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Future projections of precipitation change over tropical land are often enhanced by vegetation responses to CO2 forcing in Earth system models. Projected decreases in rainfall over the Amazon basin and increases over the Maritime Continent are both stronger when plant physiological changes are modeled than if these changes are neglected, but the reasons for this amplification remain unclear. The responses of vegetation to increasing CO2 levels are complex and uncertain, including possible decreases in stomatal conductance and increases in leaf area index due to CO2 fertilization. Our results from an idealized atmospheric general circulation model show that the amplification of rainfall changes occurs even when we use a simplified vegetation parameterization based solely on CO2-driven decreases in stomatal conductance, indicating that this mechanism plays a key role in complex model projections. Based on simulations with rectangular continents we find that reducing terrestrial evaporation to zero with increasing CO2 notably leads to enhanced rainfall over a narrow island. Strong heating and ascent over the island trigger moisture advection from the surrounding ocean. In contrast, over larger continents rainfall depends on continental evaporation. Simulations with two rectangular continents representing South America and Africa reveal that the stronger decrease in rainfall over the Amazon basin seen in Earth system models is due to a combination of local and remote effects, which are fundamentally connected to South America’s size and its location with respect to Africa. The response of tropical rainfall to changes in evapotranspiration is thus connected to size and configuration of the continents.

Tropical precipitation change under global warming varies with season. The present study investigates the characteristics and cause of the seasonality in rainfall change. Diagnostically, tropical precipitation change is decomposed into thermodynamic and dynamic components. The thermodynamic component represents the wet-get-wetter effect and its seasonality is due mostly to that in the mean vertical velocity, especially in the monsoon regions. The dynamic component includes the warmer-get-wetter effect due to the spatial variations in sea surface temperature (SST) warming, while the seasonality is due to that of the climatological SST and can be largely reproduced by an atmospheric model forced with the monthly climatological SST plus the annual-mean SST warming pattern. In the eastern equatorial Pacific where the SST warming is locally enhanced; for example, rainfall increases only during the March–May season when the climatological SST is high enough for deep convection. To the extent that the seasonality of tropical precipitation change over oceans arises mostly from that of the climatological SST, the results support the notion that reducing model biases in climatology improves regional rainfall projections.

We propose an algorithm to detect and track coastal precipitation systems and we apply it to 18 years of the high-resolution (8 km and 30 min) Climate Prediction Center CMORPH precipitation estimates in the tropics. Coastal precipitation in the Maritime Continent and Central America contributes to up to 80% of the total rainfall. It also contributes strongly to the diurnal cycle over land with the largest contribution from systems lasting between 6 and 12 h and contributions from longer-lived systems peaking later in the day. While the diurnal cycle of coastal precipitation is more intense over land in the summer hemisphere, its timing is independent of seasons over both land and ocean because the relative contributions from systems of different lifespans are insensitive to the seasonal cycle. We investigate the hypothesis that coastal precipitation is enhanced prior to the arrival of the Madden–Julian oscillation (MJO) envelope over the Maritime Continent. Our results support this hypothesis and show that, when considering only coastal precipitation, the diurnal cycle appears reinforced even earlier over islands than previously reported. We discuss the respective roles of coastal and large-scale precipitation in the propagation of the MJO over the Maritime Continent. We also document a shift in diurnal cycle with the phases of the MJO, which results from changes in the relative contributions of short-lived versus long-lived coastal systems.

By comparing a single-column model (SCM) with closely related general circulation models (GCMs), precipitation changes that can be diagnosed from local changes in surface temperature (TS) and relative humidity (RHS) are separated from more complex responses. In the SCM setup, the large-scale tropical circulation is parameterized to respond to the surface temperature departure from a prescribed environment, following the weak temperature gradient (WTG) approximation and using the damped gravity wave (DGW) parameterization. The SCM is also forced with moisture variations. First, it is found that most of the present-day mean tropical rainfall and circulation pattern is associated with TS and RHS patterns. Climate change experiments with the SCM are performed, imposing separately surface warming and CO2 increase. The rainfall responses to future changes in sea surface temperature patterns and plant physiology are successfully reproduced, suggesting that these are direct responses to local changes in convective instability. However, the SCM increases oceanic rainfall too much, and fails to reproduce the land rainfall decrease, both of which are associated with uniform ocean warming. It is argued that remote atmospheric teleconnections play a crucial role in both weakening the atmospheric overturning circulation and constraining precipitation changes. Results suggest that the overturning circulation weakens, both as a direct local response to increased CO2 and in response to energy-imbalance driven exchanges between ascent and descent regions.

The driving of tropical precipitation by the variability of the underlying sea surface temperature (SST) plays a critical role in the atmospheric general circulation. To assess the precipitation sensitivity to SST variability, it is necessary to observe and understand the relationship between precipitation and SST. However, the precipitation–SST relationships from any coupled atmosphere–ocean system can be difficult to interpret given the challenge of disentangling the SST-forced atmospheric response and the atmospheric intrinsic variability. This study demonstrates that the two components can be isolated using uncoupled atmosphere-only simulations, which extract the former when driven by time-varying SSTs and the latter when driven by climatological SSTs. With a simple framework that linearly combines the two types of uncoupled simulations, the coupled precipitation–SST relationships are successfully reproduced. Such a framework can be a useful tool for quantitatively diagnosing tropical air–sea interactions. The precipitation sensitivity to SST variability is investigated with the use of uncoupled simulations with prescribed SST anomalies, where the influence of atmospheric intrinsic variability on SST is deactivated. Through a focus on local precipitation–SST relationships, the precipitation sensitivity to local SST variability is determined to be predominantly controlled by the local background SST. In addition, the strength of the precipitation response increases monotonically with the local background SST, with a very sharp growth at high SSTs. These findings are supported by basic principles of moist static stability, from which a simple formula for precipitation sensitivity to local SST variability is derived.

A compositing scheme that predicts changes in tropical precipitation under climate change from changes in near-surface relative humidity (RH) and temperature is presented. As shown by earlier work, regions of high tropical precipitation in general circulation models (GCMs) are associated with high near-surface RH and temperature. Under climate change, it is found that high precipitation continues to be associated with the highest surface RH and temperatures in most CMIP5 GCMs, meaning that it is the “rank” of a given GCM grid box with respect to others that determines how much precipitation falls rather than the absolute value of surface temperature or RH change, consistent with the weak temperature gradient approximation. Further, it is demonstrated that the majority of CMIP5 GCMs are close to a threshold near which reductions in land RH produce large reductions in the RH ranking of some land regions, causing reductions in precipitation over land, particularly South America, and compensating increases over ocean. Recent work on predicting future changes in specific humidity allows the prediction of the qualitative sense of precipitation change in some GCMs when land surface humidity changes are unknown. However, the magnitudes of predicted changes are too small. Further study, perhaps into the role of radiative and land–atmosphere feedbacks, is necessary.

The intrinsic atmospheric and ocean-induced tropical precipitation variability is studied using millennial control simulations with various degrees of ocean coupling. A comparison between the coupled simulation and the atmosphere-only simulation with climatological sea surface temperatures (SSTs) shows that a substantial amount of tropical precipitation variability is generated without oceanic influence. This intrinsic atmospheric variability features a red noise spectrum from daily to monthly time scales and a white noise spectrum beyond the monthly time scale. The oceanic impact is inappreciable for submonthly time scales but important at interannual and longer time scales. For time scales longer than a year, it enhances precipitation variability throughout much of the tropical oceans and suppresses it in some subtropical areas, preferentially in the summer hemisphere. The sign of the ocean-induced precipitation variability can be inferred from the local precipitation–SST relationship, which largely reflects the local feedbacks between the two, although nonlocal forcing associated with El Niño–Southern Oscillation also plays a role. The thermodynamic and dynamic nature of the ocean-induced precipitation variability is studied by comparing the fully coupled and slab ocean simulations. For time scales longer than a year, equatorial precipitation variability is almost entirely driven by ocean circulation, except in the Atlantic Ocean. In the rest of the tropics, ocean-induced precipitation variability is dominated by mixed layer thermodynamics. Additional analyses indicate that both dynamic and thermodynamic oceanic processes are important for establishing the leading modes of large-scale tropical precipitation variability. On the other hand, ocean dynamics likely dampens tropical Pacific variability at multidecadal time scales and beyond.

Future projections of the eastern-Pacific (EP) and central-Pacific (CP) types of El Niño in the twenty-first century, as well as their associated tropical circulation and precipitation variability, are investigated using historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). As inferred from CMIP5 models that best capture both El Niño flavors, EP El Niño sea surface temperature (SST) variability will become weaker in the future climate, while no robust change of CP El Niño SST is found. Models also reach no consensus on the future change of relative frequency from CP to EP El Niño. However, there are robust changes in the tropical overturning circulation and precipitation associated with both types of El Niño. Under a warmer climate, magnitudes of precipitation anomalies during EP El Niño are projected to increase, presenting significant enhancement of the dry (wet) signal over the western (central–eastern) Pacific. This is consistent with an accelerated hydrological cycle in the deep tropics; hence, a “wet get wetter” picture appears under global warming, accompanied by a weakened anomalous Walker circulation. For CP El Niño, drier-than-normal conditions will be intensified over the tropical central–eastern Pacific in the future climate, with stronger anomalous sinking related to the strengthened North Pacific local Hadley cell. These results suggest that, besides the enhanced basic-state hydrological cycle over the tropics, other elements, such as the anomalous overturning circulation, might also play a role in determining the ENSO precipitation response to a warmer background climate.

The authors evaluate the contribution of tropical cyclones (TCs) to daily precipitation extremes over land for TC-active regions around the world. From 1998 to 2012, data from the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA 3B42) showed that TCs account for an average of 3.5% ± 1% of the total number of rainy days over land areas experiencing cyclonic activity regardless of the basin considered. TC days represent between 13% and 31% of daily extremes above 4 in. day−1, but can account locally for the large majority (>70%) or almost all (≈100%) of extreme rainfall even over higher-latitude areas marginally affected by cyclonic activity. Moreover, regardless of the TC basin, TC-related extremes occur preferably later in the TC season after the peak of cyclonic activity.

This study uses 16-yr Tropical Rainfall Measuring Mission (TRMM) radar precipitation feature (RPF) data to characterize warm rain systems in the tropics with large horizontal extensions, referred to as warm organized rain systems. The systems are selected by specifying the RPFs with minimum infrared brightness temperature warmer than 0°C and rain area greater than 500 km2. ERA-Interim atmospheric fields and SST from NOAA are analyzed to highlight the environmental characteristics of warm organized rain systems. Warm organized systems occur over specific oceanic regions, including the eastern Pacific ITCZ, the eastern part of the SPCZ, and coastal regions. In contrast with ubiquitous warm isolated RPFs, warm organized systems have greater near-surface radar reflectivity. The rainfall amounts generated by warm organized systems are greater in winter than in summer. Composite analyses indicate that warm organized RPFs prefer to coexist with a dry midtroposphere associated with a strong upper-level descent, an enhanced near-surface moisture convergence, and a strong low-level large-scale ascent. The shallow meridional circulation in the eastern Pacific is significantly stronger for warm organized RPFs compared to the circulation for warm isolated RPFs. Warm organized systems over the tropical eastern Pacific occur at warm SSTs with mean value of about 27°C and a strong SST meridional gradient. The warm organized RPFs in the tropical eastern Pacific are found to be at the southern edge of deep ITCZ cores. This is probably related to the meridional asymmetrical thermodynamic structure over the eastern Pacific ITCZ with a higher low-level humidity to the south. Similar favorable large-scale environments for the warm organized RPFs are also found over the SPCZ and other regions.

Motivated by observational evidence of rainfall concentrations near tropical coastlines with a diurnal cycle, the annual mean precipitation amount was quantified in the tropics (latitudes lower than 37°) obtained as a function of coastal distance and compared between land and ocean sides. The data are from the Tropical Rainfall Measuring Mission (TRMM). Precipitation amount peaks at the coastline and decreases rapidly over a distance of 300 km from the coastline on both sides of the coastline. The precipitation inside the “coastal region” (defined by a distance

The influence of tropical precipitation variability on summertime seasonal circulation anomalies in the Euro-Atlantic sector is investigated. The dominant mode of the maximum covariance analysis (MCA) between the Euro-Atlantic circulation and tropical precipitation reveals a cyclonic anomaly over the extratropical North Atlantic, contributing to anomalously wet conditions over western Europe and dry conditions over eastern Europe and Scandinavia (in the positive phase). The related mode of tropical precipitation variability is associated with tropical Pacific SST anomalies and is closely linked to the El Niño–Southern Oscillation (ENSO). The second MCA mode consists of weaker tropical precipitation anomalies but with a stronger extratropical signal that reflects internal atmospheric variability. The teleconnection mechanism is tested in barotropic model simulations, which indicate that the observed link between the dominant mode of tropical precipitation and the Euro-Atlantic circulation anomalies is largely consistent with linear Rossby wave dynamics. The barotropic model response consists of a circumglobal wave train in the extratropics that is primarily forced by divergence anomalies in the eastern tropical Pacific. Both the eastward and westward group propagation of the Rossby waves are found to be important in determining the circulation response over the Euro-Atlantic sector. The mechanism was also analyzed in an operational seasonal forecasting system, ECMWF’s System 4. While System 4 is well able to reproduce and skillfully forecast the tropical precipitation, the extratropical circulation response is absent over the Euro-Atlantic region, which is likely related to biases in the Asian jet stream.

Shear-relative distributions of four types of precipitation/convection in tropical cyclones (TCs) are statistically analyzed using 14 years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data. The dataset of 1139 TRMM PR overpasses of tropical storms through category-2 hurricanes over global TC-prone basins is divided by future 24-h intensity change. It is found that increased and widespread shallow precipitation (defined as where the 20-dBZ radar echo height

Latent heating (LH) from precipitation systems with different sizes, depths, and convective intensities is quantified with 15 years of LH retrievals from version 7 Precipitation Radar (PR) products of the Tropical Rainfall Measuring Mission (TRMM). Organized precipitation systems, such as mesoscale convective systems (MCSs; precipitation area > 2000 km2), contribute to 88% of the LH above 7 km over tropical land and 95% over tropical oceans. LH over tropical land is mainly from convective precipitation, and has one vertical mode with a peak from 4 to 7 km. There are two vertical modes of LH over tropical oceans. The shallow mode from about 1 to 4 km results from small, shallow, and weak precipitation systems, and partially from congestus clouds with radar echo top between 5 and 8 km. The deep mode from 5 to 9 km is mainly from stratiform precipitation in MCSs. MCSs of different regions and seasons have different LH vertical structure mainly due to the different proportion of stratiform precipitation. MCSs over ocean have a larger fraction of stratiform precipitation and a top-heavy LH structure. MCSs over land have a higher percentage of convective versus stratiform precipitation, which results in a relatively lower-level peak in LH compared to MCSs over the ocean. MCSs during monsoons have properties of LH in between those typical land and oceanic MCSs. Consistent with the diurnal variation of precipitation, tropical land has a stronger LH diurnal variation than tropical oceans with peak LH in the late afternoon. Over tropical oceans in the early morning, the shallow mode of LH peaks slightly earlier than the deep mode. There are almost no diurnal changes of MCSs LH over oceans. However, the small convective systems over land contribute a significant amount of LH at all vertical levels in the afternoon, when the contribution of MCSs is small.

The midlatitude response to localized equatorial heating events that last 2 days is examined through experimentation with an atmospheric general circulation model. Such responses are argued to be important because many tropical rainfall events only last a short time and because the responses to such pulses serve as building blocks with which to study the impacts of more general heating fluctuations. The experiments indicate that short-lived heating produces responses in midlatitudes at locations far removed from the source and these responses persist much longer than the pulses themselves. Indeed pulse forcing, which is essentially white in time, produces upper-tropospheric responses that have an e-damping time of almost a week and that are detectable for more than two weeks in the experiments. Moreover the upper-tropospheric structure of the reaction to short pulses is remarkably similar to the reaction to steady tropical heating, including having a preference for occurring at special geographical locations and being composed of recurring patterns that resemble the leading patterns of responses to steady heating. This similarity is argued to be a consequence of the responses to pulses having little or no phase propagation in the extratropics. The impact of short-lived tropical heating also produces a persistent response in midlatitude surface fields and in the statistics of synoptic eddies. The implications these results have for subseasonal variability are discussed. These include 1) the potential for improving subseasonal prediction through improved assimilation and short-range forecasts of tropical precipitation and 2) the difficulties involved in attributing subseasonal midlatitude events to tropical heating.

Some previous studies were unable to detect seasonal organization to the rainfall in the Sahara Desert, while others reported seasonal patterns only in the less-arid periphery of the Sahara. In contrast, the precipitation radar on the Tropical Rainfall Measuring Mission (TRMM) satellite detects four rainy seasons in the part of the Sahara where the TRMM radar saw the least rainfall during a 15-yr period (1998–2012). According to the TRMM radar, approximately 20°–27°N, 22°–32°E is the portion of the Sahara that has the lowest average annual rain accumulation (1–5 mm yr−1). Winter (January and February) has light rain throughout this region but more rain to the north over the Mediterranean Sea. Spring (April and May) has heavier rain and has lightning observed by the TRMM Lightning Imaging Sensor (LIS). Summer rain and lightning (July and August) occur primarily south of 23°N. At a maximum over the Red Sea, autumn rain and lightning (October and November) can be heavy in the northeastern portion of the study area, but these storms are unreliable: that is, the TRMM radar detects such storms in only 6 of the 15 years. These four rainy seasons are each separated by a comparatively drier month in the monthly rainfall climatology. The few rain gauges in this arid region broadly agree with the TRMM radar on the seasonal organization of rainfall. This seasonality is reason to reevaluate the idea that Saharan rainfall is highly irregular and unpredictable.

The geographic and diurnal variability of moist convection over tropical Africa and the east Atlantic is examined using the Tropical Rainfall Measuring Mission (TRMM) satellite and related to the variability of the convective environment. The stratiform rain fraction is highest within oceanic and continental regions just north of the equator. Both regions have high column relative humidity (CRH). In both monsoon and semiarid continental regions, stratiform rain fractions are significantly higher on days when the CRH is high, which suggests a relationship between these quantities. Large convective systems with high echo tops dominate the rainfall over the Sahel. The importance of CAPE and shear to the development of these types of systems is suggested by the fact these systems are especially common on days when the CAPE and shear are unusually high. Both deep convective and stratiform conditional rain rates increase with the size and echo-top height of convective systems. According to the TRMM Precipitation Radar (PR) near-surface rain rate, the highest deep convective and stratiform conditional rain rates occur off the coast of West Africa. However, comparisons between the PR near-surface rain rate and rain rates computed from Z–R relationships from the literature suggest that deep convective conditional rain rates over the Sahel are underestimated by the TRMM precipitation algorithm. Over the Sahel, small (large) convective systems produce most of the rainfall in the afternoon (early morning). This is associated with enhanced convective rainfall in the afternoon and stratiform in the early morning. The transition from small to large convective systems as convection propagates away from topographic features is also observed.

Based on the 12-yr (1998–2009) Tropical Rainfall Measuring Mission (TRMM) precipitation feature (PF) database, both radar and infrared (IR) observations from TRMM are used to quantify the contribution of tropical cyclones (TCs) to very deep convection (VDC) in the tropics and to compare TRMM-derived properties of VDC in TCs and non-TCs. Using a radar-based definition, it is found that the contribution of TCs to total VDC in the tropics is not much higher than the contribution of TCs to total PFs. However, the area-based contribution of TCs to overshooting convection defined by IR is 13.3%, which is much higher than the 3.2% contribution of TCs to total PFs. This helps explain the contradictory results between previous radar-based and IR-based studies and indicates that TCs only contribute disproportionately large amount of overshooting convection containing mainly small ice particles that are barely detected by the TRMM radar. VDC in non-TCs over land has the highest maximum 30- and 40-dBZ height and the strongest ice-scattering signature derived from microwave 85- and 37-GHz observations, while VDC in TCs has the coldest minimum IR brightness temperature and largest overshooting distance and area. This suggests that convection is much more intense in non-TCs over land but is much deeper or colder in TCs. It is found that VDC in TCs usually has smaller environmental shear but larger total precipitable water and convective available potential energy than those in non-TCs. These findings offer evidence that TCs may contribute disproportionately to troposphere-to-stratosphere heat and moisture exchange.

In this study the response of tropical precipitation extremes to warming in organized convection is examined using a cloud-resolving model. Vertical shear is imposed to organize the convection into squall lines. Earlier studies show that in disorganized convection, the fractional increase of precipitation extremes is similar to that of surface water vapor, which is substantially smaller than the increase in column water vapor. It has been suggested that organized convection could lead to stronger amplifications. Regardless of the strength of the shear, amplifications of precipitation extremes in the cloud-resolving simulations are comparable to those of surface water vapor and are substantially less than increases in column water vapor. The results without shear and with critical shear, for which the squall lines are perpendicular to the shear, are surprisingly similar with a fractional rate of increase of precipitation extremes slightly smaller than that of surface water vapor. Interestingly, the dependence on shear is nonmonotonic, and stronger supercritical shear yields larger rates, close to or slightly larger than surface humidity. A scaling is used to evaluate the thermodynamic and dynamic contributions to precipitation extreme changes. To first order, they are dominated by the thermodynamic component, which has the same magnitude for all shears, close to the change in surface water vapor. The dynamic contribution plays a secondary role and tends to weaken extremes without shear and with critical shear, while it strengthens extremes with supercritical shear. These different dynamic contributions for different shears are due to different responses of convective mass fluxes in individual updrafts to warming.

Tropical precipitation characteristics are investigated using the Tropical Rainfall Measuring Mission (TRMM) 3-hourly estimates, and the result is compared with five reanalyses including the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim), Modern Era Retrospective Analysis for Research and Applications (MERRA), National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis (NCEP1), NCEP–U.S. Department of Energy (DOE) reanalysis (NCEP2), and NCEP–Climate Forecast System Reanalysis (CFSR). Precipitation characteristics are evaluated in terms of the mean, convectively coupled equatorial wave activity, frequency characteristics, diurnal cycle, and seasonality of regional precipitation variability associated with submonthly scale waves. Generally the latest reanalyses such as ERA-Interim, MERRA, and CFSR show better performances than NCEP1 and NCEP2. However, all the reanalyses are still different from observations. Besides the positive mean bias in the reanalyses, a spectral analysis revealed that the reanalyses have overreddened spectra with persistent rainfall. MERRA has the most persistent rainfall, and CFSR appears to have the most realistic variability. The diurnal cycle in NCEP1 is extremely exaggerated relative to TRMM. The low-frequency waves with the period longer than 3 days are relatively well represented in ERA-Interim, MERRA, and CFSR, but all the reanalyses have significant deficiencies in representing convectively coupled equatorial waves and variability in the high-frequency range.

The objective of this paper is to characterize the precipitation amounts originating from tropical cyclones (TCs) in the southeastern United States during the tropical storm season from June to November. Using 12 years of precipitation data from the Tropical Rainfall Measurement Mission (TRMM), the authors estimate the TC contribution on the seasonal, interannual, and monthly precipitation budget using TC information derived from the International Best Track Archive for Climate Stewardship (IBTrACS). Results derived from the TRMM Multisatellite Precipitation Analysis (TMPA) 3B42 showed that TCs accounted for about 7% of the seasonal precipitation total from 1998 to 2009. Rainfall attributable to TCs was found to contribute as much as 8%–12% for inland areas located between 150 and 300 km from the coast and up to 15%–20% for coastal areas from Louisiana to the Florida Panhandle, southern Florida, and coastal Carolinas. The interannual contribution varied from 1.3% to 13.8% for the period 1998–2009 and depended on the TC seasonal activity, TC intensity, and TC paths as they traveled inland. For TCs making landfall, the rainfall contribution could be locally above 40% and, on a monthly basis, TCs contributed as much as 20% of September rainfall. The probability density functions of rainfall attributable to tropical cyclones showed that the percentage of rainfall associated with TC over land increased with increasing rain intensity and represent about 20% of heavy rainfall (>20 mm h−1), while TCs account for less than 5% of all seasonal precipitation events.

The spurious double intertropical convergence zone (ITCZ) is a systematic bias affecting state-of-the-art coupled general circulation models (GCMs). Modeling studies show that the ITCZ structure is very sensitive to moist convection parameterization and in particular, to the vertical profile of convective heating and free-tropospheric moistening. To further explore this sensitivity, the authors focus in this study on the influence of lateral entrainment in convective plumes on the simulated tropical precipitation and large-scale circulation. Sensitivity studies to the entrainment parameter were performed in a hierarchy of models (coupled ocean–atmosphere GCM, atmospheric GCM, and aquaplanet GCM), in order to mitigate the double ITCZ problem in the Centre National de Recherches Météorologiques Coupled Global Climate Model, version 5 (CNRM-CM5). The sensitivity of the ITCZ structure to lateral entrainment is robust across our hierarchy of models. In response to increased entrainment, the realistic simulations exhibit a weakening of the southern side of the double ITCZ over the southeastern Pacific Ocean and a better representation of the South Pacific convergence zone (SPCZ). However, as a result of stronger moisture–convection feedbacks, precipitation is overestimated in the center of convergence zones. The change in ITCZ configuration is associated with a more realistic representation of the large-scale vertical regimes, explained by a decreased frequency of weak-to-moderate ascending regimes and an enhanced frequency of subsidence regimes. Mechanisms at play in this circulation change are examined by analyzing the vertically integrated dry static energy budget. This energetic analysis suggests that the feedback between large-scale dynamics and deep convection is crucial in controlling the probability distribution function (PDF) of midtropospheric vertical wind. This PDF, in turn, controls the precipitation distribution and, in particular, the double ITCZ bias.

Using observational rainfall data and atmospheric reanalysis data, the precipitation variations in Northwest China during July and the corresponding atmospheric teleconnection patterns are studied. The results indicate that the leading modes of July precipitation variations in Northwest China are affected by the Silk Road pattern and the Europe–China (EC) pattern. The analysis suggests that the circumglobal teleconnection (CGT) could be considered as the interannual component of the Silk Road pattern. To investigate the excitation mechanisms for the CGT pattern and EC pattern on interannual time scales, the singular value decomposition (SVD) analysis is performed between 200-hPa meridional wind velocity over the region of (30°–60°N, 30°–130°E) and tropical rainfall between (15°S and 30°N). The results suggest that the tropical heating anomalies most responsible for the CGT pattern are located over the North Indian Ocean, and the tropical heating anomalies most responsible for EC pattern are located over equatorial central Pacific, Indonesia, and tropical Atlantic. The tropical heating anomalies excite the CGT pattern and EC pattern by inducing divergent flow at the upper troposphere, and the advections of vorticity by the divergent component of the flow act as effective Rossby wave sources. Further analysis indicates that the tropical rainfall anomalies responsible for the CGT pattern and EC pattern are the leading modes of tropical rainfall variations, and these modes of tropical rainfall variations are related to the SST anomalies.

The leading intraseasonal mode of tropical atmospheric variability, the Madden–Julian oscillation (MJO), has been shown to modulate precipitation and circulation on a global and regional scale. Winter precipitation in Chile has been connected to a variety of synoptic-scale forcing mechanisms. This study explored the links between the two, first examining the intraseasonal variability of Chilean precipitation from surface gauges and the Tropical Rainfall Measuring Mission (TRMM) and then examining the variability of synoptic-scale circulation. Composites of precipitation, precipitation intensity, and lower-, middle-, and upper-tropospheric circulation were created using the Real-Time Multivariate MJO index, which divides the MJO into eight longitudinally based phases. Precipitation was found to vary across MJO phases, with positive precipitation anomalies in central and south-central Chile (30°–45°S) for MJO phases 8, 1, and 2, and negative anomalies in phases 3–7. Circulation was also found to vary across phase, in good agreement with precipitation: low geopotential height and negative omega (corresponding to upward vertical motion) anomalies were found over and upstream of Chile during the rainier phases, and the anomalies reversed during the drier phases. Surface pressure and middle- and upper-tropospheric geopotential height anomalies showed a classic equivalent barotropic wave train, indicating a teleconnection response to deep convective activity in the Maritime Continent in agreement with numerous earlier observational, modeling, and theoretical studies.

The authors investigate if atmospheric water vapor from remote sensing retrievals obtained from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and the water vapor budget from the NASA Goddard Space Flight Center (GSFC) Modern Era Retrospective-analysis for Research and Applications (MERRA) are physically consistent with independently synthesized precipitation data from the Tropical Rainfall Measuring Mission (TRMM) or the Global Precipitation Climatology Project (GPCP) and evaporation data from the Goddard Satellite-based Surface Turbulent Fluxes (GSSTF). The atmospheric total water vapor sink (Σ) is estimated from AIRS water vapor retrievals with MERRA winds (AIRS–MERRA Σ) as well as directly from the MERRA water vapor budget (MERRA–MERRA Σ). The global geographical distributions as well as the regional wavelet amplitude spectra of Σ are then compared with those of TRMM or GPCP precipitation minus GSSTF surface evaporation (TRMM–GSSTF and GPCP–GSSTF P − E, respectively). The AIRS–MERRA and MERRA–MERRA Σs reproduce the main large-scale patterns of global P − E, including the locations and variations of the ITCZ, summertime monsoons, and midlatitude storm tracks in both hemispheres. The spectra of regional temporal variations in Σ are generally consistent with those of observed P − E, including the annual and semiannual cycles, and intraseasonal variations. Both AIRS–MERRA and MERRA–MERRA Σs have smaller amplitudes for the intraseasonal variations over the tropical oceans. The MERRA P − E has spectra similar to that of MERRA–MERRA Σ in most of the regions except in tropical Africa. The averaged TRMM–GSSTF and GPCP–GSSTF P − E over the ocean are more negative compared to the AIRS–MERRA, MERRA–MERRA Σs, and MERRA P − E.

The 1997/98 El Niño–induced changes in rainfall vertical structure in the east Pacific (EP) are investigated by using collocated Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and associated daily SST and 6-hourly reanalysis data during January, February, March, and April of 1998, 1999, and 2000. This study shows that there are five key parameters, that is, surface rain rate, precipitation-top height (or temperature), and precipitation growth rates at upper, middle, and low layers to define a rainfall profile, and those five key parameters are strongly influenced by both SST and large-scale dynamics. Under the influence of 1997/98 El Niño, the precipitation-top heights in the EP were systematically higher by about 1 km than those under non–El Niño conditions, while the freezing level was about 0.5 km higher. Under the constraints of rain type, surface rain rate, and the precipitation top, the shape of rainfall profile still showed significant differences: the rain growth was relatively faster in the mid-layer (−5° to +2°C isotherm) but slower in the lower layer (below +2°C isotherm) under the influence of El Niño. It is also evident that the dependence of precipitation top height on SST was stronger under large-scale decent (non–El Niño) circulations but much weaker under large-scale ascent (El Niño) circulations. The combined effect of larger vertical extent and greater growth rate in the middle layer further shifted latent heating upward as compared with the impact of horizontal changes in the rain type fractions (convective versus stratiform). Such additional latent heating shift would certainly further elevate circulation centers and strengthen the upper-layer circulation.

This study investigates anvils from thick, nonprecipitating clouds associated with deep convection as observed in the tropics by the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) during the 10-yr period, 1998–2007. Anvils observable by the PR occur, on average, 5 out of every 100 days within grid boxes with 2.5° resolution and with a conditional areal coverage of 1.5%. Unconditional areal coverage is only a few tenths of a percent. Anvils also had an average 17-dBZ echo top of ∼8.5 km and an average thickness of ∼2.7 km. Anvils were usually higher and thicker over land compared to ocean, and occurred most frequently over Africa, the Maritime Continent, and Panama. Anvil properties were intimately tied to the properties of the parent convection. In particular, anvil area and echo-top heights were highly correlated to convective rain area. The next best predictor for anvil areal coverage and echo tops was convective echo tops, while convective reflectivities had the weakest correlation. Strong upper-level wind shear also may be associated with anvil occurrence over land, especially when convection regularly attains echo-top heights greater than 7 km. Some tropical land regions, especially those affected by monsoon circulations, experience significant seasonal variability in anvil properties—strong interannual anvil variability occurs over the central Pacific because of the El Niño–Southern Oscillation. Compared to the CloudSat Cloud Profiling Radar, the TRMM PR underestimates anvil-top height by an average of ∼5 km and underestimates their horizontal extent by an average factor of 4.

How much precipitation is contributed by warm rain systems over the tropics? What is the typical size, intensity, and echo top of warm rain events observed by the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar over different regions of the tropics? What proportion of warm raining areas is actually attached to the edges of cold systems? Are there mesoscale warm raining systems, and if so, where and when do they occur? To answer these questions, a 9-yr TRMM precipitation feature database is used in this study. First, warm rain features in 20°S–20°N are selected by specifying precipitation features 1) with minimum infrared brightness temperature > 0°C, 2) with TRMM Precipitation Radar (PR) echo top below freezing level, or 3) without any ice-scattering signature in the microwave observations, respectively. Then, the geographical, seasonal, and diurnal variations of the rain volume inside warm rain features defined in these three ways are presented. The characteristics of warm rain features are summarized. Raining pixels with cloud-top temperature above 0°C contribute 20% of the rainfall over tropical oceans and 7.5% over tropical land. However, about half of the warm pixels over oceans and two-thirds of the warm pixels over land are attached to cold precipitation systems. A large amount of warm rainfall occurs over oceans near windward coasts during winter. Most of the warm rain systems have small size < 100 km2 and weak radar echo with a modal maximum near-surface reflectivity around 23 dBZ. However, mesoscale warm rain systems with strong radar echoes do occur in large regions of the tropical oceans, more during the nighttime than during daytime. Though the mean height of the warm precipitation features over oceans is lower than that over land, there is no significant regional difference in its size and intensity.

Boreal summer intraseasonal (30–90-day time scale) sea surface temperature (SST) variability in the east Pacific warm pool is examined using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) sea surface temperatures during 1998–2005. Intraseasonal SST variance maximizes at two locations in the warm pool: in the vicinity of 9°N, 92°W near the Costa Rica Dome and near the northern edge of the warm pool in the vicinity of 19°N, 108°W. Both locations exhibit a significant spectral peak at 50–60-day periods, time scales characteristic of the Madden–Julian oscillation (MJO). Complex empirical orthogonal function (CEOF) and spectra coherence analyses are used to show that boreal summer intraseasonal SST anomalies are coherent with precipitation anomalies across the east Pacific warm pool. Spatial variations of phase are modest across the warm pool, although evidence exists for the northward progression of intraseasonal SST and precipitation anomalies. Intraseasonal SSTs at the north edge of the warm pool lag those in the vicinity of the Costa Rica Dome by about 1 week. The MJO explains 30%–40% of the variance of intraseasonal SST anomalies in the east Pacific warm pool during boreal summer. Peak-to-peak SST variations of 0.8°–1.0°C occur during MJO events. SST is approximately in quadrature with MJO precipitation, with suppressed (enhanced) MJO precipitation anomalies leading positive (negative) SST anomalies by 7–10 days. Consistent with the CEOF and coherence analyses, MJO-related SST and precipitation anomalies near the Costa Rica Dome lead those at the northern edge of the warm pool by about 1 week.

This study investigates the variability of convective and stratiform rainfall from 8 yr (1998–2005) of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and TRMM Microwave Imager (TMI) measurements, focusing on seasonal diurnal variability. The main scientific goals are 1) to understand the climatological variability of these two dominant forms of precipitation across the four cardinal seasons and over continents and oceans separately and 2) to understand how differences in convective and stratiform rainfall variations ultimately determine how the diurnal variability of the total rainfall is modulated into multiple modes. There are distinct day–night differences for both convective and stratiform rainfall. Oceanic (continental) convective rainfall is up to 25% (50%) greater during nighttime (daytime) than daytime (nighttime). The seasonal variability of convective rainfall’s day–night difference is relatively small, while stratiform rainfall exhibits very apparent day–night variations with seasonal variability. There are consistent late evening diurnal peaks without obvious seasonal variations over ocean for convective, stratiform, and total rainfall. Over continents, convective and total rainfall exhibit consistent dominant afternoon peaks with little seasonal variations—but with late evening secondary peaks exhibiting seasonal variations. Stratiform rainfall over continents exhibits a consistent strong late evening peak and a weak afternoon peak, with the afternoon mode undergoing seasonal variability. Thus, the diurnal characteristics of stratiform rainfall appear to control the afternoon secondary maximum of oceanic rainfall and the late evening secondary peak of continental rainfall. Even at the seasonal–regional scale spatially or the interannual global scale temporally, the secondary mode can become very pronounced, but only on an intermittent basis. Overall, the results presented here demonstrate the importance of partitioning the total rainfall into convective and stratiform components and suggest that diurnal modes largely arise from distinct diurnal stratiform variations modulating convective variations.

Four years of precipitation retrievals from the Tropical Rainfall Measuring Mission (TRMM) satellite are compared with data from 25 surface rain gauges on the National Oceanic and Atmospheric Administration/Pacific Marine Environment Laboratory (NOAA/PMEL) Tropical Atmosphere–Ocean Array/Triangle Trans-Ocean Buoy Network TAO/TRITON buoy array in the tropical Pacific. The buoy gauges have a significant advantage over island-based gauges for this purpose because they represent open-ocean conditions and are not affected by island orography or surface heating. Because precipitation is correlated with itself in both space and time, comparisons between the two data sources can be improved by properly averaging in space and/or time. When comparing gauges with individual satellite overpasses, the optimal averaging time for the gauge (centered on the satellite overpass time) depends on the area over which the satellite data are averaged. For 1° × 1° areas there is a broad maximum in the correlation for gauge-averaging periods of ∼2 to 10 h. Maximum correlations r are in the range 0.6 to 0.7. For larger satellite averaging areas, correlations with the gauges are smaller (because a single gauge becomes less representative of the precipitation in the box) and the optimum gauge-averaging time is longer. For individual satellite overpasses averaged over a 1° × 1° box, the relative rms difference with respect to a rain gauge centered in the box is ∼200% to 300%. For 32-day time means over 1° × 1° boxes, the relative rms difference between the satellite data and a gauge is in the range of 40% to 70%. The bias between the gauges and the satellite retrievals is estimated by correlating the long-term time-mean precipitation estimates across the set of gauges. The TRMM Microwave Imager (TMI) gives an r2 of 0.97 and a slope of 0.970, indicating very little bias with respect to the gauges. For the Precipitation Radar (PR) the comparable numbers are 0.92 and 0.699. The results of this study are consistent with the sampling error estimates from the statistical model of Bell and Kundu.

The impact of boundary layer (BL) processes on simulated tropical precipitation was studied using the National Centers for Environmental Prediction (NCEP) Medium-Range Forecast (MRF) Model. A new BL scheme, which is a nonlocal mixing concept of Noh et al. after Troen and Mahrt, was successfully incorporated into the MRF Model. In this study, simulations with 10-member ensembles were conducted for boreal summers of normal, El Nino, and La Nina years, respectively. In particular, the authors focused on the impact on tropical rainfall of the new BL scheme when two different convection schemes are utilized respectively in the model. The new BL scheme improves simulated tropical precipitation overall and in particular reduces the simulated rainfall in the central and eastern equatorial Pacific Ocean. This reduction over the eastern Pacific is a direct effect of the new BL scheme resulting in less mixing of heat and moisture and is irrespective of the convection scheme. The effect of BL processes over ...

Convective systems in sub-Saharan Africa were defined from measurements by the Tropical Rainfall Measuring Mission satellite Microwave Imager at 85 GHz for four wet seasons, May–September 1998–2001. By applying a convective–stratiform discrimination algorithm to each convective system, the pixels within were designated as convective or stratiform cloud, life cycle ages assigned, and rainfall rates calculated. The years 1998 and 1999 were wetter than the long-term (1898–2000) mean, while 2000 and 2001 were drier. The wetter years had about 10% more convective systems than the drier years, but the size and intensity distributions for the wetter and drier years were virtually identical. The wet season diurnal cycle of precipitation in the study area varied regionally, intraseasonally, and interannually. Analysis of precipitation versus time revealed different diurnal cycles for each of the three 10° zones south of the Sahara Desert. The diurnal cycle was bimodal north of 10°N and unimodal south of 1...

A recent GCM study indicates that a weakening of tropical circulation associated with a slight increase in tropical precipitation may occur when atmospheric CO2 is increased. To further understand the mechanism of atmospheric temperature and precipitation changes associated with the greenhouse gas increase, a numerical experiment was conducted using an atmospheric general circulation model to investigate the separate effects of CO2 increase and sea surface temperature (SST) increase. It has been shown that the effect of CO2 increase is a reduction of radiative cooling in the lower troposphere, leading to a reduction of tropical precipitation. When atmospheric CO2 concentration is doubled (quadrupled) without changing the SST, the tropical precipitation is reduced by about 3% (6%) in the model. The reduction of radiative cooling is a result of the overlap effect of the CO2 15-μm and water vapor absorption bands. On the other hand, the effect of SST increase is the increase in atmospheric temperatu...

The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has made it possible for the first time to evaluate global characteristics of vertical structures of rainfall over monsoon Asia. This study is intended to depict features of seasonal variations of the vertical gradient of rainfall rate using TRMM PR data from 1998 to 2000. The features of downward decreasing (DD) or downward increasing (DI) rainfall rate in the lower part of the vertical profile are focused on. Horizontal maps were made showing decreasing/increasing characteristics in the vertical gradient of rainfall rate over Asia. The pattern showed a clear land–ocean contrast and had monthly variation as the monsoon progressed in Asia. The DD pattern migrated northward around the monsoon onset and withdrew southward in the retrogressing period. The seasonal march of the DD pattern was clear especially over the India subcontinent. The DD seemed to be bordering monsoon rainfall over India. Seasonal changes in characteristic...

Severe drought over the past three years (1998‐2001), in combination with the effects of protracted sociopolitical disruption, has led to widespread famine affecting over 60 million people in central and southwest (CSW) Asia. Here both a regional and a large-scale mode of climate variability are documented that, together, suggest a possible forcing mechanism for the drought. During the boreal cold season, an inverse relationship exists between precipitation anomalies in the eastern Indian Ocean and CSW Asia. Suppression of precipitation over CSW Asia is consistent with interaction between local synoptic storms and wave energy generated by enhanced tropical rainfall in the eastern Indian Ocean. This regional out-of-phase precipitation relationship is

In this study, a statistical model is developed that exploits the slow evolution of the Madden–Julian oscillation (MJO) to predict tropical rainfall variability at long lead times (i.e., 5–20 days). The model is based on a field-to-field decomposition that uses previous and present pentads of outgoing longwave radiation (OLR; predictors) to predict future pentads of OLR (predictands). The model was developed using 30–70-day bandpassed OLR data from 1979 to 1989 and validated on data from 1990 to 1996. For the validation period, the model exhibits temporal correlations to observed bandpassed data of about 0.5–0.9 over a significant region of the Eastern Hemisphere at lead times from 5 to 20 days, after which the correlation drops rapidly with increasing lead time. Correlations against observed total anomalies are on the order of 0.3–0.5 over a smaller region of the Eastern Hemisphere. Comparing the skill values from the above OLR-based model, along with those from an identical statistical model us...

The atmospheric state simulated by the National Center for Atmospheric Research (NCAR) Community Climate Model, version 3 (CCM3), is compared to that simulated by the NCAR Climate System Model, version 1 (CSM1). CCM3 is an atmospheric general circulation model that uses specified sea surface temperatures (SSTs) for a lower boundary condition. Observed monthly mean SSTs for 1979–93 were used in the present study. CSM1 is a coupled general circulation model in which the SSTs are determined as part of the simulation and CCM3 serves as the atmospheric component. It is found that the differences between CCM3 and CSM1 are quite small in most measures of the atmospheric circulation, consistent with the accurate and drift-free simulation of the SSTs in the coupled model. There are substantial temperature differences near the surface in the Arctic and over the ocean around Antarctica, resulting from different sea-ice distributions. The tropical precipitation also has significant differences, although neit...