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Alison M. Anders
and
Stephen W. Nesbitt

Abstract

A Tropical Rainfall Measuring Mission (TRMM) climatology shows variability in surface precipitation rate–elevation relationships across the tropics. Vertical profiles of radar reflectivity and profiles of specific humidity and cross-barrier moisture fluxes during precipitation events from the Interim European Centre for Medium-Range Weather Forecasts Re-Analysis reveal four precipitation regimes with distinct precipitation mechanisms: 1) a tropical regime with a broad precipitation maximum at ~1500 m where convection is triggered by orographic lifting; 2) a trade winds regime with a near–sea level precipitation maximum dominated by forced ascent due to prevailing winds and the presence of dry air aloft; 3) a wet monsoon regime with a low-elevation precipitation maximum driven by efficient precipitation generation, large low-level cross-barrier moisture fluxes, and multiple convective modes; and 4) a dry monsoon regime with a high-elevation precipitation maximum reflecting intense convection and stratiform rain with a strong evaporation signature. In general, surface precipitation–elevation relationships across the tropics feature lower-elevation precipitation maxima relative to typical midlatitude regimes.

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Nicole J. Schiffer
and
Stephen W. Nesbitt

Abstract

This study uses an improved surge identification method to examine composites of 29 yr of surface observations and reanalysis data alongside 10 yr of satellite precipitation data to reveal connections between flow, thermodynamic parameters, and precipitation, both within and outside of the North American monsoon (NAM) region, associated with Gulf of California (GoC) moisture surges. The North American Regional Reanalysis (NARR), examined using composites of flow during all detected moisture surges at Yuma, Arizona, and so-called wet and dry surges (those producing anomalously high and low precipitation, respectively, over Arizona and New Mexico), show markedly different flow and moisture patterns that ultimately lead to the differing observed precipitation distributions in the region. Wet surges tend to be associated with moister precursor air masses over the southwestern United States, have a larger contribution of enhanced easterly cross–Sierra Madre Occidental (SMO) moisture transport, and tend to result from a transient cyclonic disturbance tracking across northern Mexico. Dry surges tend to be associated with a more southerly tracking disturbance, are associated with less convection over the SMO, and tend to be associated with a drier presurge air mass over Arizona and New Mexico.

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Stephen W. Nesbitt
and
Edward J. Zipser

Abstract

The Tropical Rainfall Measuring Mission (TRMM) satellite measurements from the precipitation radar and TRMM microwave imager have been combined to yield a comprehensive 3-yr database of precipitation features (PFs) throughout the global Tropics (±36° latitude). The PFs retrieved using this algorithm (which number nearly six million Tropicswide) have been sorted by size and intensity ranging from small shallow features greater than 75 km2 in area to large mesoscale convective systems (MCSs) according to their radar and ice scattering characteristics. This study presents a comprehensive analysis of the diurnal cycle of the observed precipitation features' rainfall amount, precipitation feature frequency, rainfall intensity, convective–stratiform rainfall portioning, and remotely sensed convective intensity, sampled Tropicswide from space.

The observations are sorted regionally to examine the stark differences in the diurnal cycle of rainfall and convective intensity over land and ocean areas. Over the oceans, the diurnal cycle of rainfall has small amplitude, with the maximum contribution to rainfall coming from MCSs in the early morning. This increased contribution is due to an increased number of MCSs in the nighttime hours, not increasing MCS areas or conditional rain rates, in agreement with previous works. Rainfall from sub-MCS features over the ocean has little appreciable diurnal cycle of rainfall or convective intensity. Land areas have a much larger rainfall cycle than over the ocean, with a marked minimum in the midmorning hours and a maximum in the afternoon, slowly decreasing through midnight. Non-MCS features have a significant peak in afternoon instantaneous conditional rain rates (the mean rain rate in raining pixels), and convective intensities, which differs from previous studies using rain rates derived from hourly rain gauges. This is attributed to enhancement by afternoon heating. MCSs over land have a convective intensity peak in the late afternoon, however all land regions have MCS rainfall peaks that occur in the late evening through midnight due to their longer life cycle. The diurnal cycle of overland MCS rainfall and convective intensity varies significantly among land regions, attributed to MCS sensitivity to the varying environmental conditions in which they occur.

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Daniel S. Harnos
and
Stephen W. Nesbitt

Abstract

Characteristics of over 15 000 tropical cyclone (TC) inner cores are evaluated coincidentally using 37- and 85-GHz passive microwave data to quantify the relative prevalence of cold clouds (i.e., deep convection and stratiform clouds) versus predominantly warm clouds (i.e., shallow cumuli and cumulus congestus). Results indicate greater presence of combined liquid and frozen hydrometeors associated with cold clouds within the atmospheric column for TCs undergoing subsequent rapid intensification (RI) or intensification. RI episodes compared to the full intensity change distribution exhibit approximately an order of magnitude increase for inner-core cold cloud frequency relative to warm cloud presence. Incorporation of an objective ring detection algorithm shows the robust presence of rings associated with hydrometeors for 85-GHz polarization corrected temperatures ( ) and 37-GHz vertically polarized brightness temperatures ( ) for differentiating RI with significance levels ≥99.99%, while 37-GHz false color rings of a combined cyan and pink appearance surrounding a region that is not cyan or pink lack statistical significance for discriminating RI against lesser intensification. Rings of depressed and enhanced tied to RI suggest the combined presence of liquid and frozen hydrometeors within the atmospheric column, indicative of cold clouds. The rings also exhibit preferences for those with collocated more widespread ice scattering signatures to be more commonly associated with RI and general intensification.

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Stephen W. Nesbitt
,
David J. Gochis
, and
Timothy J. Lang

Abstract

This study examines the spatial and temporal variability in the diurnal cycle of clouds and precipitation tied to topography within the North American Monsoon Experiment (NAME) tier-I domain during the 2004 NAME enhanced observing period (EOP, July–August), with a focus on the implications for high-resolution precipitation estimation within the core of the monsoon. Ground-based precipitation retrievals from the NAME Event Rain Gauge Network (NERN) and Colorado State University–National Center for Atmospheric Research (CSU–NCAR) version 2 radar composites over the southern NAME tier-I domain are compared with satellite rainfall estimates from the NOAA Climate Prediction Center Morphing technique (CMORPH) and Precipitation Estimation from Remotely Sensed Information Using Artificial Neural Networks (PERSIANN) operational and Tropical Rainfall Measuring Mission (TRMM) 3B42 research satellite estimates along the western slopes of the Sierra Madre Occidental (SMO). The rainfall estimates are examined alongside hourly images of high-resolution Geostationary Operational Environmental Satellite (GOES) 11-μm brightness temperatures.

An abrupt shallow to deep convective transition is found over the SMO, with the development of shallow convective systems just before noon on average over the SMO high peaks, with deep convection not developing until after 1500 local time on the SMO western slopes. This transition is shown to be contemporaneous with a relative underestimation (overestimation) of precipitation during the period of shallow (deep) convection from both IR and microwave precipitation algorithms due to changes in the depth and vigor of shallow clouds and mixed-phase cloud depths. This characteristic life cycle in cloud structure and microphysics has important implications for ice-scattering microwave and infrared precipitation estimates, and thus hydrological applications using high-resolution precipitation data, as well as the study of the dynamics of convective systems in complex terrain.

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Daniel J. Cecil
,
Edward J. Zipser
, and
Stephen W. Nesbitt

Abstract

Covering December 1997 through December 1998, 261 overpasses of 45 hurricanes by the Tropical Rainfall Measuring Mission (TRMM) satellite are used to document the observed radar reflectivity values, passive microwave ice scattering magnitudes, and total lightning (cloud to ground plus in cloud). These parameters are interpreted as describing convective vigor or intensity, with greater reflectivities (particularly aloft), greater ice scattering (lower 85- and 37-GHz brightness temperatures), and increased lightning frequency indicating more intense convection. For each parameter, the full distribution of values observed during the TRMM satellite's first year is presented for specific regions. Properties of three regions of the hurricane (eyewall, inner rainband, and outer rainband) are treated separately and compared to other tropical oceanic and tropical continental precipitation systems. Reflectivity profiles and ice scattering signatures are found to be fairly similar for both hurricane and nonhurricane tropical oceanic precipitation systems, although the hurricane inner rainband region yields the weakest of these convective signatures. When normalized by the area experiencing significant convection, the outer rainband region produces more lightning than the rest of the hurricane or nonhurricane tropical oceanic systems. As a whole, the tropical oceanic precipitation systems (both hurricane and nonhurricane) are dominated by stratiform rain and relatively weak convection.

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Stephen W. Nesbitt
,
Edward J. Zipser
, and
Christian D. Kummerow

Abstract

An evaluation of the version-5 precipitation radar (PR; algorithm 2A25) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI; algorithm 2A12) rainfall products is performed across the Tropics in two ways: 1) by comparing long-term TRMM rainfall products with Global Precipitation Climatology Centre (GPCC) global rain gauge analyses and 2) by comparing the rainfall estimates from the PR and TMI on a rainfall feature-by-feature basis within the narrow swath of the PR using a 1-yr database of classified precipitation features (PFs). The former is done to evaluate the overall biases of the TMI and PR relative to “ground truth” to examine regional differences in the estimates; the latter allows a direct comparison of the estimates with the same sampling area, also identifying relative biases as a function of storm type. This study finds that the TMI overestimates rainfall in most of the deep Tropics and midlatitude warm seasons over land with respect to both the GPCC gauge analysis and the PR (which agrees well with the GPCC gauges in the deep Tropics globally), in agreement with past results. The PR is generally higher than the TMI in midlatitude cold seasons over land areas with gauges. The analysis by feature type reveals that the TMI overestimates relative to the PR are due to overestimates in mesoscale convective systems and in most features with 85-GHz polarization-corrected temperature of less than 250 K (i.e., with a significant optical depth of precipitation ice). The PR tended to be higher in PFs without an ice-scattering signature of less than 250 K. Normalized for a subset of features with a large rain volume (exceeding 104 mm h−1 km2) independent of the PF classification, features with TMI > PR in the Tropics tended to have a higher fraction of stratiform rainfall, higher IR cloud tops, more intense radar profiles and 85-GHz ice-scattering signatures, and larger rain areas, whereas the converse is generally true for features with PR > TMI. Subtropical-area PF bias characteristics tended not to have such a clear relationship (especially over the ocean), a result that is hypothesized to be due to the influence of more variable storm environments and the presence of frontal rain. Melting-layer effects in stratiform rain and a bias in the ice-scattering–rain relationship were linked to the TMI producing more rainfall than the PR. However, noting the distinct characteristic biases Tropics-wide by feature type, this study reveals that accounting for regime-dependent biases caused by the differing horizontal and vertical morphologies of precipitating systems may lead to a reduction in systematic relative biases in a microwave precipitation algorithm.

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Ole Peters
,
J. David Neelin
, and
Stephen W. Nesbitt

Abstract

Size distributions and other geometric properties of mesoscale convective systems (MCSs), identified as clusters of adjacent pixels exceeding a precipitation threshold in satellite radar images, are examined with respect to a recently identified critical range of water vapor. Satellite microwave estimates of column water vapor and precipitation show that the onset of convection and precipitation in the tropics can be described as a phase transition, where the rain rate and likelihood of rainfall suddenly increase as a function of water vapor. This is confirmed in Tropical Rainfall Measuring Mission radar data used here. Percolation theory suggests that cluster properties should be highly sensitive to changes in the density of occupied pixels, which here translates into a rainfall probability, which in turn sensitively depends on the water vapor. To confirm this, clusters are categorized by their prevalent water vapor. As expected, mean cluster size and radius of gyration strongly increase as the critical water vapor is approached from below. In the critical region one finds scale-free size distributions spanning several orders of magnitude. Large clusters are typically from the critical region: at low water vapor most clusters are small, and supercritical water vapor values are too rare to contribute much. The perimeter of the clusters confirms previous observations in satellite, field, and model data of robust nontrivial scaling. The well-known area–perimeter scaling is fully compatible with the quantitative prediction from the plausible null model of gradient percolation, where the accessible hull is a fractal object with dimension 4/3.

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Chuntao Liu
,
Edward J. Zipser
, and
Stephen W. Nesbitt

Abstract

Cold cloud features (CCFs) are defined by grouping six full years of Tropical Rainfall Measuring Mission (TRMM) infrared pixels with brightness temperature at 10.8-μm wavelength (T B11) less than or equal to 210 and 235 K. Then the precipitation radar (PR)-observed precipitation area and reflectivity profiles inside CCFs are summarized and compared with the area and minimum temperature of the CCFs. Comparing the radar with the infrared data, significant regional differences are found, quantified, and used to describe regional differences in selected properties of deep convective systems in the Tropics. Inside 4 million CCFs, 35% (57%) of cold cloud area with T B11 ≤ 235 K (210 K) have rain detected by the PR near the surface. Only ∼1% of the area of TB11 ≤ 210 K have 20 dBZ reaching 14 km. CCFs colder than 210 K occur most frequently over the west Pacific Ocean, but 20-dBZ echoes extending above 10 km in this region are disproportionately rare by comparison to many continental regions. Ratios of PR-detected raining area to area of T B11 ≤ 235 K are higher over central Africa, Argentina, and India than over tropical oceans. After applying these ratios to the climatological Global Precipitation Index (GPI) tropical rainfall estimates, the regional distribution is more consistent with the rainfall retrieval from the PR. This suggests that the discrepancy between GPI- and PR-retrieved rainfall can be partly explained with the nonraining anvil. Categorization of CCFs based on the minimum T B11, size of CCFs, and 20-dBZ heights demonstrates that 20-dBZ echoes above 17 km occur most frequently over land, while the coldest clouds occur most frequently over the west Pacific. The vertical distances between the cloud-top heights determined from T B11 and PR 20-dBZ echo-top heights are smaller over land than over ocean and may be considered as another proxy for convective intensity.

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Robert J. Trapp
,
Geoffrey R. Marion
, and
Stephen W. Nesbitt
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