Abstract

This study compares instantaneous rainfall estimates provided by the current generation of retrieval algorithms for passive microwave sensors using retrievals from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and merged surface radar and gauge measurements over the continental United States as references. The goal is to quantitatively assess surface rain retrievals from cross-track scanning microwave humidity sounders relative to those from conically scanning microwave imagers. The passive microwave sensors included in the study are three operational sounders—the Advanced Microwave Sounding Unit-B (AMSU-B) instruments on the NOAA-15, -16, and -17 satellites—and five imagers: the TRMM Microwave Imager (TMI), the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) instrument on the Aqua satellite, and the Special Sensor Microwave Imager (SSM/I) instruments on the Defense Meteorological Satellite Program (DMSP) F-13, -14, and -15 satellites. The comparisons with PR data are based on “coincident” observations, defined as instantaneous retrievals (spatially averaged to 0.25° latitude and 0.25° longitude) within a 10-min interval collected over a 20-month period from January 2005 to August 2006. Statistics of departures of these coincident retrievals from reference measurements as given by the TRMM PR or ground radar and gauges are computed as a function of rain intensity over land and oceans. Results show that over land AMSU-B sounder rain retrievals are comparable in quality to those from conically scanning radiometers for instantaneous rain rates between 1.0 and 10.0 mm h⁻¹. This result holds true for comparisons using either TRMM PR estimates over tropical land areas or merged ground radar/gauge measurements over the continental United States as the reference. Over tropical oceans, the standard deviation errors are comparable between imager and sounder retrievals for rain intensities above 5 mm h⁻¹, below which the imagers are noticeably better than the sounders; systematic biases are small for both imagers and sounders. The results of this study suggest that in planning future satellite missions for global precipitation measurement, cross-track scanning microwave humidity sounders on operational satellites may be used to augment conically scanning microwave radiometers to provide improved temporal sampling over land without degradation in the quality of precipitation estimates.

Abstract

Rawinsonde and satellite infrared radiation (IR) data from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate mean and transient behavior and horizontal variability of the atmosphere over the western Pacific warm pool. Infrared data for the 4-mo Intensive Observing Period (IOP) and vertical motion fields indicate that the intensity of convection, height of maximum upward motion, and SST all increased from west (140°E) to east across the COARE domain. IOP-mean IR data show a double ITCZ (Intertropical Convergence Zone) structure north and south of the Intensive Flux Array (IFA, centered at 2°S, 156°E), although marked variability in the patterns occurred on a month to month basis.

Three prominent westerly wind bursts occurred over the IFA during the 4-mo IOP in association with the intraseasonal oscillations (ISOs). Strong upward motion usually occurred 1–3 weeks prior to the peak low-level westerlies. Subsidence dominated when the westerly winds prevailed. COARE data reveal that the vertical wind shear (more than 50 m s⁻¹ from 850 to 100 hPa) and the vertical extent of westerlies during the peak westerly wind bursts were far greater than previously recognized. The mean low-level equatorial flow over the western Pacific was westerly, interrupted occasionally by brief periods of easterly flow. The perturbation westerlies to the west of the disturbance associated with the ISO were usually stronger than the perturbation winds to the east. Maximum surface latent beat flux usually occurred during the peak westerlies, whereas the surface sensible heat flux peaked prior to the strongest westerlies.

The IOP-mean divergence profile over the IFA shows a very weak divergence near the surface and weak convergence at middle and low levels. The ITCZ-band divergence profiles show strong low-level convergence from the surface to about 700 hPa. The striking difference between the divergence profiles along the equator over the IFA and those north and south in the ITCZ bands suggests that, although the divergence and vertical motion profiles tend to look alike whenever and wherever the convection is strong, great care should be exercised in generalizing divergence and vertical motion profiles from one region to another over the western Pacific warm pool.

Correlations between cold clouds and vertical motion indicate that cold clouds are a good indicator of upper-level upward vertical motion but not low-level vertical motion. In a significant number of cases, low-level downward motion occurred under very cold cloud tops over the warm pool, indicating extensive optically thick anvil cloud and nonprecipitating high cirrus are a common occurrence over the warm pool.

The IOP-mean relative humidity profile over the IFA shows a primary peak at low levels at the top of the mixed layer and a secondary peak near 550 hPa (near the 0°C level). The secondary peak is not present in either ECMWF or NMC operational analyses, and the midtroposphere is much drier in the two model-assimilated results.

A synthesis of the kinematic and thermodynamic characteristics of the December–early January westerly wind burst as it passed the IFA is presented.

Abstract

Heat and moisture budgets and mesoscale circulation features for the developing, mature, and dissipating stages of an intense frontal squall line that occurred in the central United States are investigated. The slow propagating behavior of the squall line made the dataset unique since observations covered a large fraction of the squall line life cycle. Budgets have been performed at six different times at intervals of 90 minutes using 1985 OK PRE-STORM rawinsonde data.

The squall line was followed by a low-level cold front. The flow pattern normal to the squall line was generally similar to previous squall line studies except that a low-level rear inflow associated with the cold front was superimposed upon expected squall line FTR/RTF (front to rear/rear to front) flows. The midlevel RTF flow was quite weak well behind the squall line during the developing and mature stages and significantly strengthened during the dissipating stage as the stratiform region developed, suggesting that internal processes within the expanding stratiform region played an important role in RTF flow development.

A convergence band resulting from system RTF and FTR flows extended upward and rearward from low levels near the leading edge of the system. During the developing and mature stages, peak convergence was located at low levels around the leading edge. At the dissipating stage, midlevel convergence behind the convective region intensified as the stratiform region developed, while low-level convergence near the leading edge gradually weakened.

Both the apparent heat source Q ₁, and apparent moisture sink Q ₂ showed an increasing upshear tilt when the stratiform region developed, as did the vertical velocity field. The system-averaged heating peak Q ₁ was located at middle levels between 500 and 550 hPa throughout the evolution. The moisture sink Q ₂ exhibited a single drying peak, which resulted from the convective region, at low levels around 700 hPa through most of the developing and early mature stages. During the late mature and dissipating stages, a double-peak structure in Q ₂ become very pronounced. The coexistence of convective and stratiform drying appears to be the causal mechanism for the double peak in Q ₂ at these stages. At later stages, a single drying peak resulting from the stratiform region was present at middle levels around 475 pHa.

Abstract

Rawinsonde, surface, and satellite data collected from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate the distributions of beating, moistening, precipitation, and evaporation over the western Pacific warm pool. The behavior of the atmosphere and the response of the ocean surface before, during, and after the passages of westerly wind bursts are examined. The tropospheric vertical wind shear associated with tropical low-frequency oscillations strongly modulated convective beating and moistening. Heavy precipitation usually fell 1 to 3 weeks prior to the peak westerly wind bursts. SSTs reached their maximum during the undisturbed phase of the intraseasonal oscillations (ISOs), gradually decreased as convection intensified, and reached their minimum during the periods of peak westerly winds when deep convection was generally suppressed over the intensive flux array (IFA).

Surface latent heat fluxes were positively correlated with surface wind speed and varied between 50 and 100 W m⁻² during light winds to more than 200 W m⁻² during strong westerly wind bursts. Surface sensible heat fluxes, however, did not follow the pattern of surface wind speed and usually peaked during organized deep convection over the IFA.

Intensive observing period (IOP)-mean evaporation and sensible heat flux over the large-scale array (LSA) were obtained by adjusting the European Centre for Medium-Range Weather Forecasts (ECMWF) fields toward buoy estimates over the IFA. These values were then used to determine IOP-mean rainfall distribution over the LSA from the moisture budget. The results generally compare well with two satellite remote-sensing estimations, SSM/I retrievals and GPI, and the ECMWF model forecast. All four methods indicate an east-west zone of minimal rainfall along the equator. Two heavy rainfall bands coinciding with the double-ITCZ structure were located north and south of the equator. The IFA was mainly located within the minimum rainfall band. Budget-diagnosed rainfall rates over the IFA agree with SSM/I retrievals, but the GPI values are excessive, particularly at times of extensive cirrus.

Comparison of the apparent heat source Q ₁ profiles suggests that the long-term mean heating rates over the warm pool have large positive values at all levels of the atmosphere and peak between 400 and 450 hPa. In contrast, the apparent moisture sink Q ₂ profiles show distinctly different features over the warm pool. Positive values (indicating drying) exist in the ITCZ bands north and south of the IFA. However, low-level moistening is evident in the profiles over the IFA, probably due to strong evaporation and upward transport of moisture by shallow cumuli during high winds.

Abstract

The western Pacific warm pool experiences the greatest rainfall of any oceanic region on earth. While the SST is everywhere high over the warm pool, there is great spatial and temporal variability in rainfall. Sounding data from the recent TOGA COARE are used to document this variability. In particular, the vertical distributions of heating and moistening at different phases of the 30–60 day or intraseasonal oscillation are determined for different areas within the warm pool.

While heating and moistening distributions near the equator over the warm pool are often similar to those observed over the western Pacific within the convectively active ITCZ, these profiles do not prevail at all times. In particular, during westerly wind bursts and suppressed, light-wind periods, heating and moistening distributions over the COARE Intensive Flux Array frequently resemble those observed in the trade wind belts. Such profiles are characterized by relatively large negative values of apparent moisture sink Q ₂ in the lowest 2–3 km, reflecting the important moistening effects of shallow, mostly nonprecipitating cumulus clouds. A maximum in moistening commonly occurs in the lower part of the cloud layer during the westerly wind bursts, indicating many “forced” cumuli that are extensions of boundary-layer turbulence. During suppressed, light-wind periods the moistening peak shifts to the upper part of the cloud layer, indicating a larger proportion of “active” trade cumulus at that time (consistent with higher SSTs and weaker vertical wind shear).

A spectral cloud diagnostic model is used to determine the properties of the shallow cloud fields. Computed profiles of mass fluxes, entrainment, detrainment, and heat and moisture balances during these periods generally resemble those for the western Atlantic trade wind regime. These findings lead to the conclusion that, in association with the intraseasonal oscillation, the western Pacific warm pool boundary layer periodically develops trade-wind-like characteristics with abundant shallow cumulus, and these episodic “tradelike” regimes are frequent enough to impact the seasonal-mean distributions of boundary layer heating and moistening.

Abstract

A high-resolution surface rainfall product is used to estimate rain characteristics over the continental United States as a function of rain intensity. By defining data at 4-km horizontal resolutions and 1-h temporal resolutions as an individual precipitating or nonprecipitating sample, statistics of rain occurrence and rain volume including their geographical and seasonal variations are documented. Quantitative estimations are also conducted to evaluate the impact of missing light rain events due to satellite sensors’ detection capabilities.

It is found that statistics of rain characteristics have large seasonal and geographical variations across the continental United States. Although heavy rain events (>10 mm h⁻¹) only occupy 2.6% of total rain occurrence, they may contribute to 27% of total rain volume. Light rain events (<1.0 mm h⁻¹), occurring much more frequently (65%) than heavy rain events, can also make important contributions (15%) to the total rain volume.

For minimum detectable rain rates setting at 0.5 and 0.2 mm h⁻¹, which are close to sensitivities of the current and future spaceborne precipitation radars, there are about 43% and 11% of total rain occurrence below these thresholds, and they respectively represent 7% and 0.8% of total rain volume. For passive microwave sensors with their rain pixel sizes ranging from 14 to 16 km and the minimum detectable rain rates around 1 mm h⁻¹, the missed light rain events may account for 70% of rain occurrence and 16% of rain volume.

Statistics of rain characteristics are also examined on domains with different temporal and spatial resolutions. Current issues in estimates of rain characteristics from satellite measurements and model outputs are discussed.

Abstract

The radius of maximum wind (RMW) has been found to contract rapidly well preceding rapid intensification in tropical cyclones (TCs) in recent literature, but the understanding of the involved dynamics is incomplete. In this study, this phenomenon is revisited based on ensemble axisymmetric numerical simulations. Consistent with previous studies, because the absolute angular momentum (AAM) is not conserved following the RMW, the phenomenon cannot be understood based on the AAM-based dynamics. Both budgets of tangential wind and the rate of change in the RMW are shown to provide dynamical insights into the simulated relationship between the rapid intensification and rapid RMW contraction. During the rapid RMW contraction stage, due to the weak TC intensity and large RMW, the moderate negative radial gradient of radial vorticity flux and small curvature of the radial distribution of tangential wind near the RMW favor rapid RMW contraction but weak diabatic heating far inside the RMW leads to weak low-level inflow and small radial absolute vorticity flux near the RMW and thus a relatively small intensification rate. As RMW contraction continues and TC intensity increases, diabatic heating inside the RMW and radial inflow near the RMW increase, leading to a substantial increase in radial absolute vorticity flux near the RMW and thus the rapid TC intensification. However, the RMW contraction rate decreases rapidly due to the rapid increase in the curvature of the radial distribution of tangential wind near the RMW as the TC intensifies rapidly and RMW decreases.

Abstract

This study attempts to explore a comprehensive and compact approach for delineating the multiscale and multivariate characteristics of the ITCZ over the western–central Pacific based on daily satellite observations of precipitation, SSTs, and surface winds. Essentially six distinct ITCZ spatial patterns—namely, the north, south, equator, double, full, and weak—are identified according to the daily percentage coverage of deep convection within different latitudinal bands on and off the equator over the western–central Pacific. The evolving structure of the ITCZ over the western–central Pacific is investigated with a focus on the transient statistical characteristics. The relationship between these daily ITCZ patterns and SSTs, and near-surface winds, is also examined.

The north (37%), south (24%), and weak (24%) ITCZs represent the three major ITCZ daily patterns over the western–central Pacific, and combined they account for almost 85% of the total number of days within a 10-yr period. The other three ITCZ patterns, namely, the equator (3%), double (6%), and full (5%) ITCZs, occur infrequently. The climatology of the ITCZ, such as monthly, seasonal, and annual means, can be approximately determined by how often and intense these ITCZ daily spatial patterns occur within a specified period. Taking the long-term mean statistics for each ITCZ daily type into account, the double ITCZ deep convection typically observed over the western–central Pacific in monthly, seasonal, and annual mean plots appears to be mainly associated with the frequent occurrence of the north and south ITCZ patterns, instead of the double ITCZ pattern in which an ITCZ appears on each side of the equator simultaneously on a daily basis.

Consistent with the strong seasonality in their frequency of occurrence, the three major ITCZ patterns indicate a dominant correspondence with the seasonal meridional migration of warm SSTs. In contrast, the three less frequent ITCZ patterns show a close relationship with the positive or negative SST anomaly over the equatorial central and eastern Pacific, namely, the extension and retraction of the equatorial cool tongue and its strength. Surface wind divergence/convergence does not show any distinct patterns for these ITCZ spatial patterns, suggesting that little relationship between low-level convergence and precipitation can be discerned from daily data.

As an application of the method proposed, the frequency distribution of daily ITCZ patterns, as derived from a recent version of the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) general circulation model (GCM), is evaluated against observations. Preliminary comparisons indicate that the GEOS-5 GCM is capable of simulating the correct ITCZ spatial patterns, but their occurrence frequencies can be further improved, in particular, the weak ITCZ and the patterns with fewer occurrences, which may be associated with significantly different control mechanisms and/or feedbacks.

Abstract

Global microwave rainfall retrievals from a five-satellite constellation, including the Tropical Rainfall Measuring Mission Microwave Imager, Special Sensor Microwave Imager from the Defense Meteorological Satellite Program F13, F14, and F15, and the Advanced Microwave Scanning Radiometer from the Earth Observing System Aqua, are assimilated into the NASA Goddard Earth Observing System (GEOS) Data Assimilation System using a 1D variational continuous assimilation (VCA) algorithm. The physical and dynamical impact of rainfall assimilation on GEOS analyses is examined at various temporal and spatial scales. This study demonstrates that the 1D VCA algorithm, which was originally developed and evaluated for rainfall assimilations over tropical oceans, can effectively assimilate satellite microwave rainfall retrievals and improve GEOS analyses over both the Tropics and the extratropics where the atmospheric processes are dominated by different large-scale dynamics and moist physics, and also over land, where rainfall estimates from passive microwave radiometers are believed to be less accurate. Results show that rainfall assimilation renders the GEOS analysis physically and dynamically more consistent with the observed precipitation at the monthly mean and 6-h time scales. Over regions where the model precipitation tends to misbehave in distinctly different rainy regimes, the 1D VCA algorithm, by compensating for errors in the model’s moist time tendency in a 6-h analysis window, is able to bring the rainfall analysis closer to the observed. The radiation and cloud fields also tend to be in better agreement with independent satellite observations in the rainfall–assimilation run especially over regions where rainfall analyses indicate large improvements. Assimilation experiments with and without rainfall data for a midlatitude frontal system clearly indicate that the GEOS analysis is improved through changes in the thermodynamic and dynamic fields that respond to the rainfall assimilation. The synoptic structures of temperature, moisture, winds, divergence, and vertical motion, as well as vorticity, are more realistically captured across the front.

Abstract

The simulated diurnal cycle is in many ways an ideal test bed for new physical parameterizations. The purpose of this paper is to compare observations from the Tropical Rainfall Measurement Mission, the Earth Radiation Budget Experiment, the International Satellite Cloud Climatology Project, the Clouds and the Earth’s Radiant Energy System Experiment, and the Anglo-Brazilian Amazonian Climate Observation Study with the diurnal variability of the Amazonian hydrologic cycle and radiative energy budget as simulated by the Colorado State University general circulation model, and to evaluate improvements and deficiencies of the model physics. The model uses a prognostic cumulus kinetic energy (CKE) to relax the quasi-equilibrium closure of the Arakawa–Schubert cumulus parameterization. A parameter, α, is used to relate the CKE to the cumulus mass flux. This parameter is expected to vary with cloud depth, mean shear, and the level of convective activity, but up to now a single constant value for all cloud types has been used. The results of the present study show clearly that this approach cannot yield realistic simulations of both the diurnal cycle and the monthly mean climate state. Improved results are obtained using a version of the model in which α is permitted to vary with cloud depth.