Abstract

The time-dependent role of cloud liquid water in conjunction with its vertical heterogeneities on top-of-atmosphere (TOA) passive microwave brightness temperatures is investigated. A cloud simulation is used to specify the microphysical structure of an evolving cumulus cloud growing toward the rain stage. A one-dimensional multistream solution to the radiative transfer equation is used to study the upwelling radiation at the top of the atmosphere arising from the combined effect of cloud, rain, and ice hydrometeors. Calculations are provided at six window frequencies and one H₂O resonance band within the EHF/SHF microwave spectrum. Vertically detailed transmission functions are used to help delineate the principal radiative interactions that control TOA brightness temperatures. Brightness temperatures are then associated with a selection of microphysical situations that reveal how an evolving cloud medium attenuates rainfall and surface radiation. The investigation is primarily designed to study the impact of cloud microphysics on space-based measurements of passive microwave signals, specifically as they pertain to the retrieval of precipitation over water and land backgrounds.

Results demonstrate the large degree to which the relationship between microwave brightness temperature (BT) and rainrate (RR) can be altered purely by cloud water processes. The relative roles of the cloud and rain drop spectra in emissive contributions to the upwelling radiation are assessed with a normalized absorption index, which removes effects due purely to differences in the magnitudes of the cloud and rain liquid water contents. This index is used to help explain why the amplitudes of the BT-RR functions decrease with respect to cloud evolution time and why below-cloud precipitation is virtually masked from detection at the TOA.

Although cloud water tends to obscure BT-RR relationships, it does so in a differential manner with respect to frequency, suggesting that the overall impact of cloud water is not necessarily debilitating to precipitation retrieval schemes. Furthermore, it is shown how a “surface” of “probability” can be defined, which contains an optimal time-dependent BT-RR function associated with an evolving cloud at a given frequency and removes ambiguities within the BT-RR functions at the critical retrieval frequencies. The influence of a land surface having varying emissivity characteristics is also examined in the context of an evolving cloud to show how the time-dependent cloud microphysics modulates the sign and magnitude of brightness temperature differences between various frequencies.

Model results are assessed in conjunction with a Nimbus-7 SMMR case study of precipitation within an intense tropical Pacific storm. It is concluded that in order to obtain a realistic estimation and distribution of rainrates, the effects of cloud liquid water content must be considered.

Abstract

The Tropical Rainfall Measuring Mission (TRMM) Microwave Imager precipitation profile retrieval algorithm (2a12) assumes cloud model–derived vertically distributed microphysics as part of the radiative transfer–controlled inversion process to generate rain-rate estimates. Although this algorithm has been extensively evaluated, none of the evaluation approaches has explicitly examined the underlying microphysical assumptions through a direct intercomparison of the assumed cloud-model microphysics with in situ, three-dimensional microphysical observations. The main scientific objective of this study is to identify and overcome the foremost model-generated microphysical weaknesses in the TRMM 2a12 algorithm through analysis of (a) in situ aircraft microphysical observations; (b) aircraft- and satellite-based passive microwave measurements; (c) ground-, aircraft-, and satellite-based radar measurements; (d) synthesized satellite brightness temperatures and radar reflectivities; (e) radiometer-only profile algorithm retrievals; and (f) radar-only profile or volume algorithm retrievals. Results indicate the assumed 2a12 microphysics differs most from aircraft-observed microphysics where either ground or aircraft radar–derived rain rates exhibit the greatest differences with 2a12-retrieved rain rates. An emission–scattering coordinate system highlights the 2a12 algorithm's tendency to match high-emission/high-scattering observed profiles to high-emission/low-scattering database profiles. This is due to a lack of mixed-phase-layer ice hydrometeor scatterers in the cloud model–generated profiles as compared with observed profiles. Direct comparisons between aircraft-measured and model-generated 2a12 microphysics suggest that, on average, the radiometer algorithm's microphysics database retrieves liquid and ice water contents that are approximately 1/3 the size of those observed at levels below 10 km. Also, the 2a12 rain-rate retrievals are shown to be strongly influenced by the 2a12's convective fraction specification. A proposed modification of this factor would improve 2a12 rain-rate retrievals; however, fundamental changes to the cloud radiation model's ice parameterization are necessary to overcome the algorithm's tendency to produce mixed-phase-layer ice hydrometeor deficits.

Abstract

Datasets of daily high-resolution upper-air soundings and Special Sensor Microwave/Imager (SSM/I) passive microwave measurements from the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive operation period are used for large-scale diagnostic budget calculations of the apparent heat source (Q ₁), the apparent moisture sink (Q ₂), and latent heating to investigate the mechanisms of diabatic heating and moistening processes within the TOGA COARE Intensive Flux Array (IFA). Latent-heating retrievals are obtained from The Florida State University SSM/I-based precipitation profile retrieval algorithm. The estimates are correlated well with heating calculations from the soundings in which approximately 70% of the total heating arises from latent heat release. Moisture-budget processes also have a strong relationship with the large-scale environment, in which drying from condensation is mainly balanced by large-scale horizontal convergence of moisture flux. It is found that there may be more convective activity in summer than in winter over the tropical region of the western Pacific Ocean. Results also show that Q ₁ and Q ₂ exhibit a 20–30-day oscillation, in which active periods are associated with strong convection.

Comparisons of the Q ₁–Q ₂ calculations over IFA are made with a number of previously published results to help to establish the similarities and differences of Q ₁–Q ₂ between the warm pool and other regions of the Tropics. The Q ₁–Q ₂ budget analyses over IFA then are used to study quantitatively the detailed vertical heating structures. Cumulus-scale heating–moistening processes are obtained by using published radiative divergence (Q _R ) data, retrieved latent heating, and the Q ₁–Q ₂ calculations. These results show that cumulus-scale turbulent transport is an important mechanism in both heat and moisture budgets. Although daily estimates of eddy vertical moisture flux divergence are noisy, by averaging over 7-day periods and vertically integrating to obtain surface latent heat flux, good agreement with measured surface evaporation is found. This agreement demonstrates the feasibility of estimating averaged eddy heat–moisture flux profiles by combining satellite-derived rain profile retrievals with large-scale sounding and Q _R data, a methodology that helps to shed light on the role of cumulus convection in atmospheric heating and moistening.

Abstract

The behavior and various controls of diurnal variability in tropical–subtropical rainfall are investigated using Tropical Rainfall Measuring Mission (TRMM) precipitation measurements retrieved from the three level-2 TRMM standard profile algorithms for the 1998 annual cycle. Results show that diurnal variability characteristics of precipitation are consistent for all three algorithms, providing assurance that TRMM retrievals are producing consistent estimates of rainfall variability. As anticipated, most ocean areas exhibit more rainfall at night, while over most land areas, rainfall peaks during daytime; however, important exceptions are noted.

The dominant feature of the oceanic diurnal cycle is a rainfall maximum in late-evening–early-morning (LE–EM) hours, while over land the dominant maximum occurs in the mid- to late afternoon (MLA). In conjunction with these maxima are pronounced seasonal variations of the diurnal amplitudes. Amplitude analysis shows that the diurnal pattern and its seasonal evolution are closely related to the rainfall accumulation pattern and its seasonal evolution. In addition, the horizontal distribution of diurnal variability indicates that for oceanic rainfall, there is a secondary MLA maximum coexisting with the LE–EM maximum at latitudes dominated by large-scale convergence and deep convection. Analogously, there is a preponderancy for an LE–EM maximum over land coexisting with the stronger MLA maximum, although it is not evident that this secondary continental feature is closely associated with the large-scale circulation. Neither of the secondary maxima exhibit phase behavior that can be considered semidiurnal in nature. Diurnal rainfall variability over the ocean associated with large-scale convection is clearly an integral component of the general circulation.

Phase analysis reveals differences in regional and seasonal features of the diurnal cycle, indicating that underlying forcing mechanisms differ from place to place. This is underscored by the appearance of secondary ocean maxima in the presence of large-scale convection, along with other important features. Among these, there are clear-cut differences between the diurnal variability of seasonal rainfall over the mid-Pacific and Indian Ocean Basins. The mid-Pacific exhibits double maxima in spring and winter but only LE–EM maxima in summer and autumn, while the Indian Ocean exhibits double maxima in spring and summer and only an LE–EM maximum in autumn and winter. There are also evident daytime maxima within the major large-scale marine stratocumulus regions off the west coasts of continents. The study concludes with a discussion concerning how the observational evidence either supports or repudiates possible forcing mechanisms that have been suggested to explain diurnal rainfall variability.

Abstract

This study examines a mix of seven statistical and physical Special Sensor Microwave Imager (SSM/I) passive microwave algorithms that were designed for retrieval of over-ocean precipitable water (PW). The aim is to understand and explain why the algorithms exhibit a range of discrepancies with respect to measured PWs and with respect to each other, particularly systematic regional discrepancies that would produce substantive uncertainties in water vapor transports and radiative cooling in the context of climate dynamics. Data analysis is used to explore the nature of the algorithm differences, while radiative transfer analysis is used to explore the influence of several environmental variables (referred to as tangential environmental factors) that affect the PW retrievals. These are sea surface temperature (SST), surface wind speed (U _s ), cloud liquid water path (LWP), and vertical profile structure of water vapor [q(z)]. The main datasets include the Wentz matched radiosonde–SSM/I point database consisting of 42 months of globally distributed oceanic radiosonde profiles paired with coincident SSM/I brightness temperatures, and globally compiled instantaneous orbit-swath maps of SSM/I brightness temperatures for January and July 1990.

Results demonstrate that the seemingly good agreement found in past studies and herein, within the conventional framework of scatter diagram analysis that ignores regional classification, gives way to poor agreement in the framework of monthly and zonally averaged differences. It is shown how much of the disagreement inherent to statistical algorithms is due to disjoint training datasets used in deriving algorithm regression coefficients. The investigation also explores how tangential environmental factors composed of variations in SST, U _s , cloud LWP, and q(z) structure impart dissimilar errors to retrieved PWs, according to the design of the retrieval algorithms. A discussion on implications of the discrepancies vis-à-vis the Global Energy and Water Cycle Experiment program is given, with suggestions on mitigating discrepancies in algorithm designs.

Abstract

This study addresses the retrieval of tropical open-ocean latent heating using Special Sensor Microwave Imager (SSM/I) satellite measurements. The analysis is carried out for the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observation period in the western Pacific, much of it focused on the study area of the third WCRP–GPCP Algorithm Intercomparison Project (AIP-3) situated over the TOGA COARE Inner Flux Array (IFA). The retrieval algorithm is a profile-type physical inversion scheme based on the use of multispectral passive microwave (PMW) measurements. It estimates vertically distributed rain rate and latent heating by first retrieving mixing ratio profiles of liquid and frozen hydrometeors and then calculating rain fallout rates and vertical derivatives of the liquid–ice mass fluxes. Various modifications to the existing algorithm are discussed, including a combined visible–infrared–PMW–radar screening scheme for distinguishing among “clear,” “cloud without rain,” and “cloud with rain pixels” to better delineate vertical heating structure. Validation of retrieved rain rates over the AIP-3 study area indicates acceptable accuracy/precision uncertainty levels in terms of intensity, distribution, and time variation.

A procedure is developed for improving the initially retrieved heating profiles based on calibration to shipboard radar measurements. The modified algorithm and calibration scheme were applied to the IFA for estimating vertical profiles of latent heating. An optimum high-quality sounding period (1–17 February 1993) was selected for large-scale diagnostic calculations of apparent heating (Q ₁) and moistening (Q ₂) to analyze heat-moisture budgets of convective and stratiform cloud systems. Comparison and sensitivity tests indicate that the retrieved latent heating and Q ₁/Q ₂ calculations are representative. Moisture budget analyses over the IFA were carried out to study the detailed heating structures of clouds, particularly the cumulus scale heating process. This was accomplished by using residuals between the SSM/I-retrieved latent heating and the large scale Q ₂ diagnostics. Results show that estimates of daily eddy vertical moisture flux divergence contain sizable uncertainties, however, by averaging over extended periods and vertically integrating to obtain surface latent heat flux transfer, close agreement to independently derived surface evaporation rates is found. This suggests that by combining the SSM/I retrievals with large-scale sounding data, it is possible to shed light on the role of cumulus convection on diabatic heating.

Abstract

Infrared radiative cooling rates are calculated over the Asian summer monsoon between 5°S–20°N and 40°–135°E at a spatial resolution of 5° × 5° for the summer seasons of 1984 and 1987. A medium spectral resolution infrared radiative transfer model with specified temperature, moisture, clouds, and trace gas distributions is used to obtain the cooling rate profiles. Cloud distributions for the two summers are obtained from Indian National Satellite measurements. Seasonal mean and intraseasonal variations of clouds and radiative cooling rates over a 21–76-day range of periods are examined.

The analysis identifies centers over the central and eastern Indian Ocean, and western Pacific Ocean, along the equator, and along 15°N, where seasonal mean cloud amounts range from 40% to 80% with cloud tops mostly in the middle and upper troposphere. Intraseasonal variability of clouds is also large over these centers (% variances >25%). Consistently, seasonal mean cooling rates are at a maximum (3°–5°C day⁻¹) in the upper troposphere between 300 and 400 mb, related to cloud-top cooling. The cooling rates below 400 mb are between 1° and 3°C day⁻¹. The cooling rates exhibit intraseasonal amplitudes of 1.0°–1.5°C day⁻¹. The largest amplitudes are found between 300 and 500 mb, indicating that cooling rate variability is directly related to intraseasonal variability of convective clouds. Spatial distributions of clouds and cooling rates remain similar during the 1984 and 1987 summer seasons. However, during 1987, intraseasonal amplitudes of deep convective cloud amount and cooling rate over the Indian Ocean are 10%–15% larger than in 1984.

It is shown that intraseasonal variability of cooling rates over the Indian Ocean can perturb convective heating by 10%–30% in the upper and lower troposphere. Based on a one-dimensional radiative–convective equilibrium model, it is estimated that the radiative damping timescale over the Indian Ocean region is ∼3 days. Based on this damping timescale and in conjunction with a model of equatorial Kelvin waves with first baroclinic mode, it is hypothesized that the variable cloud-radiative cooling rates can alter phase speeds of Kelvin waves by up to 60%. This helps explain why the frequency range of intraseasonal oscillations is so broad.

Abstract

The role of the Tibetan Plateau on the behavior of the surface longwave radiation budget is examined, and the behavior of the vertical profile of longwave cooling over the plateau, including its diurnal variation, is quantified. The investigation has been conducted with the aid of datasets obtained during the 1986 Tibetan Plateau Meteorological Experiment (TIPMEX-86). A medium spectral-resolution infrared radiative transfer model using a simple modification for applications in idealized complex (valley) terrain is developed for the study. This study focuses on the clear-sky case where the surface effects are most significant.

The TIPMEX-86 data, obtained during the spring-summer transition into the East Asian monsoon season, are used to help validate the surface longwave radiation budget at two sites of varying elevation: Lasa (3650 m) and Naqu (4500 m). Based on the degree to which skin-temperature boundary conditions control the magnitude of infrared cooling, we define the concept of relative longwave heating and explain its influence on the vertical infrared cooling-rate profile. Relative longwave radiative heating at the higher-elevation Naqu site is found to be twice as large as that corresponding to the lower-elevation Lasa site located within a valley. Besides reducing the infrared cooling rates, it is shown that relative longwave heating extends the period of the day over which the plateau acts as a direct heat source to the atmosphere. Computational results from the infrared model help substantiate observational analyses that indicate surface longwave net radiation at the high-elevation site, on clear days, exceeds 300 W m⁻²; this is an order of magnitude greater than typical of sea-level oceanic conditions. As a result of the unique meteorological and surface conditions, total infrared flux convergence occurs within the deep planetary boundary layer (i.e., infrared heating of the cloud-free lower atmosphere) at the high-elevation site during the afternoon. An important characteristic of the daytime longwave heating process of the lower layers is how it turns off like a switch at approximately 1800 MST, transforming almost immediately to maximum cooling of the lower layers.

Atmospheric longwave cooling is significantly influenced by variations in the biophysical composition of the surface and the associated thermal diurnal cycle. It is estimated that natural variations of surface emissivity could modulate longwave cooling by up to 40%. The largest impact would occur at a time when the surface temperature is high and the relative longwave radiative heating of the lower atmosphere by the surface reaches its maximum value.

Abstract

During the summer east Asian monsoon transition period in 1979, a meteorological field experiment entitled the Qinghai-Xizang Plateau Meteorological Experiment (QXPMEX-79) was conducted over the entire Tibetan Plateau. Data collected on and around the plateau during this period, in conjunction with a medium spectral-resolution infrared radiative transfer model, are used to gain an understanding of how elevation and surface biophysical factors, which are highly variable over the large-scale plateau domain, regulate the spatial distribution of clear-sky infrared cooling during the transition phase of the summer monsoon.

The spatial distribution of longwave cooling over the plateau is significantly influenced by variations in biophysical composition, topography, and elevation, the surface thermal diurnal cycle, and various climatological factors. An important factor is soil moisture. Bulk clear-sky longwave cooling rates are larger in the southeast sector of the plateau than in the north. This is because rainfall is greatest in the southeast, whereas the north is highly desertified and relative longwave radiative heating by the surface is greatest. Another important phenomenon is that the locale of a large-scale east-west-aligned spatial gradient in radiative cooling propagates northward with time. During the premonsoon period (May–June), the location of the strong spatial gradient is found in the southeastern margin of the plateau. Due to changes in surface and atmospheric conditions after the summer monsoon commences, the high gradient sector is shifted to the central Qinghai region. Furthermore, an overall decrease in longwave cooling takes place in the lower atmosphere immediately prior to the arrival of the active monsoon.

The magnitude of longwave cooling is significantly affected by skin-temperature boundary conditions at plateau altitudes. If skin-temperature discontinuities across the surface-atmosphere interface are neglected, bulk cooling rates will be in error up to 1°C day⁻¹. The high surface skin temperatures, particularly in the afternoon, lead to significant relative longwave radiative heating in the lower atmosphere for which the impact in terms of vertical depth is shown to increase rather dramatically as a function of the elevation of the terrain. The significance of these results in the context of previous heat budget studies of the plateau suggest that the radiative heating term (Q_R ) used by previous investigators contains far too much longwave cooling, and thus in a classic formulation of the Yanai Q ₁ balance equation, would lead to underestimation of sensible heating into the atmospheric column.

Abstract

Cloud–radiative forcing calculations based on Nimbus-7 radiation budget and cloudiness measurements reveal that cloud-induced longwave (LW) warming (cloud greenhouse influence) is dominant over the tropics, whereas cloud-induced shortwave (SW) cooling (cloud albedo influence) is dominant over mid- and high latitudes. The average SW cloud cooling taken over the area of the globe from 65°N to 65°S is −27.8 W m⁻². This magnitude slightly overcomes LW cloud warming (−25.7 W m⁻²), resulting in a small net cooling effect of −2.1 W m⁻² over 93% of the earth.

A 6-year zonally averaged mean cloudy- and clear-sky net radiation flux analysis shows that there are three distinct regimes in terms of net cloud warming or cooling, that is, warming in the tropics (between 20°N and 20°S) and in the high latitudes (poleward of 55°) and cooling in the extratropical latitudes between 20° and 55° in both hemispheres. These distributions reinforce the intensities of the Hadley and Ferrel meridional circulation cells. This stems from strong warming due to high-level clouds in the tropics and strong cooling due to mid- and low-level clouds at extratropical latitudes. The magnitude of the contribution by cloud forcing is found to be of the same order as eddy heat and momentum flux forcing to the maintenance of the mean meridional circulation.

Surface–atmosphere forcing obtained by differentiating the cloud-induced effects from the measured radiative fluxes indicates that an east–west coupled North Africa–western Pacific energy transport dipole is maintained mainly by low-latitude land–ocean contrasts associated with shortwave radiation but supported by cloud controls on tropical longwave radiation. This implies that interannual variations in the net radiation balance associated with these two regions can give rise to fluctuations of the basic dipole structure and thus fundamental changes in low-latitude climate.