a set of rainfall case studies at full resolution‐instantaneous space‐timescales. The cases are drawn from over the globe during all seasons, for a period of 7 yr, over a 608N‐ 178S latitude range. Ground-based data were used for the intercomparisons, principally based on radar measurements but also including rain gauge measurements. The goals of PIP-2 are 1) to improve performance and accuracy of different SSM/I algorithms at full resolution‐instantaneous scales by seeking a better understanding of the relationship between microphysical signatures in the PMW measurements and physical laws employed in the algorithms; 2) to evaluate the pros and cons of individual algorithms and their subsystems in order to seek optimal ‘‘front-end’’ combined algorithms; and 3) to demonstrate that PMW algorithms generate acceptable instantaneous rain estimates. It is found that the bias uncertainty of many current PMW algorithms is on the order of 630%. This level is below that of the radar and rain gauge data specially collected for the study, so that it is not possible to objectively select a best algorithm based on the ground data validation approach. By decomposing the intercomparisons into effects due to rain detection (screening) and effects due to brightness temperature‐rain rate conversion, differences among the algorithms are partitioned by rain area and rain intensity. For ocean, the screening differences mainly affect the light rain rates, which do not contribute significantly to area-averaged rain rates. The major sources of differences in mean rain rates between individual algorithms stem from differences in how intense rain rates are calculated and the maximum rain rate allowed by a given algorithm. The general method of solution is not necessarily the determining factor in creating systematic rain-rate differences among groups of algorithms, as we find that the severity of the screen is the dominant factor in producing systematic group differences among land algorithms, while the input channel selection is the dominant factor in producing systematic group differences among ocean algorithms. The significance of these issues are examined through what is called ‘‘fan map’’ analysis. The paper concludes with a discussion on the role of intercomparison projects in seeking improvements to algorithms, and a suggestion on why moving beyond the ‘‘ground truth’’ validation approach by use of a calibration-quality forward model would be a step forward in seeking objective evaluation of individual algorithm performance and optimal algorithm design.

Effect of multipath fading on millimetre wave propagation: a field study

Analysis of clear air fading encountered on millimetric wave and infrared radio links are presented. The analysis is based on data obtained during a four-year field study in the city of Riyadh, Saudi Arabia. The region can be con- sidered as a typical arid climate where the rate of evaporation is higher than the rate of precipi- tation. Maximum rain rate is of the order of 30 mm/h for 0.001% of the year. A brief descrip- tion of the experimental setup is presented, together with the results of measuring clear air fading experienced by the radio links. Statistical characterisation of fading is given for both the millimetric wave links operating near 40 GHz and the infrared radio link at a wavelength of 0.88 pm. It is shown that fading is dominated by multipath, having a Rayleigh amplitude distribution and an occurrence factor similar to the microwave band. Fades were highly correlated for the two links sharing the same path and separated in frequency by 1%. Multipath fading was also measured on the infrared link. Even at the short hop length at 0.75 km, the occurrence factor was about 1%. Time duration of fades are also analysed, and fade durations were essentially exponentially distrib- uted. Small fades, however, have normal duration distribution.

Liquid water content and precipitation characteristics of stratiform clouds as inferred from satellite microwave measurements

In this paper we present an analysis of the integrated liquid water content and precipitation characteristics of stratiform clouds using data from the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) for January 1979, over the North Atlantic Ocean (40°–60°N). Concurrent analysis of the SMMR data with the U.S. Air Force 3‐Dimensional Nephanalysis (3DNEPH) allows the interpretation of the SMMR‐derived liquid water paths and precipitation characteristics in terms of cloud type, cloud fraction, and cloud height. Combining the initialized analyses from the European Center for Medium‐Range Weather Forecasting (ECMWF) with the 3DNEPH enables vertical temperature and humidity profiles to be incorporated into the retrievals. The interpretation and presentation of our results are guided by their implications for the parameterization of liquid water content of layer clouds in largescale atmospheric models. The average liquid water paths for middle and low clouds were determined to be 115 and 102 g m−2, respectively, with a maximum value of 1070 g m−2. A comparison of the SMMR‐derived values of the liquid water path with the adiabatic liquid water path determined from the 3DNEPH cloud data and the ECMWF temperature profiles indicated that these clouds were for the most part substantially diluted by a combination of precipitation, freezing, and entrainment. Analysis of the liquid water path as a function of temperature showed that clouds with average temperature below 246 K had little liquid water and were inferred to be predominantly crystalline. There was little evidence that cloud liquid water path increases with temperature for cloudiness on a large scale, suggesting that cloud thickness plays the dominant role in determining cloud liquid water path. A total of 8.5% and 4.4% of the total middle and low clouds, respectively, were determined to be raining. Liquid water paths of 350 g m−2 and 500 g m−2 for middle and low clouds, respectively, were determined to be average thresholds for the onset of precipitation. Maximum rain rates for these clouds were determined to be 7 mm h−1. The autoconversion of cloud water to rain water was determined to occur at a rate of 0.001 s−1.

The Relationship between Mesoscale Thermodynamic Variables and Convective Precipitation

Abstract Soundings. surface pressure, temperature and humidity obtained from a standard observation network were correlated with rain rates given by raingages and radar. The correlations indicate that a single thermodynamic parameter (static potential energy) explains ∼60% of the storm-to-storm variability of the mean and the maximum rain rates. During the evolution of a precipitating system the time variation of rain rate parameters follows closely the variation of the static energy. The entire distribution of rain rates is well stratified by the energy.

More on Rain Rate Distributions and Extreme Value Statistics

A new methodology is described for estimating the 5-minute rain rate distribution from yearly 5-minute maximum rain rate data and yearly accumulated rainfall data published by the National Climatic Center for U.S. locations. The method previously described gives the high rain rate portion of the distribution, whereas the extended methodology yields the complete distribution, which is assumed to be approximately lognormal. The three parameters characterizing the lognormal distribution can be calculated by application of the theory of extreme value statistics. The calculated results agree well with the 20-year data. The accuracy of the calculated results is limited by the instability of extreme rain rate data with a finite time base. Two-year rain rate data measured by a tipping bucket rain gauge at Palmetto, Georgia, are used to demonstrate that the time variation of rainfall process obeys a proportionate relationship, supporting the lognormal hypothesis.