ORCID Profile
0000-0002-0642-6876
Current Organisation
University of Reading
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Publisher: Wiley
Date: 07-04-2019
DOI: 10.1002/QJ.3439
Publisher: American Geophysical Union (AGU)
Date: 26-01-2023
DOI: 10.1029/2022EF002995
Abstract: Anthropogenic aerosol emissions from North America and Europe have strong effects on the decadal variability of the West African monsoon (WAM). Anthropogenic aerosol effective radiative forcing is model dependent, but the impact of such uncertainty on the simulation of long‐term WAM variability is unknown. We use an ensemble of simulations with HadGEM3‐GC3.1 that span the most recent estimates in simulated anthropogenic aerosol effective radiative forcing. We show that uncertainty in anthropogenic aerosol radiative forcing leads to significant uncertainty at simulating multi‐decadal trends in West African precipitation. At the large scale, larger forcing leads to a larger decrease in the interhemispheric temperature gradients, in temperature over both the North Atlantic Ocean and northern Sahara. There are also differences in dynamic changes specific to the WAM (locations of the Saharan heat low and African Easterly Jet, of the strength of the West African westerly jet, and of African Easterly Wave activity). We also assess effects on monsoon precipitation characteristics and temperature. We show that larger aerosol forcing results in a decrease of the number of rainy days and of heavy and extreme precipitation events and warm spells. However, simulated changes in onset and demise dates do not appear to be sensitive to the magnitude of aerosol forcing. Our results demonstrate the importance of reducing the uncertainty in anthropogenic aerosol forcing for understanding and predicting multi‐decadal variability in the WAM.
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15748
Abstract: & & IMPROVE is motivated by the effects of orography on Indian precipitation as part of the diurnal cycle of convection, contributing to water supply, as well as its role in extreme events.& IMPROVE considers two focal regions.& The Western Ghats, which intercept the monsoon flow across the Arabian Sea, receive some of the most frequent and heaviest rainfall during summer as well as being subject to extremes such as the 2018 Kerala floods.& Meanwhile, the Himalayas play a vital role in separating dry midlatitude flows from tropical airmasses and are subject to extremes during the summer monsoon, as well as in winter due to the passage of western disturbances.& This presentation summarizes the key results of IMPROVE.& Firstly, we examine the impact of orography on the observed convective diurnal cycle and assess its simulation in models at a range of resolutions including convection-permitting scales.& MetUM and WRF model experiments are used to identify key mechanisms and test their capability at simulating scale interactions between forcing at the large scale from the BSISO and newly identified regimes of on- and offshore convection near the Western Ghats.& An additional aspect to this work is the construction of a two-layer analytical model to test the behaviour of sheared flow perpendicular to a ridge analogous to the Western Ghats.& Secondly, the role of orography in extreme events is considered.& For the Western Ghats, this focuses on the interaction between monsoon low-pressure systems and the southwesterly flow in enhancing local rainfall.& For the Himalayas, we focus on characterising interactions between tropical lows and western disturbances in enhancing the orographic precipitation.& The work in IMPROVE works towards a deeper understanding of orographic rainfall and its extremes over India and uncovering why such mechanisms may be poorly represented in models.& &
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-12069
Abstract: & & Regional orography around India exerts a profound control on weather and climate, both in summer and winter as part of the diurnal cycle of convection, as well as in extreme events. & This poster summarizes the key results of the Indo-UK IMPROVE project (Indian Monsoon Precipitation over Orography: Verification and Enhancement of understanding).& IMPROVE considers two focal regions.& The Western Ghats intercept the monsoon flow across the Arabian Sea and receive some of the most frequent and heaviest summer rainfall, including being subject to extremes such as the 2018 Kerala floods.& Meanwhile, the Himalayas play a vital role in separating dry midlatitude flows from tropical airmasses in summer, while suffering extremes in winter due to western disturbances - cyclonic storms propagating on the subtropical westerly jet.& & & & & We examine the impact of orography on the observed convective diurnal cycle and assess its simulation in models at a range of resolutions including convection-permitting scales.& MetUM and WRF model experiments, in addition to DWR retrievals, are used to identify key mechanisms between forcing at the large scale from the BSISO and newly identified regimes of on- and offshore convection near the Western Ghats.& An additional aspect to this work is consideration of a novel Froude number approach for understanding the convective regimes.& Secondly, the role of orography in extreme events is considered, including its interactions between passing tropical depressions or western disturbances.& Finally, land-atmosphere interactions occurring during the diurnal cycle of precipitation in the Western Ghats and Himalayas regions are discussed.& IMPROVE works towards a deeper understanding of orographic rainfall and its extremes over India and uncovering why such mechanisms may be poorly represented in models.& &
Publisher: Wiley
Date: 10-2022
DOI: 10.1002/QJ.4367
Abstract: Variations in the character of monsoonal rainfall over the Western Ghats region on the west coast of India are studied using radiosondes, satellite observations, and reanalysis products. Summer monsoon rainfall over this region occurs in alternate offshore and onshore phases. It is shown that these phases are controlled primarily by the strength of the low‐level westerly jet. Thus, a classification based on the Froude number, , of the onshore flow is proposed, where, is the mountain height, is the mean wind speed, and is the mean Brunt–Väisäla frequency over depth . At low ( 0.5), onshore winds are weak and the diurnal thermal fluctuation over the orography is strong the land–sea and mountain–valley circulations are enhanced, leading to a stronger diurnal control over the rainfall. A nocturnal offshore propagation of rainfall from the west coast is seen during this phase. Rainfall over the rainshadow region to the east of the Western Ghats also increases, due to a weaker lee effect, while it decreases over the Western Ghats, due to a greater blocking effect. At high ( 1), orographic blocking of the low‐level winds is weak. Thus, rainfall is enhanced over the Western Ghats and reduced over the rainshadow region due to a stronger lee effect. In this phase, the diurnal thermal fluctuation over the orography is weak. The bulk Richardson number is less than 1, suggesting a dominance of vertical wind shear over the buoyancy forces. The level of free convection and convective inhibition over the west coast are also very low. Hence, at high , rainfall over the west coast results mainly from mechanical uplifting of the westerly winds by the Western Ghats, with no preference for a particular time of day. These findings will help in improving the representation of orographic effects and the diurnal cycle of rainfall in numerical models.
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-1628
Abstract: & & An increase in European and North American anthropogenic aerosol emissions in the 1970s and 1980s led to a decrease in Sahel precipitation during the same time. Although significant, the effect of anthropogenic aerosols on Sahel precipitation is uncertain across a set of CMIP6 single-forcing simulations. However, understanding the cause of this uncertainty in simulated effects of anthropogenic aerosols on West African precipitation in CMIP6 models is difficult, largely due to the relatively small number of large-ensembles with single-forcing simulations. Here, we use a single-model ensemble that spans much of the range in anthropogenic aerosol effective radiative forcing from the CMIP5 and CMIP6 multi-model ensembles. The simulations are performed with HadGEM3-GC3.1 and the different forcings are achieved by scaling emissions in anthropogenic aerosols. We show that changes in anthropogenic aerosols have strong effects on the drivers of the West African monsoon, and on precipitation extremes. Further, we show that the magnitude and even the occurrence of the West African drought (1970s-1980s) strongly depend on the simulated effective anthropogenic aerosol radiative forcing in the model simulations. Our results show that a better understanding of the effects of anthropogenic aerosols on climate is necessary to improve predictions of decadal trends in Sahel precipitation.& & &
Publisher: Copernicus GmbH
Date: 23-06-2020
DOI: 10.5194/ACP-2020-478
Abstract: Abstract. Anthropogenic aerosols are dominant drivers of historical monsoon rainfall change. However, large uncertainties in the radiative forcing associated with anthropogenic aerosol emissions, and the dynamical response to this forcing, lead to uncertainty in the simulated monsoon response. We use historical simulations in which aerosol emissions are scaled by factors from 0.2 to 1.5 to explore the monsoon sensitivity to aerosol forcing uncertainty (−0.38 W m−2 to −1.50 W m−2). Hemispheric asymmetry in emissions generates a strong relationship between scaling factor and both hemispheric temperature contrast and meridional location of tropical rainfall. Increasing the scaling from 0.2 to 1.5 reduces the global monsoon area by 3 % and the global monsoon intensity by 2 % over the period 1950–2014, and switches the dominant influence on the 1950–1980 monsoon rainfall trend between greenhouse gas and aerosol. Regionally, aerosol scaling has a pronounced effect on Northern Hemisphere monsoon rainfall.
Publisher: Wiley
Date: 29-08-2023
DOI: 10.1002/QJ.4550
Abstract: A transition from a predominantly offshore to an onshore rainfall phase over the west coast of India was simulated using three one‐way nested domains with 12, 4, and 1.33 km horizontal grid spacing in the Weather Research and Forecasting model. The mechanism of offshore–onshore rainfall oscillation and the orographic effects of the Western Ghats are studied. A convective parametrization scheme was employed only in the 12 km domain. A trough extending offshore from the west coast facilitates offshore rainfall. This trough is absent during the onshore phase, and rainfall occurs over the coast mainly via orographic uplift by the Western Ghats. The model overestimates rainfall over the Western Ghats at all resolutions as it consistently underestimates the boundary‐layer stratification along the coast. Weaker stratification weakens the blocking effect of the Western Ghats, resulting in anomalous deep convection and rainfall over its windward slopes. The 4 and 1.33 km domains simulate the offshore‐to‐onshore transition of rainfall but fail to capture a sufficient contrast in rainfall between land and sea compared with observations. The 12 km domain produces light rainfall, anchored along the coast, throughout the simulation period and, hence, gravely underestimates the offshore rainfall. The offshore rainfall persisted in the 4 and 1.33 km domains in a sensitivity experiment in which the Western Ghats were flattened. This suggests that orographic effects do not significantly influence offshore rainfall. In another experiment, the convective parametrization scheme in the 12 km domain was turned off. This experiment simulated the offshore and onshore rainfall phases correctly to some extent but the rainfall intensity was unrealistically high. Thus, a model with a horizontal grid spacing of , in which convection evolves explicitly, is desired for simulating the west‐coast rainfall variations. However, improvements in the representation of boundary‐layer processes are needed to capture the land–sea contrast.
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15444
Abstract: & & Precipitation distribution around an orographic barrier is controlled by the Froude Number (Fr) of the impinging flow. Fr is essentially a ratio of kinetic energy and stratification of winds around the orography. For Fr & 1 (Fr & ), the flow is unblocked (blocked) and precipitation occurs over the mountain peaks and the lee region (upwind region). While idealized modelling studies have robustly established this relationship, its widespread real-world application is h ered by the dearth of relevant observations. Nevertheless, the data collected in the field c aigns give us an opportunity to explore this relationship and provide a testbed for numerical models. A realistic distribution of precipitation over a mountainous region in these models is necessary for flash-flood and landslide forecasting. The Western Ghats region is a classic ex le where the orographically induced precipitation leads to floods and landslides during the summer monsoon season. In the recent INCOMPASS field c aign, it was shown that the precipitation over the west coast of India occurred in alternate offshore and onshore phases. The Western Ghats received precipitation predominantly during the onshore phase which was characterized by a stronger westerly flow. Here, using the radiosonde data from a station over the Indian west coast and IMERG precipitation product, we show that climatologically, these phases can be mapped over an Fr-based classification of the monsoonal westerly flow. Classifying the flow as 'High Fr' (Fr & ), 'Moderate Fr' ( 0.5 & Fr & #8804 1) and 'Low Fr' ( Fr & #8804 0.5 ) gives three topographical modes of precipitation -- 'Orographic', 'Coastal' and 'Offshore', respectively. & Moreover, these modes are not sensitive to the choice of radiosonde station over the west coast.& &
Publisher: Copernicus GmbH
Date: 03-12-2020
DOI: 10.5194/ACP-20-14903-2020
Abstract: Abstract. Anthropogenic aerosols are dominant drivers of historical monsoon rainfall change. However, large uncertainties in the radiative forcing associated with anthropogenic aerosol emissions, as well as the dynamical response to this forcing, lead to uncertainty in the simulated monsoon response. We use historical simulations from the “SMURPHS” project, run using HadGEM3-GC3.1, in which the time-varying aerosol emissions are scaled by factors from 0.2 to 1.5 to explore the monsoon sensitivity to historical aerosol forcing uncertainty (present-day versus preindustrial aerosol forcing in the range −0.38 to −1.50 W m−2). The hemispheric asymmetry in emissions generates a strong relationship between scaling factor and both hemispheric temperature contrast and meridional location of tropical rainfall. Averaged over the period 1950–2014, increasing the scaling factor from 0.2 to 1.5 reduces the hemispheric temperature contrast by 0.9 ∘C, reduces the tropical summertime land–sea temperature contrast by 0.3 ∘C and shifts tropical rainfall southwards by 0.28∘ of latitude. The result is a reduction in global monsoon area by 3 % and a reduction in global monsoon intensity by 2 %. Despite the complexity of the monsoon system, the monsoon properties presented above vary monotonically and roughly linearly across scalings. A switch in the dominant influence on the 1950–1980 monsoon rainfall trend between greenhouse gases and aerosol is identified as the scalings increase. Regionally, aerosol scaling has a pronounced effect on Northern Hemisphere monsoon rainfall, with the strongest influence on monsoon area and intensity located in the Asian sector, where local emissions are greatest.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Andrew Turner.