Vertical Moist Static Energy Advection Research Articles

Moist Thermodynamics of Convectively Coupled Waves over the Western Hemisphere

Abstract Convectively coupled waves (CCWs) over the Western Hemisphere are classified based on their governing thermodynamics. It is found that only the tropical depressions (TDs; TD waves) satisfy the criteria necessary to be considered a moisture mode, as in the Rossby-like wave found in an earlier study. In this wave, water vapor fluctuations play a much greater role in the thermodynamics than temperature fluctuations. Only in the eastward-propagating inertio-gravity (EIG) wave does temperature govern the thermodynamics. Temperature and moisture play comparable roles in all the other waves, including the Madden–Julian oscillation over the Western Hemisphere (MJO-W). The moist static energy (MSE) budget of CCWs is investigated by analyzing ERA5 data and data from the 2014/15 observations and modeling of the Green Ocean Amazon (GoAmazon 2014/15) field campaign. Results reveal that vertical advection of MSE acts as a primary driver of the propagation of column MSE in westward inertio-gravity (WIG) wave, Kelvin wave, and MJO-W, while horizontal advection plays a central role in the mixed Rossby gravity (MRG) and TD wave. Results also suggest that cloud radiative heating and the horizontal MSE advection govern the maintenance of most of the CCWs. Major disagreements are found between ERA5 and GoAmazon. In GoAmazon, convection is more tightly coupled to variations in column MSE, and vertical MSE advection plays a more prominent role in the MSE tendency. These results along with substantial budget residuals found in ERA5 data suggest that CCWs over the tropical Western Hemisphere are not represented adequately in the reanalysis. Significance Statement In comparison to other regions of the globe, the weather systems that affect precipitation in the tropical Western Hemisphere have received little attention. In this study, we investigate the structure, propagation, and thermodynamics of convectively coupled waves that impact precipitation in this region. We found that slowly evolving tropical systems are “moisture modes,” i.e., moving regions of high humidity and precipitation that are maintained by interactions between clouds and radiation. The faster waves are systems that exhibit relatively larger fluctuations in temperature. Vertical motions are more important for the movement of rainfall in these waves. Last, we found that reanalysis and observations disagree over the importance of different processes in the waves that occurred over the Amazon region, hinting at potential deficiencies on how the reanalysis represents clouds in this region.

Evaluation of the Madden–Julian oscillation in HiRAM

The aim of this study was to understand the cause of Madden–Julian oscillation (MJO) bias in the High Resolution Atmospheric Model (HiRAM) driven by observed SST through process-oriented diagnosis. Wavenumber-frequency power spectrum and composite analyses indicate that HiRAM underestimates the spectral amplitude over the MJO band and mainly produces non-propagating rather than eastward-propagating intraseasonal rainfall anomalies, as observed. Column-integrated moist static energy (MSE) budget analysis is conducted to understand the MJO propagation bias in the simulation. It is found that the bias is due to the lack of a zonally asymmetric distribution of the MSE tendency anomaly in respect to the MJO convective center, which is mainly attributable to the bias in vertical MSE advection and surface turbulent flux. Further analysis suggests that it is the unrealistic simulation of MJO vertical circulation anomalies in the upper troposphere as well as overestimation of the Rossby wave response that results in the bias.摘要本研究评估了高分辨率大气环流模式HiRAM模拟的MJO. 结果表明, HiRAM模拟的MJO东传很弱. 我们通过计算整层积分的湿静力能 (MSE) 收支来诊断MJO东传模拟偏差的原因. 结果发现, MSE倾向相对于MJO对流中心的纬向非对称分布很弱是导致东传模拟偏弱的原因, 这主要是由MSE垂直平流和地表湍流通量的模拟偏差造成的. 进一步研究表明, 对流层上层MJO垂直环流结构的模拟偏差和MJO对流西侧的Rossby波环流偏强共同导致了模式的偏差. 本研究中指出的MJO传播模拟偏差的原因与之前基于多模式结果的结论不同, 这意味着要想了解特定模式的模拟偏差, 有必要对该模式进行具体分析.

Seasonality in Prediction Skill of the Madden‐Julian Oscillation and Associated Dynamics in Version 2 of NASA's GEOS‐S2S Forecast System

AbstractThe prediction skill of the Madden‐Julian Oscillation (MJO) in Version 2 of NASA's Global Earth Observing System Subseasonal to Seasonal (GEOS‐S2S) forecast system is investigated for winter and summer focusing on moistening‐related processes crucial for eastward propagating MJO activity. It is found that the annual bivariate correlation of the Real‐time Multivariate MJO time series between prediction and observation is ∼0.70, ∼0.57, and ∼0.50 at 20‐, 25‐, and 30‐day forecast leads. Correlation at long‐leads (>30 days) is noticeably higher for boreal summer initial conditions (June‐September [JJAS]), with correlations remaining above 0.5 at 35–40 days leads. Correlations are lower for boreal winter initial conditions from January through March (JFM), dropping to ∼0.5 at 25‐day lead, still comparable to the skills in the other reliable S2S forecast systems.The predicted eastward MJO propagation across the Indo‐western Pacific sector is well captured in JJAS, but is slower than observed in JFM. Investigations of the moisture field and advection and moisture sink, moist static energy (MSE) budget, and tropical circulation/pressure responses to the MJO convective heating reveal that, in JFM, those responses and moistening processes, especially the vertical MSE advection, are underestimated over and to the east of the Maritime Continent when the MJO anomaly approaches from the west. In contrast, those processes are well represented in JJAS, although moistening is overestimated due to large surface evaporation. This study suggests that improvement of the moistening tendency over that region in the boreal winter could contribute to further increases in MJO prediction skill of the GEOS‐S2S system.

Effects of MJO Vertically Tilted Structure on Its Phase Speed from the Moisture Mode Theory Perspective

AbstractThe effect of vertically tilted structure (VTS) of the MJO on its phase propagation speed was investigated through the diagnosis of ERA-Interim reanalysis data during 1979–2012. A total of 84 eastward propagating MJO events were selected. It was found that all MJO events averaged throughout their life cycles exhibited a clear VTS, and the tilting strength was significantly positively correlated to the phase speed. The physical mechanism through which the VTS influenced the phase speed was investigated. On the one hand, a stronger VTS led to a stronger vertical overturning circulation and a stronger descent in the front, which caused a greater positive moist static energy (MSE) tendency in situ through enhanced vertical MSE advection. The stronger MSE tendency gradient led to a faster eastward phase speed. On the other hand, the enhanced overturning circulation in front of MJO convection led to a stronger easterly/low pressure anomaly at the top of the boundary layer, which induced a stronger boundary layer convergence and stronger ascent in the lower troposphere. This strengthened the boundary layer moisture asymmetry and favored a faster eastward propagation speed.

Intercomparison of MJO Column Moist Static Energy and Water Vapor Budget among Six Modern Reanalysis Products

AbstractThis study conducts an intercomparison of the column-integrated moist static energy (MSE) and water vapor budget of the Madden–Julian oscillation (MJO) among six modern global reanalysis products (RAs). Inter-RA differences in the mean MSE, MJO MSE anomalies, individual MSE budget terms, and their relative contributions to the propagation and maintenance of MJO MSE anomalies are examined. Also investigated is the relationship between the MJO column water vapor (CWV) budget residuals with the other CWV budget terms as well as with the two parameters that characterize cloud–radiation feedback and moisture–convection coupling. Results show a noticeable inter-RA spread in the mean-state MSE, especially its vertical structure. In all RAs, horizontal MSE advection dominates the propagation of the MJO MSE while column-integrated longwave radiative heating and vertical MSE advection are found to be the key processes for MJO maintenance. The MSE budget terms directly affected by the model parameterization schemes exhibit high uncertainty. The differences in anomalous vertical velocity mainly contribute to the large differences in vertical MSE advection among the RAs. The budget residuals show large inter-RA differences and have nonnegligible contributions to MJO maintenance and propagation in most RAs. RAs that underestimate (overestimate) the strength of cloud–radiation feedback and the convective moisture adjustment time scale tend to have positive (negative) MJO CWV budget residual, indicating the critical role of these processes in the maintenance of MJO CWV anomalies. Our results emphasize that a correct representation of the interactions among moisture, convection, cloud, and radiation is the key for an accurate depiction of the MJO MSE and CWV budget in RAs.

Evidence that Horizontal Moisture Advection Regulates the Ubiquitous Amplification of Rainfall Variability over Tropical Oceans

Abstract The column moist static energy (MSE) budget equation approximates the processes associated with column moistening and drying in the tropics, and is therefore predictive of precipitation amplification and decay. We use ERA-Interim (ERA-I) and TRMM 3B42 data to investigate day-to-day convective variability and distinguish the roles of horizontal MSE (or moisture) advection versus vertical advection, sources, and sinks. Over tropical convergence zones, results suggest that horizontal moisture advection is a primary driver of day-to-day precipitation fluctuations; when drying via horizontal moisture advection is smaller (greater) than Chikira’s “column process,” precipitation tends to amplify (decay). In the absence of horizontal moisture advection, precipitation tends to increase spontaneously almost universally through a positive column process feedback. This bulk positive feedback is characterized by negative effective gross moist stability (GMS), which is maintained throughout the tropical convergence zones. How this positive feedback is achieved varies geographically, depending on the shape of vertical velocity (omega) profiles. In regions where omega profiles are top-heavy, the effective GMS is negative primarily owing to strong feedbacks between convection and diabatic MSE sources (radiative and surface fluxes). In these regions, vertical MSE advection stabilizes the atmosphere (positive vertical GMS). Where omega profiles are bottom-heavy, by contrast, a positive feedback is primarily driven by import of MSE through a shallow circulation (negative vertical GMS). The diabatic feedback and vertical GMS are in a seesaw balance, offsetting one another. Our results suggest that ubiquitous convective variability is amplified by the same mechanism as moisture-mode instability.

Precipitation and the Associated Moist Static Energy Budget off Western Australia in Conjunction with Ningaloo Niño

Ningaloo Nino is a well-known ocean-atmosphere coupled climate event in the southeastern Indian Ocean that interacts with large-scale atmospheric circulations at the interannual time scale. When a Ningaloo Nino develops, remarkably enhanced precipitation anomalies occur primarily off the coast of northwestern Australia (NWA) rather than over the offshore area of western Australia (WA), where the most significant sea surface temperature warming prevails. This enhanced NWA precipitation manifests as jointly intensified stratiform and convective precipitation accompanied by an increase in high cloud cover. Further analyses of the column-integrated moist static energy (MSE) budget reveal that during Ningaloo Nino events, positive vertical MSE transport moistens and heats the atmosphere in the WA region. However, these moistening and heating effects are mostly offset by the inhibitory effect of the negative horizontal MSE advection, leading to the relatively weak and nonsignificant variation in local precipitation anomalies off the WA coastal region. In contrast, off the coast of the NWA region, the recharge of column-integrated MSE, which is induced by the significant positive radiative and surface heating, horizontal MSE advection, and vertical MSE advection, contributes to the heaviest austral summer precipitation associated with Ningaloo Nino. Therefore, the distinct MSE transport processes result in the particular rainfall pattern in which the most enhanced rainfall is associated with Ningaloo Nino events over the NWA region.

Tropical Precipitation Evolution in a Buoyancy-Budget Framework

AbstractObservations have shown that tropical convection is influenced by fluctuations in temperature and moisture in the lower free troposphere (LFT; 600–850 hPa), as well as moist enthalpy (ME) fluctuations beneath the 850 hPa level, referred to as the deep boundary layer (DBL; 850–1000 hPa). A framework is developed that consolidates these three quantities within the context of the buoyancy of an entraining plume. A “plume buoyancy equation” is derived based on a relaxed version of the weak temperature gradient (WTG) approximation. Analysis of this equation using quantities derived from the Dynamics of the Madden–Julian Oscillation (DYNAMO) sounding array data reveals that processes occurring within the DBL and the LFT contribute nearly equally to the evolution of plume buoyancy, indicating that processes that occur in both layers are critical to the evolution of tropical convection. Adiabatic motions play an important role in the evolution of buoyancy both at the daily and longer time scales and are comparable in magnitude to horizontal moisture advection and vertical moist static energy advection by convection. The plume buoyancy equation may explain convective coupling at short time scales in both temperature and moisture fluctuations and can be used to complement the commonly used moist static energy budget, which emphasizes the slower evolution of the convective envelope in tropical motion systems.

Structures and Northward Propagation of the Quasi-Biweekly Oscillation in the Western North Pacific

AbstractThis study investigates the northward-propagating quasi-biweekly oscillation (QBWO) in the western North Pacific by examining the composite meridional structures. Using newly released reanalysis and remote sensing data, the northward propagation is understood in terms of the meridional contrasts in the planetary boundary layer (PBL) moisture and the column-integrated moist static energy (MSE). The meridional contrast in the PBL moisture, with larger values north of the convection center, is predominantly attributed to the moisture convergence associated with barotropic vorticity anomalies. A secondary contribution comes from the meridional moisture advection, for which advections by mean and perturbation winds are almost equally important. The meridional contrast in the MSE tendency, due to the recharge in the front of convection and discharge in the rear of convection, is jointly contributed by the meridional and vertical MSE advections. The meridional MSE advection mainly depends on the moisture processes particularly in the PBL, and the vertical MSE advection largely results from the advection of the mean MSE by vertical velocity anomalies, wherein the upper-troposphere ascending motion related to the stratiform heating in the rear of the convection plays the major role. In addition, partial feedback from sea surface temperature (SST) anomalies is evaluated on the basis of MSE budget analysis. SST anomalies tend to enhance the surface turbulent heat fluxes ahead of the convention center and suppress them behind the convention center, thus positively contributing approximately 20% of the meridional contrast in the MSE tendency.

Diagnosing the column-integrated moist static energy budget associated with the northward-propagating boreal summer intraseasonal oscillation

Column-integrated moist static energy (MSE) budgets associated with the northward-propagating boreal summer intraseasonal oscillation (BSISO) are diagnosed for the Bay of Bangel (BoB) and western North Pacific (WNP) regions. While an active BSISO convection is in phase with a positive column-integrated MSE perturbation, the MSE tendency exhibits a marked north–south asymmetry about the convection, with positive (negative) anomaly to its north (south). An MSE budget diagnosis reveals that the advection of anomalous MSE by the background southwesterly plays a dominant role in causing such an asymmetry and thus promotes the northward propagation, as the anomalous rainband associated with the BSISO has a northwest-southeast tilted structure. The anomalous BSISO wind plays a negative role, because the low-level easterly anomaly to the north of the BSISO convection tends to suppress the surface evaporation through reduced near-surface wind speed, inhibiting the north–south asymmetry. In addition, as maximum background MSE appears over the Asian land region, the advection by the low-level easterly anomaly contributes negatively over the WNP while it has little impact over the BoB. When the BSISO convection is near the equator, a much stronger subsidence anomaly appears to the north than the south due to the equatorial asymmetry of the summer mean state. This leads to a greater MSE tendency north of the equator due to greater anomalous vertical MSE advection, supporting the initiation of the northward propagation.

Effect of vertical moist static energy advection on MJO eastward propagation: sensitivity to analysis domain

Previous studies have shown great uncertainty in assessing the effect of vertical moist static energy (MSE) advection term to the zonal asymmetry of MSE tendency. This study addresses this issue by qualitatively assess the fractional contribution of the vertical MSE advection to the zonal asymmetric pattern of the MSE tendency field, and how its contribution depends on the choice of the analysis domain, based on both observational and numerical simulation results. It is shown that the vertical MSE advection indeed plays a critical role in generating the zonal asymmetry of MSE tendency, accounting for 60% of the total MSE tendency field in observation and even more in aqua-planet simulations. It is indicated that the underestimated contribution from vertical MSE advection by some previous studies is attributed to the unphysical selection of analysis domain for the zonal asymmetric MSE tendency pattern.

Understanding Monsoonal Water Cycle Changes in a Warmer Climate in E3SMv1 Using a Normalized Gross Moist Stability Framework

AbstractOne of the grand challenges of climate science is understanding the changes of the tropical rain belts and monsoon systems owing to CO2‐induced warming. A promising path forward links the fluxes of energy and moisture to tropical circulation features. To this end, we make use of the Energy Exascale Earth System Model version 1, where the divergence of moist static energy and moisture have been calculated online, and employ a normalized gross moist stability (NGMS) diagnostic framework to understand the linkages between changes in the flow of energy and moisture within the monsoons. We focus on the Asian Summer Monsoon system and utilize a series of atmosphere‐land and atmosphere‐land‐ocean simulations to understand the connection between fluxes and monsoons. Uncoupled simulations with prescribed sea surface temperatures indicate that decreases in NGMS over land are important in explaining precipitation increases in response to both sea surface temperature and CO2 increases. In fully coupled experiments, NGMS decreases remain an important contributor to the increase in P‐E, but the coupled simulations highlight the importance of consistent ocean and land responses in interpreting the monsoon changes. This study indicates that transient eddy fluxes play an important role in NGMS decreases and that a time‐mean view of the monsoon circulations is insufficient to quantify the link between future changes in the fluxes of energy and moisture. Compensation between dynamic and thermodynamic components of vertical moist static energy advection occurs, with the thermodynamic contribution dominating. The compensation is shown to be sensitive to relative humidity, with higher relative humidity leading to a stronger thermodynamic component.

A coupled moisture-dynamics model of the Madden–Julian oscillation: convection interaction with first and second baroclinic modes and planetary boundary layer

A theoretical model with a single prognostic variable, column-integrated moist static energy (MSE), was constructed to understand the dynamics of MJO eastward propagation and planetary scale selection. A key process in the model is the interaction of MSE-dependent convection with free-atmospheric first, second baroclinic modes and planetary boundary layer. Under a realistic parameter regime, the model reproduces the most unstable mode at zonal wavenumber 1 with a slow eastward phase speed of about 5 ms−1. The slow eastward phase speed in the model arises from the competition of eastward moving MSE tendencies caused by horizontal MSE advection, vertical MSE advection and boundary layer moistening with westward moving tendencies contributed by surface latent heat flux and atmospheric longwave radiative heating. The planetary scale selection is primarily attributed to the phase lag of longwave radiative heating associated with upper-level stratiform clouds that occur in the rear of MJO deep convection.

Impacts of Vertical Structure of Large-Scale Vertical Motion in Tropical Climate: Moist Static Energy Framework

Abstract Interactions between cumulus convection and its large-scale environment have been recognized as crucial to the understanding of tropical climate and its variability. In this study, the moist static energy (MSE) budget is employed to investigate the potential impact of the vertical structure of large-scale vertical motion in tropical climate based on results from both reanalysis data and model simulation. Two domains are selected over the western and eastern Pacific with vertical motion profiles that are dominated by top-heavy and bottom-heavy structures, respectively. The bottom-heavy structure is climatologically associated with more shallow convection, while the top-heavy structure is related to more deep convection. The column-integrated vertical MSE advection of top-heavy vertical motion is positive, while that of bottom-heavy vertical motion tends to be negative. Controlling factors responsible for the above vertical MSE advection contrast are discussed based on a simple decomposition of the MSE budget equation. It was found that the sign of vertical MSE advection is determined mainly by the vertical moisture transport, the magnitude of which is very sensitive to the structure of vertical motion. A top-heavy (bottom heavy) structure of vertical motion favors an export (import) of MSE and a positive (negative) value of the vertical MSE advection.

Response of the Superparameterized Madden–Julian Oscillation to Extreme Climate and Basic-State Variation Challenges a Moisture Mode View

Abstract The climate sensitivity of the Madden–Julian oscillation (MJO) is measured across a broad range of temperatures (1°–35°C) using a convection-permitting global climate model with homogenous sea surface temperatures. An MJO-like signal is found to be resilient in all simulations. These results are used to investigate two ideas related to the modern “moisture mode” view of MJO dynamics. The first hypothesis is that the MJO has dynamics analogous to a form of radiative convective self-aggregation in which longwave energy maintenance mechanisms shut down for SST ≪ 25°C. Inconsistent with this hypothesis, the explicitly simulated MJO survives cooling and retains leading moist static energy (MSE) budget terms associated with longwave destabilization even at SST < 10°C. Thus, if the MJO is a form of longwave-assisted self-aggregation, it is not one that is temperature critical, as is observed in some cases of radiative–convective equilibrium (RCE) self-aggregation. The second hypothesis is that the MJO is propagated by horizontal advection of column MSE. Inconsistent with this view, the simulated MJO survives reversal of meridional moisture gradients in the basic state and a striking role for horizontal MSE advection in its propagation energy budget cannot be detected. Rather, its eastward motion is balanced by vertical MSE advection reminiscent of gravity or Kelvin wave dynamics. These findings could suggest a tight relation between the MJO and classic equatorial waves, which would tend to challenge moisture mode views of MJO dynamics that assume horizontal moisture advection as the MJO’s propagator. The simulation suite provides new opportunities for testing predictions from MJO theory across a broad climate regime.

Gross Moist Stability Assessment during TOGA COARE: Various Interpretations of Gross Moist Stability

Abstract Daily averaged TOGA COARE data are analyzed to investigate the convective amplification/decay mechanisms. The gross moist stability (GMS), which represents moist static energy (MSE) export efficiency by large-scale circulations associated with the convection, is studied together with two quantities, called the critical GMS (a ratio of diabatic forcing to the convective intensity) and the drying efficiency [a version of the effective GMS (GMS minus critical GMS)]. The analyses reveal that convection intensifies (decays) via negative (positive) drying efficiency. The authors illustrate that variability of the drying efficiency during the convective amplifying phase is predominantly explained by the vertical MSE advection (or vertical GMS), which imports MSE via bottom-heavy vertical velocity profiles (associated with negative vertical GMS) and eventually starts exporting MSE via top-heavy profiles (associated with positive vertical GMS). The variability of the drying efficiency during the decaying phase is, in contrast, explained by the horizontal MSE advection. The critical GMS, which is moistening efficiency due to the diabatic forcing, is broadly constant throughout the convective life cycle, indicating that the diabatic forcing always tends to destabilize the convective system in a constant manner. The authors propose various ways of computing quasi-time-independent “characteristic GMS” and demonstrate that all of them are equivalent and can be interpreted as (i) the critical GMS, (ii) the GMS at the maximum precipitation, and (iii) a combination of feedback constants between the radiation, evaporation, and convection. Those interpretations indicate that each convective life cycle is a fluctuation of rapidly changing GMS around slowly changing characteristic GMS.

The moist static energy budget in NCAR CAM5 hindcasts during DYNAMO

AbstractThe Dynamics of the MJO (DYNAMO) field campaign took place in the Indian Ocean during boreal fall and winter of 2011–2012 to collect observations of Madden‐Julian Oscillation (MJO) initiation. Hindcast experiments are conducted with an atmospheric general circulation model with varying values of a dilute CAPE entrainment rate parameter for the first two MJO events of DYNAMO from 1 October 2011 to 15 December 2011. Higher entrainment rates better reproduce MJO precipitation and zonal wind, with RMM skill up to 20 days. Simulations with lower entrainment rapidly diverge from observations with no coherent MJO convective signal after 5 days, and no MJO predictive skill beyond 12 days. Analysis of the tropical Indian Ocean column moist static energy (MSE) budget reveals that the simulations with superior MJO performance exhibit a mean positive MSE tendency by vertical advection; inconsistent with reanalysis that indicates a weak negative tendency. All simulations have weaker mean MSE source tendency and significantly weaker cloud‐radiative feedbacks. The vertical gross moist stability (VGMS) is used to interpret these MSE budget results in a normalized framework relevant to moisture mode theory. VGMS in the high entrainment runs is far too low compared to ERAi, indicating that it cannot be used in isolation as a measure of model success in producing a realistic MJO hindcast, contrary to previous studies. However, effective VGMS that includes radiative feedbacks is similar among the high entrainment runs and ERAi. We conclude that the MJO is erroneously improved by increasing the entrainment parameter because moistening by vertical MSE advection compensates for the overly weak cloud‐radiative feedbacks.

Geographic variability in the export of moist static energy and vertical motion profiles in the tropical Pacific

Column‐integrated moist static energy (MSE) budgets were calculated using daily data from three reanalyses (1998–2001) and ISCCP radiative cooling. The MSE export by the circulation was separated into vertical and horizontal MSE advection components in order to examine how observationally‐derived vertical motion profiles affect gross moist stability calculations. In a 2000 km‐wide region in the central‐eastern Pacific ITCZ, vertical motions import MSE, implying a negative gross moist stability. Horizontal MSE advection is negative and of similar magnitude to vertical MSE advection in rainy regions. Geographic differences in export of MSE by the vertical circulation are primarily due to differences in the shape of the vertical motion profile. In the west Pacific warm pool rainy regions, mean horizontal convergence extends up to 300 mb, while in parts of the Pacific ITCZ where meridional SST gradients are strong, the vertical motion profile is bottom‐heavy, with convergence below 800 mb and divergence above. On daily timescales, the mode of vertical motion variability has little dependence on precipitation rate and is consistent with observed regional differences in the vertical structure of TRMM PR reflectivities.