Convective Rain Cells Research Articles

Lightning-based propagation of convective rain fields

Abstract. This paper describes a new multi-sensor approach for continuously monitoring convective rain cells. It exploits lightning data from surface networks to propagate rain fields estimated from multi-frequency brightness temperature measurements taken by the AMSU/MHS microwave radiometers onboard NOAA/EUMETSAT low Earth orbiting operational satellites. Specifically, the method allows inferring the development (movement, morphology and intensity) of convective rain cells from the spatial and temporal distribution of lightning strokes following any observation by a satellite-borne microwave radiometer. Obviously, this is particularly attractive for real-time operational purposes, due to the sporadic nature of the low Earth orbiting satellite measurements and the continuous availability of ground-based lightning measurements – as is the case in most of the Mediterranean region. A preliminary assessment of the lightning-based rainfall propagation algorithm has been successfully made by using two pairs of consecutive AMSU observations, in conjunction with lightning measurements from the ZEUS network, for two convective events. Specifically, we show that the evolving rain fields, which are estimated by applying the algorithm to the satellite-based rainfall estimates for the first AMSU overpass, show an overall agreement with the satellite-based rainfall estimates for the second AMSU overpass.

Total CNIDR degradation during rain fade conditions of a dual polarized satellite link suffering from double adjacent satellite interference

AbstractAtmospheric impairments have a significant influence on the performance of modern satellite communication networks, working at Ku, Ka and potentially Q/V frequency bands. Both differential rain attenuation from an adjacent satellite system, operating at the same frequency range, and cross‐polarization phenomena on dual polarized satellite systems due to rain and ice crystals, induce a further aggravation on the already limited signal‐to‐noise‐plus‐total‐interference ratio (CNIDR), due to the frequency and orbital congestion of today's global communication satellite constellation. In the present paper, a new statistical methodology, towards the modelling of CNIDR under rain fade conditions, is proposed to include interference effects by two adjacent satellites, incorporating the impact of correlated fading channels (spatial rainfall inhomogeneity) of multiple slant paths. The method is based on a convective raincell model and the lognormal assumption for both the point rain‐rate statistics and the slant‐path rain attenuation. The obtained numerical results indicate the significant impact of the second interfering satellite on the aggravation of total interference effects. Some simple mathematical formulas for the prediction of the CNIDR, based on the derived theoretical results, and demonstrating the influence of various link parameters on the total link availability statistics, are also presented. Copyright © 2009 John Wiley & Sons, Ltd.

Hydrologic response of a semi-arid watershed to spatial and temporal characteristics of convective rain cells

Abstract. Rain can be measured and represented in many ways such as point data from rain gauges, grid data from meteorological radar, or interpolated data. In this paper we represent rain fields by implementing a rain cell model of convective rain cells. The rain fields are used as an input to a hydrological model to test the watershed response to spatial and temporal characteristics of the rain cells. As a case study we tested an extreme storm event over a semi-arid watershed in southern Israel. The rain cell model was found to simulate the rain storm adequately. The use of these modeled cells allowed us to test the sensitivity of the watershed hydrological response to rain cell characteristics and it was found that the watershed is mainly sensitive to the starting location of the rain cell. Relatively small changes in the rain cell's location, speed and direction may increase watershed peak discharge by three-fold.

Study of wind fields associated with subtropical squall lines using Envisat Synthetic Aperture Radar images and ground-based weather radar data

Squall lines are lines of organized convective rain cells accompanied by wind shear and high wind gusts. They are usually studied by using ground-based weather radar. In this paper, three squall line events, which occurred over the South China Sea in 2006 and 2007, are investigated by using simultaneously acquired Synthetic Aperture Radar (SAR) images of the European Envisat satellite and ground-based Doppler weather radar data of the Hong Kong Observatory. Both radars yield wind fields: the SAR near-sea surface wind fields and the Doppler radar upper-air wind fields. Both wind fields are of high spatial resolution (sub-kilometre scale), making them useful to study fine-scale features in squall lines. Three squall lines events, each having distinct features, are presented. They show exemplarily the advantage of synergistic use of both SAR and ground-based weather radar in the study of squall lines.

Analysis of Short Space–Time-Scale Variability of Oceanic Rain Using TOPEX/Jason

Abstract Information on the spatial and temporal variability of rain rate is important not only for meteorology and hydrology but also for the design of remote sensing and in situ measuring or of millimeter wave communication systems. The Ocean Topography Experiment (TOPEX)/Jason Tandem Mission (TM) collocated rain dataset is used in this study to determine the small space-scale (5 km) and time-scale (70 s) rain variability. TOPEX and Jason dual-frequency (Ku and C bands) radar altimeter data have been extensively used during the past decade to detect and study oceanic precipitations. During the TM, designed to intercalibrate and validate the two altimeters, the two satellites were put on the same orbit with a 70-s time separation. With combined use of altimeter and passive microwave radiometers (also available on board altimeter missions), rain intensity, rain attenuation, rain layer height, rain event length, and surface winds can also be estimated and provide valuable coincident geophysical contextual information. The size of the TM collocated rain database (140 000 samples) is large enough to allow a meaningful statistical analysis of the time–space variability of rain over the World Ocean. The analysis of the different terms contributing to the variability of rain attenuation, from which rain rate is inferred, shows that the geophysical and/or instrumental noise is small enough to allow a meaningful estimation of the variability of the measured rain rate. The analysis of the time and space variabilities and their relation reveals two well-defined regimes. The first one, corresponding to convective rain cells (i.e., rain rate greater than 3–4 mm h−1, length smaller than 50 km, and freezing level greater than 3.5 km), is characterized by high temporal and spatial variabilities (greater than 2–3 mm h−1) that increase with increasing rain intensity and decreasing cell length. Horizontal variability is significantly larger than the temporal one and surface wind has a very limited impact. The second regime corresponds to stratiform and/or weak rain cells. The temporal and spatial variabilities are relatively low (on the order of 1–2 mm h−1) and vary little with rain intensity and cell length. The temporal variability increases with surface wind and largely exceeds the spatial variability (ratio of 2 or more); the ratio strongly increases with increasing wind speed.

Analysis of the 26 July 2005 heavy rain event over Mumbai, India using the Weather Research and Forecasting (WRF) model

AbstractA case‐study of the 26 July 2005 Mumbai heavy‐rain event that recorded 944 mm rainfall in 24 h with significant spatial variability was carried out using the Weather Research and Forecasting (WRF) model. The event was poorly forecasted by operational models and resulted in large human and economic losses. The present results indicate that the WRF system was able to reproduce the heavy rainfall event and the associated dynamical and thermo‐ dynamical features. A number of experiments were conducted with the WRF model that suggest the highly localized, heavy rain was the result of an interaction of synoptic‐scale weather systems with the mesoscale, coastal land‐surface features. These experiments indicate that the large‐scale rising motion over the Mumbai region was synoptically forced. Analysis of the model‐simulated intense, but short‐lived, convective rain cells forming in the large‐scale rising motion over Mumbai traces their moisture source to the north and northwesterly flow from the Arabian Sea. Synthetic sensitivity simulations without topography and without a land surface (land replaced with water) show that the large‐scale synoptic flow positioned the low‐pressure system over the Arabian Sea, while the mesoscale land‐surface (including topography and latent heating) feedback modulated the location and intensity of the rain by changes in the winds and regional moisture convergences. Another important feature captured in the high‐resolution model analysis is the formation of a mesoscale vortex over Mumbai that appears to have enhanced the conditions for localized, heavy rainfall over Mumbai. Copyright © 2008 Royal Meteorological Society

Spatial and temporal characteristics of precipitation using an extensive network of ground gauge in the Korean Peninsula

The aim of the present study is to investigate the spatial and temporal structures of precipitation over the Korean Peninsula using extensive AWS (automatic weather stations) observation network data for the summertime from May to September. Additionally TRMM/PR precipitation data in the southern part of peninsula was used to investigate the vertical structure. For the spatial and temporal scales of hourly precipitation, the e-folding threshold approach was employed to cut off the correlation in terms of distance in km and time in hours. From a correlation analysis of AWS precipitation in terms of time and space, it was found out that the e-folding distance and e-folding time in correlation coefficients ranged from 50 km–110 km and 1 h–2 h. The shortest distance and time in e-folding values were found to be in July and August. Precipitation structures in May and September tended to be isotropic, a cell-type structure, and those of July and August had an apparent band type, from the southwest to northeast. In the case of the vertical feature of precipitation, the correlation with height showed that the vertically efficient height was within 5 km as convective rain cells with a monthly difference of 1.2 km. In this study, the coastal effect tended to slightly increase threshold values.

Adjacent Satellite Interference Effects on the Outage Performance of a Dual Polarized Triple Site Diversity Scheme

The design of reliable, modern satellite communication networks, in which both frequency and orbital congestion are increasing, requires modeling of interference effects. The dominant sources of aggravation of nominal interference due to propagation phenomena are assumed to be differential rain attenuation from an adjacent satellite communication network operating at the same frequency and cross polarization due to rain and ice-crystals. A physical methodology to predict the statistics of the carrier-to-noise-plus-total-interference (CNIDR), which has already been applied to single and double-site systems, is extended to include triple-site diversity reception schemes. This method is based on a model of convective raincells model and the lognormal assumption for both the point rainfall statistics and slant path rain attenuation. The statistical properties of spatial inhomogeneity of rain attenuation over six satellite slant paths is firstly here presented. A set of simple, approximate formulas are presented which follow from a regression analysis on the previous theoretical results. The results serve to examine the influence of various parameters upon the total availability performance.

The mesoscale convection life cycle: Building block or prototype for large-scale tropical waves?

A cumulonimbus cloud may ascend and spawn its anvil cloud, precipitation, and downdrafts within an hour or so. This paper inquires why a similar progression of events (life cycle) is observed for tropical weather fluctuations with time scales of hours, days, and even weeks. Regressions using point data illustrate the characteristic unit of rain production: the mesoscale convective system (MCS), covering tens of kilometers and lasting several hours, with embedded convective rain cells. Meanwhile, averages over larger spatial areas indicate a self-similar progression from shallow to deep convection to stratiform anvils on many time scales. Synthetic data exercises indicate that simple superpositions of fixed-structure MCS life cycles (the Building Block hypothesis) cannot explain why longer period life cycles are similar. Rather, it appears that an MCS may be a small analogue or prototype of larger scale waves. Multiscale structure is hypothesized to occur via a Stretched Building Block conceptual model, in which the widths (durations) of zones of shallow, deep, and stratiform anvil clouds in MCSs are modulated by larger scale waves. Temperature ( T) and humidity ( q) data are examined and fed into an entraining plume model, in an attempt to elucidate their relative roles in these large-scale convection zone variations. T profile variations, with wavelengths shorter than troposphere depth, appear important for high-frequency ( ∼ 2–5-day period) convectively coupled waves, as density directly links convection (via buoyancy) and large-scale wave dynamics (via restoring force). Still, the associated q anomalies are several times greater than adiabatic, suggesting a strong amplification by shallow convective feedbacks. For lower frequency (intraseasonal) variability, q anomalies are considerably larger compared to T, and may be dominant.

Spatial characterization of raincell horizontal profiles from C-band radar measurements at mid-latitude

Abstract. A spatial characterization of mid-latitude mesoscale rain fields from C-band radar measurements is performed by means of a systematic analysis and modelling of convective raincell shapes. To this aim a large rainfall dataset, derived from an operational C-band dual-polarized radar, has been continuously collected from 1996 to 1999. The radar-derived rain fields consist of 1558 grids of 256×256 km2 with a spatial resolution of 1 km. A new accurate and adaptive algorithm for raincell identification is introduced and thoroughly discussed. From this analysis, a quality-controlled set of 2601 raincells, together with the radial rain intensities (or raincell horizontal profiles), is extracted. Three one-dimensional analytical models of rainfall horizontal profile are reviewed and tested by best fitting their parameters against estimated raincell data. The statistical results of this intercomparison are quantitatively analyzed and discussed in terms of mean rainfall horizontal profiles and root mean square errors.

The structure of convective rain cells at mid-latitudes

Abstract. Rain cells are structures which represent an important component of convective precipitation and a study of their properties represents a necessary step both towards improved stochastic models of small-scale precipitation and for the verification of deterministic high resolution local-area models. The case of intense convective precipitation in the tropics has been analysed in a recent study (von Hardenberg et al., 2003). Here we extend the analysis to mid-latitudes and we present results on the structure of convective rain cells observed by radar measurements in Italy. In particular we consider the average shape of precipitation cells and its dependence on radar resolution and the distributions of ellipticities.

Rainfall Estimation from C-Band Polarimetric Radar in Okinawa, Japan: Comparisons with 2D-Video Disdrometer and 400 MHz Wind Profiler

This paper presents an inter-comparison of rainfall parameters (median volume diameter and rain rate) using C-band polarimetric radar, a 2D-video disdrometer and a 400 MHz profiler for the Baiu front event of 8-9 June 2005 in Okinawa, Japan. These instruments are part of the Okinawa Sub-Tropical Environment Remote Sensing Center, operated by the National Institute of Information and Communications Technology (NICT). The 2D-video disdrometer is used to derive the mean axis ratio of raindrops versus drop diameter, as well as the drop size distribution for the Baiu event. The data are then used to simulate various relations between polarimetric scattering parameters such as: specific attenuation (Ah), and specific differential attenuation (Adp), versus specific differential phase (Kdp) which are required to correct the measured reflectivity at horizontal polarization (Zh), and the differential reflectivity (Zdr) for rain attenuation. The 2D-video disdrometer data are also used to arrive at retrieval formulas for median volume diameter (D0) from radar Zdr and rain rate from radar Kdp.The intense Baiu event of 8-9 June 2005 was composed of heavy convective rain cells embedded in large areas of stratiform rain. The inter-comparison of D0 and rain rate (R) between instruments was conducted for 12 hours (03:00-07:00, 11:00-19:00 UTC on 8th June 2004). The C-band radar retrievals were found to be in excellent agreement with the 2D-video disdrometer for the entire period. The 400 MHz profiler retrievals of D0 and R were in good agreement with 2D-video disdrometer during the more steady rain periods, with more scatter observed during the heavier convective rain periods. These inter-comparisons demonstrate the accuracy of C-band polarimetric radar to retrieve important rainfall parameters, as well as the accurate correction for rain attenuation using differential propagation phase.

Aggravation of radio interference effects on a dual polarized Earth‐space link by two adjacent interfering satellites under rain fades

Interference phenomena affecting the performance and availability of an Earth‐space system are of utmost importance for the reliable design of a satellite communication network. Commercial satellite networks operate or will operate at Ku, Ka, and V frequency bands, in which rain is the dominant fading mechanism. Nominal interference induced on an Earth‐space link by an adjacent satellite network operating at the same frequency is further aggravated because of the potential existing differential rain attenuation, as well as the rain and ice crystals depolarization valid for a frequency reuse system. In the present paper, an existing model, predicting the degradation of the total carrier‐to‐interference ratio under rain fade conditions, is properly modified considering interference effects by two adjacent satellites. The method is based on a convective rain cell model and the lognormal assumption for the point rain rate statistics. The obtained numerical results indicate the significant impact of the second interfering satellite on the aggravation of radio interference effects. Also presented are some simple mathematical formulas for the prediction of the carrier‐to‐interference ratio, based on the above theoretical results, appropriate for use by the system designer for back of the envelope computations.

Comparison of Measured Rain Attenuation in the 12-GHz Band with Predictions by ITU-R Methods

The rain attenuation in the 12-GHz band and one-minute-rain rate were measured in Tokyo over a four-year period (2000-2003). The statistical characteristics of this data are presented. The one-minute-rain rates at 0.01% of time percentage of year in Tokyo and Osaka are compared to other past and recent values. The comparison of measured rain attenuation in the 12-GHz band in Tokyo and Osaka with prediction by ITU-R methods is conducted. The root-mean-square prediction error of rain attenuation for the prediction by ITU-R Rec.P.618 is evaluated. Convective rain cell effects can be seen in the scatter diagram of one-minute-rain rate and rain attenuation. However, it is found that the effect is not properly accounted for by the slant path length adjustment factor of P.618-8. A reliable rain attenuation prediction requires some revisions of the slant path length adjustment factor with taking local weather characteristics into account for the P.618-8.

Dynamics of intense convective rain cells

Abstract. Intense precipitation events are often convective in nature. A deeper understanding of the properties and the dynamics of convective rain cells is, therefore, necessary both from a physical and operational point of view. The aim of this work is to study the spatial-temporal properties of convective rain cells by using a fully parameterized nonhydrostatic code (Lokal Model) in simplified model configurations. High resolution simulations are performed and it is expected that the deep moist convection and the feedback mechanisms affecting larger scales of motion can then be resolved explicitly and some of the critical constraints of parameterization schemes can be relaxed. The sensitivity of the spatio-temporal properties of simulated cells to spatial resolution and microphysics schemes is investigated and discussed through a direct comparison with typical intense convective cells measured by radars.

Rainfall modeling for integrating radar information into hydrological model

AbstractA spatial rainfall model was applied to radar data of air mass thunderstorms to yield a rainstorm representation as a set of convective rain cells. The modeled rainfall was used as input into hydrological model, instead of the standard radar‐grid data. This approach allows a comprehensive linkage between runoff responses and rainfall structures. Copyright © 2005 Royal Meteorological Society

Atmospheric front over the East China Sea studied by multisensor satellite and in situ data

A frontal feature visible on a synthetic aperture radar (SAR) image acquired by the Radarsat satellite over the East China Sea on 19 November 2000 is analyzed in conjunction with data acquired by Quikscat, TOPEX/Poseidon, Tropical Rain Measurement Mission (TRMM), Defense Meteorological Satellite Program (DMSP), and National Oceanic and Atmospheric Administration (NOAA) satellites, and with data obtained from ship measurements. Although this frontal feature is located close to the Kuroshio front, it is demonstrated that it is not a sea surface manifestation of an oceanic front, but rather of an atmospheric front extending over 800 km from an area of the Pacific Ocean northeast of Taiwan to the southern coast of Korea. It is a cold front moving in the southeast direction with a speed of approximately 45–50 km/hour and associated with a 40‐km‐wide rainband trailing the front. The Radarsat image, which has a resolution of 50 m, reveals fine‐scale structures of the atmospheric front, in particular small‐scale convective rain cells embedded in the front. Conclusion is drawn that accurate interpretation of frontal features in SAR images requires use of additional meteorological and remote sensing data and information.

A model for the estimation of the carrier-to-noise plus total interference ratio between two adjacent dual polarized satellite links sharing the same frequency band

In modern satellite communications, matters such as frequency congestion, transmission of dual polarized carriers and increase of the number of geostationary satellites in use, necessitate the implementation of interference analysis in neighbouring networks so as to ensure satisfactory quality of service. In this paper, a recently proposed model for the prediction of the degradation of the carrier-to-interference ratio due to differential rain attenuation and cross-polarization, which is applicable only for the noise-dominant case, has been properly modified to include the general case. The proposed model is based on the lognormal assumption for the point rainfall rate statistics and the convective raincell model. Due to complexity of the proposed analysis, some simple regression-derived formulas have been generated forming a very useful tool for the system engineer. The elaborations of numerical results examine the influence of various parameters upon the total availability performance and the optimum utilization of the geostationary orbit. Copyright © 2005 John Wiley & Sons, Ltd.

The shape of convective rain cells

The shape of rainfall cells is a crucial ingredient of stochastic point‐process models that describe intense precipitation events. Here we study the shape of the individual rain cells measured during the TOGA/COARE and GATE radar experiments. We find that, on average, rainfall intensity decreases exponentially from the cell's center. An exponential profile provides a good estimate of the precipitation intensity in the bulk of the cell. By contrast, the standard assumption of a Gaussian shape leads to significant underestimation of the precipitation intensity in the central portion of the cell and of the associated hydrologic response.

Statistics of differential rain attenuation on converging terrestrial propagation paths

Rain attenuation is considered to be one of the main causes of interference impairments for terrestrial communication systems operating at frequencies above 10 GHz. A method to predict the cumulative distribution of differential rain attenuation on two converging microwave links is proposed. It is based on a convective raincells model for the structure of the rainfall medium, as well as the lognormal assumption for the point rainfall distribution. The degradation of the carrier-to-interference ratio under rainy conditions is also examined. Numerical results are compared with a set of available experimental data for the differential rain attenuation over pairs of terrestrial links in Brazil.