Severe Rainfall Events Research Articles

Numerical simulations of a severe rainfall event over the Eastern Cape coast of South Africa: sensitivity to sea surface temperature and topography

The intense precipitation event over East London, South Africa, on 15–16 August 2002 is investigated using a mesoscale model (MM5) and its sensitivity to sea surface temperature (SST) and topography tested. Over 300 mm of rain fell in 24 h over East London as compared to the August average of 78 mm. The intense precipitation resulted from a cut-off low system aloft and the formation of a low-level jet (LLJ) near the surface.The MM5 control simulation indicated that the intense precipitation was due to the ascent of moist air at the leading edge of the LLJ, which advected warm moist air at low levels onto the coastal mountains and was enhanced by the formation of a meso-beta scale low-pressure system just off the coast. The ascent at the leading edge of the LLJ formed the ascending arm of a convective circulation cell.Sensitivity experiments with MM5 indicated that the warm core of the Agulhas Current contributed to the formation of the meso-beta scale low, while the mesoscale structure of the SST field was important in determining its location and that of the LLJ, and therefore the precipitation in the model. Most of the moisture originated from the Agulhas Current region. The South African topography was found to influence the location of the LLJ via trapping a near-surface warm trough on the eastern side of the Drakensberg mountains. We suggest that deficiencies in the control experiment may have been due to inaccuracies in using a weekly mean SST field to force the model at the lower boundary.

Orographic effects on convective precipitation and space-time rainfall variability: preliminary results

Abstract. In the EFFS Project, an attempt has been made to develop a general framework to study the predictability of severe convective rainfall events in the presence of orography. Convective activity is embedded in orographic rainfall and can be thought as the result of several physical mechanisms. Quantifying its variability on selected area and time scales requires choosing the best physical representation of the rainfall variability on these scales. The main goal was (i) to formulate a meaningful set of experiments to compute the oscillation of variance due to convection inside model forecasts in the presence of orography and (ii) to give a statistical measure of it that might be of value in the operational use of atmospheric data. The study has been limited to atmospheric scales that span the atmosphere from 2 to 200 km and has been focused on extreme events with deep convection. Suitable measures of the changing of convection in the presence of orography have been related to the physical properties of the rainfall environment. Preliminary results for the statistical variability of the convective field are presented.

Numerical simulation of topography effects on the “00.7” server rainfall in Beijing

Abstract In an effort to study the severe rainfall event of 4–5th July 2000 in Beijing (with 24 h accumulated precipitation of 240 mm), we perform numerical simulations to investigate this event using the MM5v3.6 model. The model is initialized with the MM5/WRF 3DVAR analysis, which incorporates the ground-based GPS precipitable water vapor, automatic and conventional meteorological observations in its assimilation step. For 24 h accumulated precipitaion forecast, the threat scores are 0.72, 0.76, 0.67 and 0.63, for thresholds of 1, 5, 10, and 20 mm, respectively. Holding other factors unchanged, sensitivity experiments were conducted with different topographic resolutions of 110, 50 and 3.7 km to investigate the topographic effects on precipitation over the Beijing area. In these Sensitivity experiments, we attempt to preserve the realistic orographic distribution in the model topography under the condition of keeping the dynamic-themodynamic consistency of the initial model atmosphere to the extent poss...

Precipitation assessment of a superensemble forecast over South‐East Asia

AbstractA superensemble forecast that consists of six operational general circulation models is tested to simulate weather parameters over the equatorial South‐East Asian region. The forecast technique, developed at Florida State University, is a unique ensemble that has proved to be one of the better prognosticators for weather forecasting. In this paper, the root mean square errors of the superensemble precipitation for a case of a severe tropical rainfall event during the winter monsoon of 2001 were smaller than the other global operational member or ensemble models used, indicating a skilled forecast. This trait is also reflected in the equitable threat score which indicates the association of the common rainfall threshold distribution between the forecast and observed best rainfall analysis from the Tropical Rainfall Measurement Mission (TRMM) and Special Sensor Microwave Imager (SSM/I) dataset, where although it is slightly underpredicted for light rain thresholds, its results are still superior to other individual member or even the ensemble mean forecasts. Higher correlation between the superensemble forecast and the observed is maintained for up to 3‐day forecasts, which indicates its potential as a reliable and good forecast tool for predicting equatorial precipitation. Copyright © 2005 Royal Meteorological Society

A procedure for drainage network identification from geomorphology and its application to the prediction of the hydrologic response

Identifying channel initiation points is central to geomorphology and hydrology as they relate morphology, climate, and soil properties at the boundary between different surface runoff paths. Since catchment response is strongly influenced by the dynamics of water movement on the hillslope and in the channel, rainfall-runoff modeling is one of the fields in which improving the identification of channel initiation can lead to benefits. Among the various filtering criteria that can be found in the literature for channel recognition from digital elevation models, the one using contributing area and topographic slope shows interesting features in this context. Nevertheless, the area-slope criterion has been poorly applied. This is mainly due to the difficulties in objectively defining appropriate threshold values. This study proposes a new procedure to assess the area-slope threshold value. The resulting channel network is then used as input to a semi-distributed, event-based rainfall-runoff model able to describe severe rainfall events in small, steep basins. This model accounts for network and hillslope contributions to the total dispersion in the routing process, a key factor in determining the main features of the hydrologic response. In a geomorphologically homogeneous region, the set of model parameters shows interesting invariance properties with respect to storm and basin characteristics.

Verification of mesoscale modeling for the severe rainfall event over southern Ontario in May 2000

A coupled atmospheric‐hydrological model (CAHM) with a high‐resolution, self‐nesting and one‐way coupling capability is employed to simulate the severe rainfall event that lead to a flood in May 2000 over southern Ontario. Three verification approaches are carried out to evaluate the atmospheric mesoscale model performance. The results show that the 48‐h accumulated peak precipitation simulated by a mesoscale model successfully captures the observed peak rainfall recorded over a spatially dense rain gauge network in southern Ontario. Furthermore, the quantitative evaluation of the model predicted precipitation demonstrates that there is a systematic improvement in terms of the accuracies and skills when the model resolution is increased. In addition, an independent verification by comparing the CAHM simulated streamflow with the observed hourly streamflow shows the excellent agreement between the simulations and the observations in terms of magnitudes and timing of peak streamflows, indicating that precipitation is well simulated by the atmospheric mesoscale model.

A Nudging Scheme for the Assimilation of Precipitation Data into a Mesoscale Model

Abstract A nudging procedure for the assimilation of rainfall data into a mesoscale model [the Bologna Limited Area Model (BOLAM)] has been developed in order to improve short-range forecasting. The scheme modifies the model specific humidity profiles at every time step, according to the difference between observed and forecast precipitation. Different relaxation procedures are applied depending on the precipitation type (large-scale or convective rain), as estimated by the model itself. Optimizations of the nudging parameters and assessment of the scheme's performance was carried out in an idealized framework [(Observing System Simulation Experiment) OSSE-type strategy], implementing a lagged forecast scheme. Two events were selected for this purpose, both characterized by heavy precipitation in the Mediterranean Basin. The first was a severe orographic rainfall event, associated with the passage of a frontal system over the Alps during the Mesoscale Alpine Programme (MAP) field phase, in September 1999....

Poultry Litter and Grazing Animal Waste Effects on Runoff Water Quality

AbstractIn complex landscapes with multiple land uses, it is often difficult to identify the source of contaminant loadings. The objective of this study was to compare nutrient runoff as affected by grazing animal depositions vs. poultry litter application. Simulated rainfall was applied twice to 1.5 by 6.0 m runoff plots of tall fescue (Festuca arundinacea Schreb.) with treatments of no waste (CT), dairy calf feces and urine (DFU), poultry litter (PL), and dairy calf feces and urine with poultry litter (DFU + PL). Chemical properties of the runoff samples including pH, electrical conductivity (EC), C, soluble reactive phosphorus (SRP), total nitrogen (TN), NH4‐N, NO3‐N, K, Mg, S, B, Cu, Fe, Mn, Mo, Na, and Zn were determined. Plots receiving poultry litter had significantly greater losses of most nutrient parameters for both rainfall simulations. For the nutrient parameters of primary interest with regard to water quality, 5.0, 29.5, and 21.9% of the TN, NH4‐N, and SRP applied in the PL treatment were transported in runoff during the first rainfall simulation as compared to 3.9, 5.0, and 15.3%, respectively, for the DFU treatment. Comparable percentages of the applied nutrients were lost from the PL and DFU treatments even though the PL treatment, with the exception of NH4‐N, provided at least six times the amount of each nutrient. A severe rainfall event shortly after poultry litter application produces significantly greater nutrient losses as compared to similar application of grazing animal depositions at the rates used in the experiment.

A study on prediction error in radar rainfall forecasting and its application to on-line operation of a detention storage

Detention storages work for inundation contorol and water quality contorol. First flush is stored in the storage, treated after a rainfall event, and released to public water bodies like lakes, rivers, bays, etc. When rainfall forecasting by a radar is available and once a severe rainfall event is predicted, some amount of the stored first flush can be pre-released, and the resultant vacant capacity can be used for peak cut of the severe rainfall runoff to eliminate or reduce inundation volume. However large estimation error is inevitable in rainfall forecasting. In this paper, a basic study on on-line pre-release operation of a detention storage is reported, taking account of the large estimation error.

Landslides caused by rapid groundwater changes

Abstract A severe rainfall event occurs once every two years in Hong Kong, causing widespread landslides which result in human casualties and much damage. It has recently been established that these landslides are caused largely by short-period, high-intensity rainfall, with nearly all failures occurring within a few hours of peak intensity. The failures are therefore attributable mainly to rapid transient variations in groundwater conditions. These findings are in contradiction to previously accepted theories on failure mechanisms, and it is therefore necessary for these to be reconsidered. A new theoretical model is now being examined in Hong Kong to explain the rapid landslide response following high-intensity rainfall. This is based on well-defined hillslope hydrological theories. Of particular significance in this context are thought to be subsurface erosion pipes and tunnels which have been found to exist in some of Hong Kong’s hillslopes, especially those carpeted with colluvium.