Rainfall Enhancement Research Articles

Diurnal Variation of Rainfall Associated with Tropical Depression in South China and its Relationship to Land-Sea Contrast and Topography

Convective precipitation associated with tropical depression (TD) is one primary type of post-flooding season rainfall in South China (SC). Observations of the Tropical Rainfall Measuring Mission (TRMM) satellite have shown specific diurnal features of convective rainfall in South China, which is somewhat different from that in other seasons or regions of China. Convective precipitation is usually organized into a rainfall band along the southeastern coast of South China in the early morning hours. The rainfall band develops and intensifies quickly in the morning, then moves inland in the afternoon and, finally, diminishes at night. The daily convective rainfall along the coast is much more than that in the inland region, and heavy rainfall is often found along the coast. A long-duration heavy rainfall event associated with tropical depression “Fitow” during the period from 28 August to 6 September 2001, is selected in this study to explore the diurnal feature of convective rainfall and its formation mechanism. Modeling results of the 10-day heavy rainfall event are compared with both rain-gauge observation and satellite-retrieved rainfall. Total precipitation and its spatial distribution, as well as diurnal variations are reasonably simulated and agree well with observations. Further analysis reveals that the development and movement of convective precipitation is mainly related to the land and sea breezes. The anomalous height-latitudinal circulation in the morning-to-noon hours is completely reversed in the afternoon-to-late-evening hours, with the convective rainfall swinging back and forth, following its updraft branch. Sensitivity experiments show that the afternoon convective rainfall in the inland region of SC is caused by the diurnal variation of solar radiation forcing. The mountain range along the coast and the complex topography in the inland region of SC plays a critical role in the enhancement of diurnal convective rainfall everywhere. The formation of a heavy rainfall band along the southeastern coast of SC and the diurnal variation of the rainfall pattern are mainly the results of the land-sea thermal contrast.

A multi-city analysis of the UHI-influence on warm season rainfall

This study evaluates the influence of the Urban Heat Island (UHI) on warm season precipitation for several US cities by using contemporary high-resolution observations. Spatial (local and downwind) rainfall anomalies associated with propagating and non-propagating storms are analyzed using statistical methods. A diurnal trend analysis suggests that the UHI has a dominant influence on precipitation for inland cities (Minneapolis and Washington D.C.), especially during afternoon and nocturnal hours. Propagating storms appear to converge over the urban region, but the rainfall intensification occurs downwind of the city. The nocturnal precipitation anomaly, which has received less attention in the past, is significant and comparable to the daytime anomaly. For coastal cities (New York City, Providence and Cleveland), a local increase in the frequency and intensity of extreme positive rainfall anomalies may occur during daytime due to the UHI-sea breeze interaction. For non-propagating storms, nocturnal rainfall enhancement is evident downwind of an inland city (Dallas), whereas a local daytime increase in the frequency of heavy rainfall events is observed for a coastal city (Houston). Downwind and nocturnal UHI impacts on rainfall appear less obvious for coastal cities. Future studies are clearly needed to understand the variability in processes that produce urban rainfall enhancement.

Consistent responses of East Asian summer mean rainfall to global warming in CMIP5 simulations

East Asia summer rainfall is of great social–economic importance. Based on observations, reanalysis and simulations of 16 Coupled Models Intercomparison Project phase 5 (CMIP5) models, the responses of East Asia summer precipitation, as well as some relevant features, to global warming are investigated. The CMIP5 historical simulation reasonably reproduces the climatology of summer rainfall, the associated circulation, the moisture and its transportation, and the mid-troposphere horizontal advection of temperature as well. Under global warming, the rainfall enhancement is robustly projected in the state-of-the-art models over North China, Northeast China, northern coast of Japan and the Kuroshio. As well, the total summer rainfall over East Asia is consistently increased in the models. For the consistent responses, the moisture budget analysis based on the simulations shows that two factors are responsible: one is increased moisture. As East Asia is a climatological ascent region in northern summer, increased moisture induced by global warming leads to more moisture transported upward and thus the rainfall rise. The other is enhanced evaporation, which may be caused by surface warming and provides more precipitable water to the atmosphere column. Furthermore, the results may provide some implications to the long-term variability of East Asia summer rainfall over the last several decades.

Improvement of watershed flood forecasting by typhoon rainfall climate model with an ANN-based southwest monsoon rainfall enhancement

Summary This paper improves the typhoon flood forecasting over a watershed in a mountainous island of Taiwan. In the presence of the stiff topography in Taiwan, the typhoon rainfall is often phased-locked with terrain and the typhoon rainfall in general is best predicted by the typhoon rainfall climate model (TRCM) ( Lee et al., 2006 ). However, the TRCM often underestimates the rainfall amount in cases of slowing moving storms with strong southwest monsoon supply of water vapor flux. We apply an artificial neural network (ANN) based southwest monsoon rainfall enhancement (AME) to improve TRCM rainfall forecasting for the Tsengwen Reservoir watershed in the southwestern Taiwan where maximum typhoon rainfall frequently occurred. Six typhoon cases with significant southwest monsoon water vapor flux are used for the test cases. The precipitations of seven rain gauge stations in the watershed and the southwest monsoon water vapor flux are analyzed to get the spatial distribution of the effective water vapor flux threshold, and the threshold is further used to build the AME model. The results indicate that the flux threshold is related to the topographic lifting of the moist air, with lower threshold in the upstream high altitude stations in the watershed. The lower flux threshold allows a larger rainfall amount with AME. We also incorporated the rainfall prediction with a state space neural network (SSNN) to simulate rainfall-runoff processes. Our improved method is robust and produces better flood predictions of total rainfall and multiple rainfall peaks. The runoff processes in the watershed are improved in terms of coefficient of efficiency, peak discharge, and total volume.

The 1970 Clean Air Act and termination of rainfall suppression in a U.S. urban area

The purpose of this paper is to determine the impact of reduced atmospheric particulate resulting from the Clean Air Act of 1970 on changes in summer rainfall in the Atlanta, Georgia USA region. In order to determine if rainfall at nine candidate stations in the metropolitan area was influenced by changes in particulate concentrations within the 1948–2009 period, predicted rainfall characteristics were derived from rainfall frequencies at nine reference stations located more than 80 km from downtown Atlanta. Both parametric and non-parametric tests were used to test for significant differences between observed values and predicted values within 34 overlapping 30-year periods. For the country as a whole, emissions of PM10 (i.e. particulates with a diameter less than or equal to 10 μm) decreased by approximately 40% from 1970 to 1975. The reduction in emissions caused a rapid rebound in summer rainfall in the Atlanta region. There was suppression of rainfall over and downwind of the Atlanta urbanized area during 30-yr periods that comprise all or portions of the decades of the 1950s, 1960s, and 1970s. This suppression occurred even while urban-related factors that promote rainfall enhancement were present. During the 1948–1977 suppression period, there was a decrease in rainfall of at least 40 mm at affected locales, which is substantial given that the mean seasonal rainfall was approximately 300 mm. The rainfall suppression involved a decrease of heavy-rainfall days. Atlanta is most likely not a unique case; therefore, particulate-induced rainfall suppression might have occurred over and downwind of other U.S. urban areas prior to the late 1970s.

Objective tracking of African Easterly Waves in Met Office models

AbstractA simple and computationally efficient objective tracking technique is used to assess the representation of African Easterly Waves (AEWs) in the Met Office Unified Model (MetUM). The algorithm takes Hovmöller diagrams of curvature vorticity at 700 hPa and uses object‐orientated image processing to identify the propagating waves. We use the tracking to evaluate how well AEWs are captured by the MetUM in comparison to ERA‐Interim reanalysis and GPCP rainfall data. The waves are inspected from a dynamical perspective and also from a rainfall‐coupling perspective. The results show that the MetUM is able to capture AEW features at weather forecasting and climate time‐scales and has reasonable representation of the large‐scale dynamics. Some rainfall coupling to waves is also captured in the forecasts. However, it is found that the rainfall enhancement due to wave passage is underestimated by the models, as is the strength and frequency of AEWs. Potential causes of forecast deficiency are investigated and both resolution and changes to convective parametrisation are found to have impacts on model performance. The implications for future model development over Africa are discussed and it is suggested that good convection representation within models is of key importance to realistic simulations of AEWs and their coupling to West African rainfall.

Numerical simulation of deep convective cloud seeding using liquid carbon dioxide

In this study, a one-dimensional transient cumulonimbus cloud is modeled to be seeded by liquid CO2. The model includes microphysical and dynamical processes associated with glaciogenic seeding by homogenous ice nucleation and two thermal terms associated with seeding by −90 ºC liquid CO2. For this model, the study concentrates on five types of hydrometeors, namely, cloud droplet, cloud ice, snow, hail/graupel, and rain. Point and horizontal seeding methods are implemented to observe their implications for rainfall enhancement, amount of hail/graupel production, vertical cloud extension, and radar’s reflectivity. In addition, the seeding temperature effects on the rainfall and microphysical processes are investigated. The results of the study show that, the rainfall enhancement and rainfall intensity in the point seeding case are more than those in the horizontal seeding. Moreover, the study reveals that, there is a vertical cloud extension enhancement of 0.5 km for clouds with top height of 10.5 km. The most important sources of the rain water production are found to be the accretion of cloud water by rain (P RACW) and by snow (P SACW), and for the graupel production is dry growth of the graupel (P GDRY). The results of this study are confirmed by the results of other investigators and are found to be comparable with the recorded data at rain gauge stations.

Urbanization and climate change impacts on future urban flooding in Can Tho city, Vietnam

Abstract. Urban development increases flood risk in cities due to local changes in hydrological and hydrometeorological conditions that increase flood hazard, as well as to urban concentrations that increase the vulnerability. The relationship between the increasing urban runoff and flooding due to increased imperviousness is better perceived than that between the cyclic impact of urban growth and the urban rainfall via microclimatic changes. The large-scale, global impacts due to climate variability and change could compound these risks. We present the case of a typical third world city – Can Tho (the biggest city in Mekong River Delta, Vietnam) – faced with multiple future challenges, namely: (i) the likely effect of climate change-driven sea level rise, (ii) an expected increase of river runoff due to climate change as estimated by the Vietnamese government, (iii) increased urban runoff driven by imperviousness, and (iv) enhancement of extreme rainfall due to urban growth-driven, microclimatic change (urban heat islands). A set of model simulations were used to construct future scenarios, combining these influences. Urban growth of the city was projected up to year 2100 based on historical growth patterns, using a land use simulation model (Dinamica EGO). A dynamic limited-area atmospheric model (WRF), coupled with a detailed land surface model with vegetation parameterization (Noah LSM), was employed in controlled numerical experiments to estimate the anticipated changes in extreme rainfall patterns due to urban heat island effect. Finally, a 1-D/2-D coupled urban-drainage/flooding model (SWMM-Brezo) was used to simulate storm-sewer surcharge and surface inundation to establish the increase in the flood hazard resulting from the changes. The results show that under the combined scenario of significant change in river level (due to climate-driven sea level rise and increase of flow in the Mekong) and "business as usual" urbanization, the flooding of Can Tho could increase significantly. The worst case may occur if a sea level rise of 100 cm and the flow from upstream happen together with high-development scenarios. The relative contribution of causes of flooding are significantly different at various locations; therefore, detailed research on adaptation are necessary for future investments to be effective.

Impact of mineral dust on cloud formation in a Saharan outflow region

Abstract. We present a numerical modelling study investigating the impact of mineral dust on cloud formation over the Eastern Mediterranean for two case studies: (i) 25 September 2008 and (ii) 28/29 January 2003. In both cases dust plumes crossed the Mediterranean and interacted with clouds forming along frontal systems. For our investigation we used the fully online coupled model WRF-chem. The results show that increased aerosol concentrations due to the presence of mineral dust can enhance the formation of ice crystals. This leads to slight shifts of the spatial and temporal precipitation patterns compared to scenarios where dust was not considered to act as ice nuclei. However, the total amount of precipitation did not change significantly. The only exception occurred when dust entered into an area of orographic ascent, causing glaciation of the clouds, leading to a local enhancement of rainfall. The impact of dust particles acting as giant cloud condensation nuclei on precipitation formation was found to be small. Based on our simulations the contribution of dust to the CCN population is potentially significant only for warm phase clouds. Nevertheless, the dust-induced differences in the microphysical structure of the clouds can contribute to a significant radiative forcing, which is important from a climate perspective.

Kernel-Based Mapping of Orographic Rainfall Enhancement in the Swiss Alps as Detected by Weather Radar

In this paper, we develop a data-driven methodology to characterize the likelihood of orographic precipitation enhancement using sequences of weather radar images and a digital elevation model (DEM). Geographical locations with topographic characteristics favorable to enforce repeatable and persistent orographic precipitation such as stationary cells, upslope rainfall enhancement, and repeated convective initiation are detected by analyzing the spatial distribution of a set of precipitation cells extracted from radar imagery. Topographic features such as terrain convexity and gradients computed from the DEM at multiple spatial scales as well as velocity fields estimated from sequences of weather radar images are used as explanatory factors to describe the occurrence of localized precipitation enhancement. The latter is represented as a binary process by defining a threshold on the number of cell occurrences at particular locations. Both two-class and one-class support vector machine classifiers are tested to separate the presumed orographic cells from the nonorographic ones in the space of contributing topographic and flow features. Site-based validation is carried out to estimate realistic generalization skills of the obtained spatial prediction models. Due to the high class separability, the decision function of the classifiers can be interpreted as a likelihood or susceptibility of orographic precipitation enhancement. The developed approach can serve as a basis for refining radar-based quantitative precipitation estimates and short-term forecasts or for generating stochastic precipitation ensembles conditioned on the local topography.

Estimating areal rainfall over Lake Victoria and its basin using ground-based and satellite data

A gridded monthly rainfall dataset having a spatial resolution of 2km and covering the period 1960–2004 was derived for the Lake Victoria basin. The lake and its basin support more than 30million people and also contribute substantially to the River Nile flow. The major challenge in the estimation of the Lake Victoria water balance is the estimation of the rainfall over the lake, which is further complicated by the varying quality and spatial coverage of rain-gauge data in the basin. In this study, these problems were addressed by using rain-gauge data for 315 stations around the basin and satellite-derived precipitation data from two products to derive a monthly precipitation dataset for the entire basin, including the lake. First, the rain-gauge data were quality controlled. Thereafter short gaps were filled in the daily data series which resulted in 9429 additional months of data. Two spatial interpolation methods were used for generating the gridded rainfall dataset and the universal kriging method performed slightly better than the inverse distance weighting method. The enhancement of rainfall over the lake surface was addressed by estimating a relationship between rain-gauge and satellite data. Two satellite rainfall products, TRMM 3B43 and PERSIANN were compared to the interpolated monthly rain-gauge data for the land part of the basin. The bias in the TRMM 3B43 rainfall estimates was higher than the bias for PERSIANN but its correlation was higher with a better representation of the intra-annual variability. The TRMM 3B43 product showed an enhancement of lake rainfall over basin rainfall of 33% while the PERSIANN product gave a much higher enhancement of up to 85%.

The Indian summer monsoon during peaks in the 11 year sunspot cycle

An analysis of sea‐level pressure anomalies at 14 sunspot peaks in the 11 year solar cycle in the Indian region in summer shows that the mean sea level pressure anomalies consist of relatively high pressure over land surrounded by low pressure anomalies over the sea. This signal is robust enough to appear when the data are divided into two segments. The accompanying mean rainfall anomalies, with anomalies as high as 20% above normal, have maxima on the coasts and over water and are an enhancement of the mean Indian monsoon rainfall. In the sunspot peaks the Findlater Jet appears to shift east and strengthen somewhat, consistent with the lower sea level pressure and stronger monsoon rainfall.

Moist Dynamics of Extended Monsoon Breaks over South Asia

In the present research to identify moist processes that initiate and maintain extended monsoon breaks over South Asia moisture and moist static energy (MSE) budgets are performed on the newly available European Centre for Medium-Range Weather Forecasts Interim reanalysis (ERA-Interim) and ensemble integrations from a coupled model. The hypothesis that interaction between moist physics and regional circulation and the role of cloud–radiation feedbacks are important is tested. Budget diagnostics show that dry advection is the principal moist process to initiate extended breaks. Its sources are (i) regional anticyclonic circulation anomalies forced by equatorial Indian Ocean negative rainfall anomalies advect low MSE air from north to central India, and (ii) rainfall enhancement over tropical west Pacific forces cyclonic circulation anomalies to its northwest as a Rossby wave response, and the northerlies at the poleward flank of this circulation advect air of low MSE content from north. The dominance of anomalous wind acting on climatological moisture gradient is confirmed from an examination of the moisture advection equation. A partition of various flux terms indicates that over central India, due to an increase in upwelling shortwave and longwave fluxes, radiative cooling increases during extended breaks. Here, enhanced rainfall over the equatorial Indian Ocean promotes anomalous radiative warming due to trapping of upwelling fluxes. The differential radiative heating anchors a local Hadley circulation with descent over central India. A direct implication of this research is that observational efforts are necessary to monitor the three-dimensional moisture distribution and cloud–radiation interaction over the monsoon region that would aid in better understanding, modeling, and predicting extended monsoon breaks.

Changes in the frequencies of northeast monsoon rainy days in the global warming

The frequencies of the days having different rainfall limits in northeast monsoon season over South Peninsular [SP] India comprising of five meteorological subdivisions are evaluated using the grid‐point rainfall data of India Meteorological Department [IMD] for the period 1951–2008. The study period is divided into two parts 1951–1969 [pre-global warming period] and 1970–2008 [global warming period] as the Earth shows a perceptible rise in the temperature from the beginning of 1970s. The differences in the frequencies of the days having different rainfall limits [zero, 0 to 20mm, 20–60mm, 60–100mm and greater than 100mm] for these two periods are also evaluated. Further, the differences in the mean rainfall amounts over SP India and zonal wind at 850hPa for the domain 74°E–85°E, 8°N–20°N in northeast monsoon season using NCEP–NCAR reanalysis data sets for these two periods are calculated and tested with the Z‐statistic.It is observed that there is an enhancement in the northeast monsoon rainfall over the major area of SP India in the global warming period which is associated with the intensification of easterly wind belt in the lower troposphere over the major part of northeast monsoon region, decrease in the frequency of no rainfall days and increasing tendencies in the frequencies of the days having all the limits of rainfall except for 20–60mm limit.

Hydroclimatology of flash flooding in Atlanta

The objective of this study is to characterize the climatology of extreme rainfall and flash flooding in Atlanta, Georgia using high‐resolution land surface, rainfall, and discharge datasets. We examine nine urban watersheds in the Atlanta area that range in size from 3.7 to 225 km2 and exhibit a range of urban development and land‐use characteristics. We develop a high‐resolution 15 min, 1 km2 radar rainfall data set for the 2002–2010 period using the Hydro‐NEXRAD system with volume scan reflectivity observations from the Atlanta WSR‐88D radar and rainfall observations from a dense network of 72 U.S. Geological Survey rain gauges. Bias‐corrected radar rainfall fields accurately capture the spatial and temporal structure of heavy rainfall. There is enhancement of heavy rainfall within and east of the urban core, and a rainfall minimum north and northwest of the city. There has been an increase in variability of annual flood peaks in Atlanta since the 1960s associated with urban impacts on runoff production. Flood response is dependent on a combination of basin size, drainage network structure, spatial distribution of land use, and basin storage in urban soils and storm water detention ponds. Future studies of urban rainfall modification in Atlanta and elsewhere should consider the influence of regional topography and other geographic features on the storm environment.

Impacts of Mountain–Plains Solenoid on Diurnal Variations of Rainfalls along the Mei-Yu Front over the East China Plains

Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) model are performed to examine the impact of a thermally driven mountain–plains solenoid (MPS) on the diurnal variations of precipitation and mesoscale convective vortices along the mei-yu front over the east China plains during 1–10 July 2007. The focus of the analyses is a 10-day simulation that used the 10-day average of the global analysis at 0000 UTC as the initial condition and the 10-day averages every 6 h as lateral boundary conditions (with diurnal variations only). Despite differences in the rainfall intensity and location, this idealized experiment successfully simulated the observed diurnal variation and eastward propagation of rainfall and mesoscale convective vortices along the mei-yu front. It was found that the upward branch of the MPS, along with the attendant nocturnal low-level jet, is primarily responsible for the midnight-to-early-morning rainfall enhancement along the mei-yu front. The MPS is induced by differential heating between the high mountain ranges in central China and the low-lying plains in east China. Diabatic heating from moist convection initiated and/or enhanced by the solenoid circulation subsequently leads to the formation of a mesoscale convective vortex that further organizes and amplifies moist convection while propagating eastward along the mei-yu front. The downward branch of the MPS, on the other hand, leads to the suppression of precipitation over the plains during the daytime. The impacts of this regional MPS on the rainfall diurnal variations are further attested to by another idealized WRF simulation that uses fixed lateral boundary conditions.

Modoki, Indian Ocean Dipole, and western North Pacific typhoons: Possible implications for extreme events

[1] An interesting feature during El Niño Modoki summers (June–August) is the occurrence of surplus rainfall anomalies over the South China Sea (SCS) and off-equatorial tropical Northwest Pacific. Based on analyses of observed and reanalyzed data, it is noted that anomalous warming in central tropical Pacific during an El Niño Modoki induces a quasi-stationary Rossby wave response in the form of a low-level cyclonic flow over the northern SCS. This anomalous response favors rainfall enhancement over the SCS and also promotes the seasonal typhoon activity. We find that this scenario over SCS and sub-tropical NW Pacific is further amplified when an El Niño Modoki co-occurs with a positive Indian Ocean Dipole (IOD) as in 1994. The combination is apparently more conducive for extreme convection events over the region as compared to a pure El Niño Modoki case. The respective impacts of positive IOD and El Niño, on the other hand, are opposite to one another. The modulation of the typhoon activity by IOD events, however, is prominent only since late 1970s.

Winter daily precipitation variability over Cumbria, Northwest England

Daily precipitation totals at 55 sites were used to investigate geographic variability in winter (DJF) rainfall over Cumbria, NW England, over an 11-year period. Winter is the wettest season (>800 mm in the mountainous Lake District), with rainfall mechanisms closely linked to North Atlantic forcing. The Lamb weather type catalogue was used to identify rainfall distributions under different wind directions. Precipitation magnitude over Cumbria is much more sensitive to a change in wind direction than the geographic pattern in rainfall, with southwesterly (easterly) winds producing the highest (lowest) spatially averaged daily rainfall totals of 8.2 mm (0.6 mm). S-mode principal components analysis was used to identify the main patterns of precipitation variability. Three principal components (PCs) were retained as being statistically significant (cumulative explained variance for unrotated PCs = 84.3%), with a correlated PC structure (direct oblimin rotation) best describing the spatial variance in rainfall. PC 1 has a very high index of strength (variance measure = 40.9), indicating that there is one dominant rainfall pattern. PC 1 shows a gradient between wetter conditions in southwest Cumbria and over the central Lake District and drier conditions in NE Cumbria, and is usually caused by active zonal west to southwest flows. Almost of equal importance to PC 1 is PC 3 (variance measure = 39.3), which has a more uniform rainfall distribution than PC 1 and is usually caused by fronts stalling over the region. PC 2, which shows an east to west decline in rainfall totals, is much less important than PCs 1 and 3 (variance measure = 18.6). PC 2’s rainfall pattern can be caused by easterly flows with high pressure over Scandinavia and low pressure over the Continent, or by strong southwesterly flows, with depressions often centred over Scotland. Finally, cluster analysis was carried out to identify precipitation regions for all days and for each wind direction. Clusters were found to be largely stable to changes in wind direction, with stations in the central Lake District often clustered together, thus highlighting the importance of orographic enhancement of rainfall in this region.

Rain on small tropical islands

[1] A high-resolution rainfall climatology based on observations from the Tropical Rainfall Measuring Mission's Precipitation Radar (PR) instrument is used to evaluate the influence of small tropical islands on climatological rainfall. Islands with areas between one hundred and several thousand km2 are considered in both the Indo-Pacific Maritime Continent and Caribbean regions. Annual mean climatological (1997–2007) rainfall over each island is compared with that over the surrounding ocean region, and the difference is expressed as a percentage. In addition to total rainfall, rain frequency and intensity are also analyzed. Results are stratified into two 12 h halves of the diurnal cycle as well as eight 3 h periods, and also by a measure of each island's topographic relief. In both regions, there is a clear difference between larger islands (areas of a few hundred km2 or greater) and smaller ones. Both rain frequency and total rainfall are significantly enhanced over larger islands compared to the surrounding ocean. For smaller islands the enhancement is either negligibly small, statistically insignificant, or, in the case of Caribbean rain frequency, negative. The enhancement in total rainfall over larger islands is partly attributable to greater frequency and partly to greater intensity. A diurnal cycle in island enhancement is evident in frequency but not intensity, except over small Caribbean islands where the converse is true. For the larger islands, higher orography is associated with greater rainfall enhancements. The orographic effect is larger (percentagewise) in the Caribbean than in the Maritime Continent. Orographic precipitation enhancement manifests more strongly as increased frequency of precipitation rather than increased intensity and is present at night as well as during the day. The lack of a clear diurnal cycle in orographic enhancement suggests that much of the orographic rainfall enhancement is attributable to mechanically forced upslope flow rather than elevated surface heating.

Comment on “‘Elevated heat pump’ hypothesis for the aerosol-monsoon hydroclimate link: ‘Grounded’ in observations?” by S. Nigam and M. Bollasina

In their recent paper, Nigam and Bollasina [2010] (hereinafter NB) claimed to have found observational evidences that are at variance with the elevated heat pump (EHP) hypothesis regarding the possible impacts of absorbing aerosols on the South Asian summer monsoon [Lau et al., 2006; Lau and Kim, 2006]. We found NB's arguments and inferences against the EHP hypothesis flawed, stemming from their own out of context interpretation of the hypothesis. NB argued that the simultaneous negative correlation of aerosol with rainfall, and correlations with other quantities in May, are evidence against the EHP hypothesis. Their argument cannot be justified. First, Lau and Kim [2006] (hereinafter LK06) never stated that the main rainfall response to EHP is in May. Second, the EHP is about responses of the entire Indian monsoon system that are nonlocal in space and time with respect to the aerosol forcing. As shown in Figure 4 of LK06, while the aerosol anomalies are strongest in April-May, the strongest rainfall response is in June-July, with the enhanced rainfall fed by an induced thermally driven circulation which brings additional moisture from the ocean to the Indian subcontinent. Third, the increased rainfall over the Bay of Bengal as shown in Figure 1a of NB and the increased low-level convergence in Figure 1f of NB do not necessarily reflect responses associated with EHP but rather the large ]scale circulation that provides the buildup of the aerosols before the onset of the monsoon rainfall over India. Because aerosol can only accumulate where there is little or no washout by rain, the negative correlation is a necessary condition for increased atmospheric loading of aerosols. For the same reason, the spatial distributions of rainfall and aerosol generally are offset with each other, i.e., high aerosol in regions of low rainfall. This is evident in Figure 1, which shows the climatological mean of the MODIS aerosol optical depth (AOD), and TRMM rainfall over India in May. The maximum AOD is found over the Indo-Gangetic Plain and the desert regions of northwest India and Pakistan