Stratiform Rain Rate Research Articles

Microphysical characteristics of torrential predecessor rain events over the Yangtze River Delta Area and the related tropical cyclones

The precipitation structures and microphysical characteristics of predecessor rain events (PREs) over the Yangtze River Delta area and related tropical cyclones (TCs) from 2014 to 2019 were investigated using Dual-frequency Precipitation radar data from Global Precipitation Measurement (GPM) for drop size distributions (DSDs). Results showed that the total mean rain rate of PREs was larger than that of TCs, primarily due to higher convective and stratiform rain rates, with enhanced fractional coverage of convective rain in PREs. Examination of microphysical characteristics revealed that a greater quantity of small-sized droplets and a smaller quantity of medium- as well as large-sized droplets contributed towards PREs in comparison with TCs. The conclusion still holds when partitioning DSDs based on different precipitation rate categories. Further investigation of DSDs using gamma functions illustrated that precipitation in PREs had lower average mass-weighted diameters (Dm) and enhanced normalized number concentration (Nw) compared with TCs; partitioning precipitation into convective and stratiform components illustrated a larger Dm and lower Nw in TCs than PREs. The analysis of microphysical and thermodynamical processes using the reanalysis data indicates that relatively intense convective activity with drier conditions may be favorable to enhancing raindrop growth through collision-coalescence processes, as a result of larger Dm in TCs than PREs. The empirical relations (Z–R algorithms) applied in different rain regimes (stratiform, convective, and total PREs) revealed significant diversities, relying on weather conditions and geographical locations. Plain language summaryThe Yangtze River Delta area is an important economic belt in China. Under climate change, observed frequent occurrences of weather extremes of heavy rainfall and tropical cyclones (TCs) exert adverse effects on economic development in this region. Thus, a deep understanding of the mechanism of TCs torrential rainfall in the Yangtze River Delta area is urgently necessary. In this study, we investigated the precipitation patterns and microphysical characteristics of predecessor rain events (PREs) in the Yangtze River Delta region, and their association with TCs in the South China Sea-Western North Pacific Ocean (SCS-WNPO) area from 2014 to 2019. We found that PREs had a higher total mean precipitation rate than TCs. Further examination using gamma functions demonstrated that PREs exhibited lower average mass-weighted diameters (Dm) and higher normalized intercept parameters (Nw) than TCs. This pattern persisted when distinguishing between convective and stratiform precipitation components. We believe that our study makes a significant contribution to the literature because these results provide valuable insights into the distinct precipitation characteristics of PREs and TCs in the study region and contribute to a better understanding of tropical cyclone-related rainfall patterns, and act as a scientific basis for disaster mitigation.

Fundamental Characteristics of Tropical Rain Cell Structures as Measured by TRMM PR

Rain cells are the most elementary unit of precipitation system in nature. In this study, fundamental geometric and physical characteristics of rain cells over tropical land and ocean areas are investigated by using 15-yr measurements of the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). The rain cells are identified with a minimum bounding rectangle (MBR) method. The results indicate that about 50% of rain cells occur at length of about 20 km and width of 15 km. The proportion of rain cells with length > 200 km and width > 100 km is less than 1%. There is a a log-linear relationship between the mean length and width of rain cells. Usually, for the same horizontal geometric parameters, rain cells tend to be square horizontally and lanky vertically over land, while vertically squatty over ocean. The rainfall intensity of rain cells varies from 0.4 to 10 mm h−1 over land to 0.4–8 mm h−1 over ocean. Statistical results indicate that the occurrence frequency of rain cells decreases as the areal fraction of convective precipitation in rain cells increases, while such frequency remains almost invariant when the areal fraction of stratiform precipitation varies from 10% to 80%. The relationship between physical and geometric parameters of rain cells shows that the mean rain rate of rain cells is more frequently associated with the increase of their area, with the increasing rate over land greater than that over ocean. The results also illustrate that heavy convective rain rate prefers to occur in larger rain cells over land while heavy stratiform rain rate tends to appear in larger rain cells over ocean. For the same size of rain cells, the areal fraction and the contribution of convective precipitation are about 10%–15% higher over land than over ocean.

Cloud‐Resolving Model Intercomparison of an MC3E Squall Line Case: Part II. Stratiform Precipitation Properties

AbstractIn this second part of a cloud microphysics scheme intercomparison study, we focus on biases and variabilities of stratiform precipitation properties for a midlatitude squall line event simulated with a cloud‐resolving model implemented with eight cloud microphysics schemes. Most of the microphysics schemes underestimate total stratiform precipitation, mainly due to underestimation of stratiform precipitation area. All schemes underestimate the frequency of moderate stratiform rain rates (2–6 mm/hr), which may result from low‐biased ice number and mass concentrations for 0.2–2‐mm diameter particles in the stratiform ice region. Most simulations overestimate ice water content (IWC) at altitudes above 7 km for temperatures colder than −20 °C but produce a decrease of IWC approaching the melting level, which is opposite to the trend shown by in situ observations. This leads to general underestimations of stratiform IWC below 5‐km altitude and rainwater content above 1‐km altitude for a given rain rate. Stratiform precipitation area positively correlates with the convective condensate detrainment flux but is modulated by hydrometeor type, size, and fall speed. Stratiform precipitation area also changes by up to 17%–25% through alterations of the lateral boundary condition forcing frequency. Stratiform precipitation, rain rate, and area across the simulations vary by a factor of 1.5. This large variability is primarily a result of variability in the stratiform downward ice mass flux, which is highly correlated with convective condensate horizontal detrainment strength. The variability of simulated local microphysical processes in the stratiform region plays a secondary role in explaining variability in simulated stratiform rainfall properties.

Potential Impacts of Sahara Dust Aerosol on Rainfall Vertical Structure Over the Atlantic Ocean as Identified From EOF Analysis

AbstractThe current study investigates the potential impacts of Sahara dust on rainfall vertical structure over the Atlantic Ocean by employing multisensor satellite observations. The variabilities in rainfall vertical structure are decomposed by empirical orthogonal function (EOF) analysis. For a given near surface rain rate, the mean storm height of stratiform rain under dust‐laden condition is significantly higher than that under the dust‐free condition. The stratiform rain rate at the layers well above the freezing level is substantially enhanced under dust‐laden condition. Those changes may result from dynamic and thermodynamic effect of Sahara Air Layer effect and/or the effect that dust aerosol acting as additional ice nuclei. For convective rain, there is no significant difference in the leading three normalized EOF modes between the dust‐laden and dust‐free sectors, implying that dust effect, if exists, is not strong enough to cause evident changes in convective rainfall profiles. For stratiform rain, the EOF1 shows no signals of dust effect either. However, significant difference is found in the EOF2 and EOF3 modes between dust‐laden and dust‐free conditions. The normalized EOF2 and EOF3 under dust‐laden condition both represent enhanced rain rate at the upper layer well above the freezing level, where heterogeneous ice nucleation prevails. The statistical study shows consistent results with the case study, except that the aerosol‐related changes in stratiform rainfall vertical structure is isolated in the EOF3 only. This study suggests that EOF analysis is a promising way for isolating the potential and relatively weak aerosol effects on precipitation from the strong dynamic effects.

Revisiting the iris effect of tropical cirrus clouds with TRMM and A‐Train satellite data

AbstractJust as the iris of human eye controls the light influx (iris effect), tropical anvil cirrus clouds may regulate the Earth's surface warming by controlling outgoing longwave radiation. This study examines this possible effect with monthly satellite observations such as Tropical Rainfall Measuring Mission (TRMM) precipitation, Moderate Resolution Imaging Spectroradiometer cirrus fraction, and Clouds and the Earth's Radiant Energy System top‐of‐the‐atmosphere radiative fluxes averaged over different tropical domains from March 2000 to October 2014. To confirm that high‐level cirrus is relevant to this study, Cloud‐Aerosol Lidar with Orthogonal Polarization high cloud observations were also analyzed from June 2006 to December 2015. Our analysis revealed that the increase in sea surface temperature in the tropical western Pacific tends to concentrate convective cloud systems. This concentration effect very likely induces the significant reduction of both stratiform rain rate and cirrus fraction, without appreciable change in the convective rain rate. This reduction of stratiform rain rate and cirrus fraction cannot be found over its subregion or the tropical eastern Pacific, where the concentration effect of anvil cirrus is weak. Consistently, over the tropical western Pacific, the higher ratio of convective rain rate to total rain rate (i.e., precipitation efficiency) significantly correlates with warmer sea surface temperature and lower cirrus fraction. The reduced cirrus eventually increased outgoing longwave radiation to a greater degree than absorbed solar radiation. Finally, the negative relationship between precipitation efficiency and cirrus fraction tends to correspond to a low global equilibrium climate sensitivity in the models in the Coupled Model Intercomparison Project Phase 5. This suggests that tropical anvil cirrus clouds exert a negative climate feedback in strong association with precipitation efficiency.

Interannual variability of summer monsoon convective and stratiform precipitations in East Asia during 1998–2013

ABSTRACTIn this paper, interannual variations of convective and stratiform precipitations in summer East Asian monsoon (SEAM) dominion are analysed using 16‐year (1998–2013) Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) rain products (2A25 and 3A25). It is found that the climatological mean patterns of convective and stratiform rain rates (RRs) are generally similar to each other, but the area fraction (AF) of stratiform precipitation can always reach about 80% and dominates in summer East Asia. Nonetheless, convective precipitation produces similar rainfall quantity with stratiform rainfall. For the RR, AF and contribution fraction (CF), their interannual standard deviation and coefficient of variation (CV) spatial patterns are similar between summer convective and stratiform precipitations in East Asia, because of the fact that they are all maybe influenced by SEAM. Differently, only the intensity of interannual variability (i.e. CV) for convective precipitation AF is much greater than that for stratiform precipitation, while CV for both convective and stratiform precipitations RR/CF are similar in most areas. Moreover, the interannual variability of SEAM is more highly correlated with that of RR for convective precipitation than that for stratiform precipitation in most land regions. While the opposite conclusion is found in ocean regions. Generally, the SEAM intensity in the lower troposphere mainly controls interannual variability of the atmospheric stability and both stratiform and convective precipitations RR. Then, atmospheric stability variation further modulates the AF and CF variations of both stratiform and convective precipitations on the interannual scale. This work mainly provides possible evidence of linkage between large‐scale circulation and convective and stratiform precipitations on the interannual scale. It suggests that the possible dominant mechanism should be explored using more observations and together with model simulations in the future work.

The Study of Atmospheric Physics Parameters over Iran Using Satellite TRMM-TMI Data

There are a limited number of ground-based hydro-meteorological observation networks in developing countries. However, precipitation estimates can be obtained using aeronautical and remote sensing techniques, and from radiometric observations by satellites with a suitable coverage. In this research, Tropical Rainfall Measuring Mission- TRMM Microwave Imager (TRMM-TMI) data are applied to estimate and investigate the surface, convective, and strati form rain rates using an analysis of surface and cloud liquid water, cloud ice, precipitation water, and precipitation ice in 14 vertical atmospheric levels(from the surface to 18 km above the surface). The TRMM-TMI data has a monthly temporal resolution and 0.5° spatial resolutions, and covers the period 1998-2005 in a binary format. Results show that surface precipitation estimated from TRMM-TMI has the greatest correlation with ground-based data for spring and autumn, but overestimates precipitation rates in the winter, spring, and summer. It was also found that convective and strati form precipitation patterns are the same in autumn and winter over Iran, but are different during the spring and summer seasons, and that most of the surface precipitation is related to convective precipitation. Furthermore the height of the melting level is estimated to be 3-4 km above the surface of the country.

Evaluation of cloud‐resolving and limited area model intercomparison simulations using TWP‐ICE observations: 2. Precipitation microphysics

AbstractTen 3‐D cloud‐resolving model (CRM) simulations and four 3‐D limited area model (LAM) simulations of an intense mesoscale convective system observed on 23–24 January 2006 during the Tropical Warm Pool–International Cloud Experiment (TWP‐ICE) are compared with each other and with observations and retrievals from a scanning polarimetric radar, colocated UHF and VHF vertical profilers, and a Joss‐Waldvogel disdrometer in an attempt to explain a low bias in simulated stratiform rainfall. Despite different forcing methodologies, similar precipitation microphysics errors appear in CRMs and LAMs with differences that depend on the details of the bulk microphysics scheme used. One‐moment schemes produce too many small raindrops, which biases Doppler velocities low, but produces rainwater contents (RWCs) that are similar to observed. Two‐moment rain schemes with a gamma shape parameter (μ) of 0 produce excessive size sorting, which leads to larger Doppler velocities than those produced in one‐moment schemes but lower RWCs. Two‐moment schemes also produce a convective median volume diameter distribution that is too broad relative to observations and, thus, may have issues balancing raindrop formation, collision‐coalescence, and raindrop breakup. Assuming a μ of 2.5 rather than 0 for the raindrop size distribution improves one‐moment scheme biases, and allowing μ to have values greater than 0 may improve excessive size sorting in two‐moment schemes. Underpredicted stratiform rain rates are associated with underpredicted ice water contents at the melting level rather than excessive rain evaporation, in turn likely associated with convective detrainment that is too high in the troposphere and mesoscale circulations that are too weak. A limited domain size also prevents a large, well‐developed stratiform region like the one observed from developing in CRMs, although LAMs also fail to produce such a region.

Evaluation of cloud-resolving model intercomparison simulations using TWP-ICE observations: Precipitation and cloud structure

The Tropical Warm Pool.International Cloud Experiment (TWP ]ICE) provided extensive observational data sets designed to initialize, force, and constrain atmospheric model simulations. In this first of a two ]part study, precipitation and cloud structures within nine cloud ]resolving model simulations are compared with scanning radar reflectivity and satellite infrared brightness temperature observations during an active monsoon period from 19 to 25 January 2006. Seven of nine simulations overestimate convective area by 20% or more leading to general overestimation of convective rainfall. This is balanced by underestimation of stratiform rainfall by 5% to 50% despite overestimation of stratiform area by up to 65% because of a preponderance of very low stratiform rain rates in all simulations. All simulations fail to reproduce observed radar reflectivity distributions above the melting level in convective regions and throughout the troposphere in stratiform regions. Observed precipitation ]sized ice reaches higher altitudes than simulated precipitation ]sized ice despite some simulations that predict lower than observed top ]of ]atmosphere infrared brightness temperatures. For the simulations that overestimate radar reflectivity aloft, graupel is the cause with one ]moment microphysics schemes whereas snow is the cause with two ]moment microphysics schemes. Differences in simulated radar reflectivity are more highly correlated with differences in mass mean melted diameter (Dm) than differences in ice water content. Dm is largely dependent on the mass ]dimension relationship and gamma size distribution parameters such as size intercept (N0) and shape parameter (m). Having variable density, variable N0, or m greater than zero produces radar reflectivities closest to those observed.

TRMM 위성의 강수레이더에서 관측된 동아시아 여름 강수의 특성

The characteristics and vertical structure of the rainfall are examined in terms of rain types using TRMM (Tropical Rainfall Measuring Mission) PR (Precipitation Radar) data during the JJA period of 2002-2006 over three different regions; midlatitude region around the Korean Peninsula (EA1), subtropical East Asia (EA2), and tropical East Asia (EA3). The convective rain fraction in the EA1 region is 12.2%, which is smaller by 6% than those in the EA2 and EA3 regions. EA1 shows less frequent convective rain events, which are about 0.5 times as many as those in EA3. EA1 produces the mean convective rain rate of 10.4 mm/h that is about 40% larger than EA2 and EA3 while all regions have similar mean stratiform rain rate. The relationships between storm height and rain rate indicate that the rain rate is proportional to the storm height. Based on the vertical structure of radar reflectivity, EA1 produces deeper and stronger convective clouds with higher rain rate compared to the other regions. In EA3, radar reflectivity increases distinctly toward the land surface at altitude below 5 km, indicating more dominant coalescence-collision processes than the other regions. Furthermore, the bright band of stratiform rain clouds in EA3 is very distinct. In convective rain clouds, the first EOFs of radar reflectivity profiles are similar among the three regions, while the second EOFs are slightly different. The larger variability exists at upper layers for EA1 while it exits at lower levels for EA3.

Effects of vertical wind shear and cloud radiative processes on responses of rainfall to the large-scale forcing during pre-summer heavy rainfall over southern China

The pre-summer heavy rainfall over southern China during 3–8 June 2008 is simulated using a two-dimensional cloud-resolving model. The model is integrated with imposed zonally uniform vertical velocity, zonal wind, horizontal temperature and vapour advection from National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS) data. The effects of vertical wind shear and cloud radiative processes on the response of rainfall to large-scale forcing are analysed through the comparison of two sensitivity experiments with the control experiment. One sensitivity experiment excludes the large-scale vertical wind shear and the other excludes the cloud radiative effects. During the decay phase of convection, the increase in model domain-mean surface rain-rate resulting from the exclusion of vertical wind shear is associated with the slowdown in the decrease of perturbation kinetic energy due to the exclusion of barotropic conversion from mean kinetic energy to perturbation kinetic energy. The increase in domain-mean rain-rate from the exclusion of cloud radiative effects is related to the enhancement of condensation and associated latent heat as a result of strengthened radiative cooling. The increase in the domain-mean surface rain-rate is mainly associated with the increase of convective rainfall, which is in turn related to the local atmospheric change from moistening to drying. During the onset and mature phases of convection, the domain-mean surface rain-rates are generally insensitive to vertical wind shear and cloud radiative effects whereas convective and stratiform rain-rates are sensitive to vertical wind shear and cloud radiative effects. The decrease in convective rain-rate and the increase in stratiform rain-rate are primarily associated with the enhanced transport of hydrometeor concentration from convective regions to raining stratiform regions. Copyright © 2011 Royal Meteorological Society

Microphysical and radiative effects of ice clouds on responses of rainfall to the large-scale forcing during pre-summer heavy rainfall over southern China

Ice clouds are an important part of precipitation systems and their microphysical and radiative effects may significantly impact rainfall simulations. This study investigates these effects using a two-dimensional cloud-resolving model simulation of a pre-summer heavy rainfall event over southern China from 3–8 June 2008, forced with NCEP/GDAS data. The microphysical and radiative effects of ice clouds on rainfall responses to the large-scale forcing during pre-summer heavy rainfall over southern China are analyzed by comparing the control experiment and two sensitivity experiments with the exclusion of ice radiative effects and with the total exclusion of ice microphysics (the exclusion of both ice radiative and microphysical effects), respectively. During the development phase, the total exclusion of ice microphysics decreased the model domain mean surface rain rate primarily by suppressing the convective rain rate through the exclusion of ice radiative effects on 4 June 2008 and by reducing the stratiform rain rate through the exclusion of ice microphysical effects on 5 June. During the peak phase on 6 June, the mean rain rate significantly increased from the previous day. The reduction in the mean rain rate by the total exclusion of ice microphysics continued from the previous day due to the continuous decrease in the stratiform rain rate caused by the exclusion of ice microphysical effects. During the decay phase on 7 June, the total exclusion of ice microphysics increased the mean rain rate by enhancing convective rainfall through the exclusion of ice microphysical effects. These results indicate that the microphysical and radiative effects of ice clouds play equally important roles in pre-summer heavy rainfall events.

Verification of Tropical Cyclone–Related Satellite Precipitation Estimates in Mainland China

Abstract To evaluate the abilities of satellite retrievals in reflecting precipitation features related to tropical cyclones (TCs) affecting mainland China, four years of 6- and 24-h precipitation retrievals from three datasets, namely the Tropical Rainfall Measuring Mission satellite algorithm 3B42, version 6 (3B42), Climate Prediction Center morphed (CMORPH) product, and one based on the Geostationary Meteorological Satellite-5 infrared brightness temperature (GMS5-TBB), are compared statistically with direct measurements from surface gauge rainfall data during the periods affected by TCs. The GMS5-TBB dataset was set up by a method of considering the GMS5-TBB characteristics, hourly precipitation intensity, and horizontal distribution for landfalling TCs. The results show that in a general sense, all three satellite-retrieved rainfall datasets give quite reasonable 6- and 24-h rainfall distributions, with skill decreasing with the increase in both latitude and rainfall amount. The 3B42 has a little bit better skill than CMORPH, which is likely related to the fact that the 3B42 product has a rain gauge adjustment and CMORPH does not. Further analyses show that both 3B42 and CMORPH considerably underestimate the moderate and heavy rainfall and overestimate the very light precipitation. The overestimation of the GMS5-TBB data for the light rain is larger than that for 3B42 and CMORPH, probably due to the fact that the GMS5-TBB method considers stratiform and convective rainfall separately with a fixed stratiform rain rate of 2 mm h−1. For the heavy rainfall events, the GMS5-TBB data perform much better than the 3B42 and CMORPH data with an almost halved bias, owing to the fact that the GMS5-TBB method adopted the adjustment of the convective rain rate by considering TBB characteristics of landfalling TCs and using hourly gauge rainfall in the setup process. Since the heavy rainfall events associated with landfalling TCs are of the most concern, the compared GMS5-TBB data could be useful as an operational/research reference.

Cloud microphysical and rainfall responses to zonal perturbations of sea surface temperature: A cloud-resolving modeling study

The cloud microphysical and rainfall responses to zonal perturbations of sea surface temperature (SST) are investigated by analyzing the equilibrium simulation data (from day 31–40) obtained from a series of two-dimensional cloud-resolving simulations with a zonal model domain of 768 km. Four experiments imposed by zonal SST perturbations of wavenumbers 1 (SST29Z1), 2 (SST29Z2), 4 (SST29Z4), and 8 (SST29Z8) are compared to the control experiment imposed by zonally uniform SST (SST29). The model domain mean SST is 29 °C, and the two-dimensional cloud-resolving model with a cyclic lateral boundary is also imposed by zero vertical velocity and constant zonal wind. The time and model domain mean surface rain rates in SST29Z1, SST29Z2, and SST29Z8 are about 10% larger than those in SST29, whereas the mean surface rain rates in SST29Z4 and SST29 are similar. The analysis of mean surface rainfall budgets shows that local water vapor and hydrometeor changes play important roles in determining the differences and similarities in mean surface rain rate between the perturbation experiments and the control experiment. Both convective and stratiform rain rates are larger in SST29Z1 and SST29Z2 than in SST29 due to the smaller advection of rain from convective regions into raining stratiform regions and the larger vapor condensation rates associated with the larger water vapor convergence over raining stratiform regions in SST29Z1 and SST29Z2. The convective rain rates are larger in SST29Z4 and SST29Z8 than in SST29 because of the larger condensation rates associated with the larger water vapor convergence over convective regions in SST29Z4 and SST29Z8. The stratiform rain rates in SST29Z4 and SST29Z8 are smaller than in SST29 due to the smaller vapor condensation rates and smaller collection rates of cloud water by rain over raining stratiform regions in SST29Z4 and SST29Z8.

Responses of tropical deep convective precipitation systems and their associated convective and stratiform regions to the large‐scale forcing

AbstractResponses of convective and stratiform rainfall to large‐scale forcing in the tropical deep convective regime are investigated by analysing data from a two‐dimensional cloud‐resolving model simulation. The model is forced by large‐scale vertical velocity, zonal wind, horizontal advection, and sea‐surface temperature observed and derived from TOGA COARE. The imposed large‐scale vertical velocity shows that the ascending motion averaged from 22–27 December 1992 is significantly stronger than that averaged from 3–8 January 1993. Thus, the periods of 22–27 December 1992 and 3–8 January 1993 are, respectively, identified as strong‐forcing (SF) and weak‐forcing (WF) phases. Although the convective rain rates are similar in the two phases, stratiform rain rate is larger in the SF phase than in the WF phase, which leads to larger time and spatial mean surface rain rate in the SF phase. Similar rates of water vapour convergence and vapour condensation and collection of cloud water by rain over water‐hydrometeor‐dominated convective regions are responsible for similar convective rain rates in the two phases. Water vapour divergence is larger in the WF phase than in the SF phase, which causes smaller vapour condensation and deposition rates in the WF phase. As a result, the collection rate of cloud water by rain and the melting rate of graupel to rain are smaller in the WF phase than in the SF phase, which accounts for the smaller stratiform rain rate in the WF phase. Copyright © 2008 Royal Meteorological Society

A cloud‐resolving modeling study of diurnal variations of tropical convective and stratiform rainfall

Diurnal variations of convective and stratiform rainfalls over the tropical ocean are investigated on the basis of data from a two‐dimensional cloud‐resolving simulation. The model is integrated for 40 days to a quasi‐equilibrium state with imposed zero zonally uniform vertical velocity, a constant zonal wind, and a constant sea surface temperature of 29°C. The convective rain rate contributes more to the diurnal variation of model domain‐mean surface rain rate than the stratiform rain rate does. The calculations of the surface rainfall equation show that more vapor convergence yields higher convective rain rate and that more vapor convergence and more local vapor loss cause higher stratiform rain rate in early morning than in afternoon. Analysis of cloud microphysics budgets reveals that variations in vapor condensation and associated water microphysical processes are responsible for the diurnal variations of both convective and stratiform rain rates.

Stratiform Rain in the Tropics as Seen by the TRMM Precipitation Radar*

Across the Tropics (20°N–20°S), the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) indicates that for reflectivities ≥17 dBZ, stratiform precipitation accounts for 73% of the area covered by rain and 40% of the total rain amount over a 3-yr period (1998–2000). The ratio of the convective rain rate to the stratiform rain rate is 4.1 on average at the horizontal resolution of the PR data. Convective rain rates remain constant or decrease as the stratiform contribution to total rain increases, implying that stratiform rain production is not very dependent on the strength of convection. This relationship is especially evident over the ocean, where there are weaker convective rain rates than over land but relatively larger stratiform rain amounts. The ocean environment appears more efficient in the production of stratiform precipitation through either the sustainability of convection by a warm, moist boundary layer with only a weak diurnal variation and/or by the near–moist adiabatic stratification of the free atmosphere. Factors such as wind shear and the relative humidity of the large-scale environment can also affect the production of stratiform rain. Over land, higher stratiform rain fractions often occur during the season of maximum insolation and with the occurrence of very large, organized precipitation systems (i.e., mesoscale convective complexes). Monsoon regions show the largest seasonal variations in stratiform rain fraction, with very low values in the season before the monsoon and higher values during the monsoon. A strong gradient in stratiform rain fraction exists across the Pacific, with a minimum ∼25% over the Maritime Continent and a maximum ∼60% in the intertropical convergence zone (ITCZ) of the eastern-central Pacific. This near-equatorial trans-Pacific gradient becomes exaggerated during El Niño. A higher stratiform rain fraction concentrates latent heating at upper levels, which implies a stronger upper-level circulation response to the heating. Thus, the variations in stratiform rain fraction that occur before the monsoon and during the monsoon, across the Pacific basin, and between La Niña and El Niño imply vertical variations in the large-scale circulation response to tropical precipitating systems that would not occur if the stratiform rain fraction was temporally and spatially uniform across the Tropics.

The Challenges of Modeling Climate Variability and Change in West Africa

This paper overviews observations and examines modeling issues associated with the mean state, climate variability and climate change in West Africa. The Tropical Rain Measuring Mission (TRMM) satellite allows for the first time estimates of Unconditional, Convective and Stratiform rain rates in West Africa. The 1998 estimated TRMM rates are compared to long-term observed rain rates and a merged rain data set (CMAP) during 1998. Further, the TRMM estimates are compared to the simulated rain rates from the Community Climate Model Version 3.6. The TRMM Precipitation Radar rain estimates are generally lower than either the long-term observations or the CMAP rates during 1998. Moreover, the TRMM rain estimates show a significant fraction of the total rain (convective + stratiform) is characterized as stratiform rain (30–40%). The CCM3 simulates primarily convective rain and negligible amounts of non-convective rain for West Africa. Furthermore, the TRMM high-resolution rain patterns strongly imply that rain in West Africa occurs on mesoscales in association with mesoscale convective systems (squall lines, mesoscale convective complexes and non-squall tropical clusters). We demonstrate this by briefly examining two mesoscale convective systems during May 1998 with METEOSAT data. Regional climate models may offer the best solution to understanding climate change in West Africa because of their ability to capture mesoscale systems and better their representation of orographic features. Adequate boundary conditions from Global Climate Models are still necessary for regional climate model simulations to successfully reproduce mean climate conditions and provide understanding with respect to future climate change. Observations in West Africa should be maintained or increased for monitoring climate variability and possibility of climate change in West Africa, proper initialization of numerical weather prediction models and the validation of climate models.

TRMM Observations of Polarization Difference in 85 GHz: Information about Hydrometeors and Rain Rate.

Observations made by the Precipitation Radar (PR) and the Microwave Imager (TMI) radiometer on board the Tropical Rainfall Measuring Mission (TRMM) satellite help us to show the significance of the 85 GHz polarization difference, PD85, measured by TMI. Rain type, convective or stratiform, deduced from the PR allows us to infer that PD85 is generally positive in stratiform rain clouds, while PD85 can be markedly negative in deep convective rain clouds. Furthermore, PD85 increases in a gross manner as stratiform rain rate increases. On the contrary, in a crude fashion PD85 decreases as convective rain rate increases. From the observations of TMI and PR, we find that PD85 is a weak indicator of rain rate. Utilizing information from existing polarimetric radar studies, we infer that negative values of PD85 are likely associated with vertically-oriented small oblate or wet hail that are found in deep convective updrafts.