Understanding the mechanism of land-cover related climate change in the low latitudes

Rainfall variability in the low latitudes in general and over tropical and sub-tropical Africa in particular, is largely affected by land surface characteristics like, vegetation cover, albedo and soil moisture. Understanding the local and dynamical effects of land-cover changes is crucial to future climate prediction, given ongoing population growth and increasing agricultural needs in Africa. Here, a set of sensitivity studies with a synoptic-scale regional climate model is presented, prescribing idealized scenarios of reduced vegetation cover over Africa. Beside the vegetation ratio itself, the leaf area index, forest ratio, surface albedo and roughness length are changed as well, in order to obtain a consistent scenario of land surface degradation. In addition, a second set of experiments is realized with altered soil parameters as expected to be coming alongwith a reduction in vegetation cover. Seasonal rainfall amount decreases substantially when the present-day vegetation continuously disappears. The strongest changes are found over the Congo Basin and subsaharan West Africa, where the summer monsoon precipitation diminishes by up to 2000 mm and 600 mm, respectively. The rainfall response to vegetation changes is non-linear and statistically significant over large parts of subsaharan Africa. Convective precipitation is more sensitive than large-scale precipitation. The most prominent effect of land degradation is a decrease (increase) of latent (sensible) heat fluxes. As a consequence, the large-scale thermal gradients, as a key factor in the monsoonal flow over Africa, are modified leading to a southward shift of the intertropical convergence zone and enhanced moisture advection over the southernmost part of West Africa and the central Congo Basin. The mid-tropospheric jet and wave dynamics are barely affected by land-cover changes. Although the large-scale dynamical response is favourable to increasing rainfall amount, the moisture budget is predominantly governed by reduced evapotranspiration, overcompensating the positive dynamical effect and inducing a weakening of the regional-scale water recycling. The related changes in the soil properties may additionally contribute to a reduction in rainfall amount, albeit of lower amplitude.

Evaluation of different convective schemes on simulation of thunderstorm event over Delhi by ARPS Model

Three difference cumulus parameterization schemes namely, Kain-Fritsch, New Kain-Fritsch and the Betts-Miller-Janjic are used to simulated convective rainfall associated with two thunderstorm events over Delhi by Advanced Regional Prediction Model (ARPS). An inter comparison of model simulated precipitation in respect of each convection scheme is made with reference to observed precipitation. The study shows that for the Delhi thunderstorm events, the Kain-Fritsch scheme provides more realistic results. This scheme is able to capture the temporal distribution of rainfall and the timely development of thunderstorm in both the cases. While the other two schemes fail to capture these features. However, the Kain-Fritsch scheme is found to overestimate the rainfall amount.

Value addition in district level dynamical forecast during monsoon depressions and storms

ABSTRACT. The trials of district level forecasts yielded encouraging results during 2005 monsoon. The purpose of this paper is to document the methodology followed in the value addition during the periods of monsoon depressions and storms. The focus is on the use of Mean Sea Level (MSL) positions and the 850 hPa circulation features predicted by different model centres, especially the European Centre for Medium-Range Weather Forecasts (ECMWF). The ECMWF-predicted 72 hr MSL position of the monsoon depression centre was found to be significantly correlated to the actual position of the system and the central location of the realized rainfall zone associated with the system. Even the predicted location of the system at 850 hPa by the ECMWF has been found useful in identifying the districts that received heaviest rainfall associated with the monsoon systems.MM5 and T-80 – predicted locations of the system at 850 hPa yielded lower correlations with the location of the actual rainfall zone associated with the system. As ECMWF – predicted rainfall was not available the rainfall predicted by MM5 and T-80 were used in the computations of the correlations with actual rainfall amounts associated with monsoon depressions and storms. The correlations between MM5 and T-80 – predicted average and maximum rainfall associated with systems and corresponding actual were poor. Though it is not difficult to identify the districts that are likely to be affected by the heavy rainfall associated with monsoon depressions/storms, the prediction of exact rainfall amount for each district (beyond heavy, very heavy or exceptionally heavy categories) is difficult from the model outputs which makes such forecasts a very challenging task. Therefore, the value addition using other inputs such as satellite information, synoptic charts, climatology etc. are very useful in the prediction of rainfall amounts associated with monsoon systems.

Holocene wet episodes recorded by magnetic minerals in stalagmites from Soreq Cave, Israel

This study demonstrates the feasibility of speleothem magnetism as a paleo-hydrology proxy in speleothems growing in semi-arid conditions. Soil-derived magnetic particles in speleothems retain valuable information on the physicochemical conditions of the overlying soil, and changes in bedrock hydrology. Yet, the link between magnetic and isotopic proxies of speleothems has been only partly established. We reveal strong coupling between the inflow of magnetic particles (quantified using the magnetic flux index, IRMflux) and δ13C in two Holocene speleothems from Soreq Cave (Israel). The stalagmite record spans from ca. 9.7 to ca. 5.4 ka, capturing the warm-humid conditions associated with the early Holocene and the transition to mid-Holocene wet-dry cycles. Extremely low IRMflux during the early Holocene, indicating minimal contribution from the overlying soil, is accompanied by anomalously high δ13C (approaching bedrock values) hypothesized to be caused by high rainfall and soil erosion. By contrast, IRMflux during the mid-Holocene covaries with the saw-tooth cyclicity of δ13C and δ18O, interpreted as rapid fluctuations in rainfall amount. The peaks in IRMflux precede the negative (wet) δ13C peaks by ~60–120 yr. The apparent lag is explained as a rapid physical translocation of overlying soil particles via groundwater (high IRMflux) as a response to increasing rainfall, compared with slower soil organic matter turnover rates (10–102 yr).

A meso-scale study of groundwater fluctuations of Delhi in relation to rainfall

A study of groundwater level at Delhi in relation to rainfall has been conducted. Groundwater data for the period 1956 to 1966 in respect of 97 bores/holes in Delhi have been processed and synchronised and maps of mean ground-water level for urban Delhi have been prepared at fortnightly intervals for the monsoon period and at monthly intervals for the remaining months of the year. Monthly variations of groundwater levels against distance from river Yamuna have been studied and the distribution of the groundwater slope evaluated. Inter-relationship between the rainfall arid the groundwater levels has been studied graphically. A regression equation for the estimation of fortnightly groundwater level fluctuations from rainfall amount, duration of rainfall and the level of water in river Yamuna has been formulated and tested for its applicability.

Estimation of Temporal R-factor Based on Monthly Precipitation Data

This study assesses temporal variation in rainfall erosivity of Gurushikhar, Rajasthan, (India) on a monthly precipitation basis in the form of the USLE/RUSLE R-factor. The objective of the paper is to theoretically calculate rainfall erosivity when the unavailability of high temporal resolution pluviographic rainfall data such as Indian condition. In the study, the rainfall erosivity has been calculated using the Modified Fourier Index. The results show that the annual rainfall erosivity factor (R) value highest in the year 2017 and lowest in 1974. Conferring to an examination through NASA, earth’s global superficial temperatures in 2017 ranked as second warmest since 1880. Therefore, the rainfall amount was more in 2017 compared to past years, and also rainfall erosivity value suddenly increased in 2017, achieved the highest value. They concluded that the heavy precipitation events in the year are lead to an increase in rainfall erosivity value and risk of soil erosion.

Lake water quality observed after extreme rainfall events: implications for water quality affected by stormy runoff

To investigate the effects of extreme rainfall events (EREs) on lake water quality, we analyzed the lake monthly monitoring data at the stations close to inflow rivers. We tested the hypothesis that the effects depend on rainfall magnitude, season, distance from the rivers, etc. The highest correlations with many water-quality indicators were obtained for the rainfall amount for the preceding 10 days before the water sampling days; i.e., negatively with Na+, Cl−, chlorophyll a (Chla), and COD, and positively with ortho-phosphate. We used the threshold of rainfall amount ≥ 140 mm to choose EREs and evaluate the water quality on such occasions because quite low values of Na+, Cl−, and Chla thresholds (≤ 25th percentile) were observed in some of the EREs. The event samples (group 1; G1) meeting the above thresholds probably represented the inflow waters during EREs (generally occurred in Oct–May), and the others (group 2; G2) indicated significant changes in the lakes (Jun to Sep), probably depending on the lake’s state of primary production. In the events of lower rainfall events (50 mm to 140 mm for 10 days), fast recoveries of algal production usually occurred even in Oct to May. The values of COD, total phosphorus, and ortho-phosphate in G1 were higher than the values of river water quality in the baseflow conditions, whereas both the G1 and baseflow values of nitrogen were nearly the same. The effects of ERFs on lake water quality from the viewpoints of tempo-longitudinal distributions, future monitoring, and climate change were discussed.

Something fishy going on? Evaluating the Poisson hypothesis for rainfall estimation using intervalometers: results from an experiment in Tanzania

A new type of rainfall sensor (the intervalometer), which counts the arrival of raindrops at a piezo electric element, is implemented during the Tanzanian monsoon season alongside tipping bucket rain gauges and an impact disdrometer. The aim is to test the validity of the Poisson hypothesis underlying the estimation of rainfall rates using an experimentally determined raindrop size distribution parameterisation based on Marshall and Palmer (1948)'s exponential one. These parameterisations are defined independently of the scale of observation and therefore implicitly assume that rainfall is a homogeneous Poisson process. The results show that 28.3 % of the total intervalometer observed rainfall patches can reasonably be considered Poisson distributed and that the main reasons for Poisson deviations of the remaining 71.7 % are non-compliance with the stationarity criterion (45.9 %), the presence of correlations between drop counts (7.0 %), particularly at higher arrival rates (ρa>500 m-2s-1), and failing a χ2 goodness-of-fit test for a Poisson distribution (17.7 %). Our results show that whilst the Poisson hypothesis is likely not strictly true for rainfall that contributes most to the total rainfall amount, it is quite useful in practice and may hold under certain rainfall conditions. The parameterisation that uses an experimentally determined power law relation between N0 and rainfall rate results in the best estimates of rainfall amount compared to co-located tipping bucket measurements. Despite the non-compliance with the Poisson hypothesis, estimates of total rainfall amount over the entire observational period derived from disdrometer drop counts are within 4 % of co-located tipping bucket measurements. Intervalometer estimates of total rainfall amount overestimate the co-located tipping bucket measurement by 12 %. The intervalometer principle shows potential for use as a rainfall measurement instrument.