It has long been known that trade wind cumulus and deep cumulonimbus represent primary components of the broad spectrum of cumulus clouds in the Tropics, which has led to the concept of a bimodal distribution of tropical clouds. However, recent analyses of shipboard radar data from Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE) provide evidence of abundant populations of a third cloud type, cumulus congestus. Congestus clouds constitute over half the precipitating convective clouds in COARE and contribute over one-quarter of the total convective rainfall. Global Atmospheric Research Program Atlantic Tropical Experiment studies reveal a similar midlevel peak in the distribution of radar-echo tops. These findings lead to the conclusion that shallow cumulus, congestus,and cumulonimbus are all prominent tropical cumulus cloud types. They are associated with trimodal distributions of divergence, cloud detrainment, and fractional cloudiness in the Tropics. The peaks in the distributions of radar-echo tops for these three cloud types are in close proximity to prominent stable layers that exist over the Pacific warm pool and the tropical eastern Atlantic: near 2 km (the trade stable layer), ;5 km (near 08C), and ;15‐16 km (the tropopause). These stable layers are inferred to inhibit cloud growth and promote cloud detrainment. The 08C stable layer can produce detrainment from cumulonimbi (attendant shelf clouds) and help retard the growth of precipitation-laden and strongly entraining congestus clouds. Moreover, restriction of growth of congestus clouds to just above the 08C level limits further enhancement of cloud buoyancy through glaciation. The three cloud types are found to vary significantly during COARE on the timescale of the 30‐60-day intraseasonal oscillation. The specific roles of clouds of the congestus variety in the general circulation are not yet clear, but some (the shallower ones) contribute to moistening and preconditioning the atmosphere for deep convection; others (the deeper ones) contribute an important fraction of the total tropical rainfall, and both likely produce many midlevel clouds, thereby modulating the radiative heating of the tropical atmosphere.

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Trimodal Characteristics of Tropical Convection

Precipitation is a key link in the global water cycle and a proxy for changing climate; therefore, proper assessment of the urban envi- ronment's impact on precipitation (land use, aerosols, thermal properties) will be increasingly important in ongoing climate diagnostics and prediction, Glob- al Water and Energy Cycle (GWEC) analysis and modeling, weather forecast- ing, freshwater resource management, urban planning-design, and land- atmosphere-ocean interface processes. These facts are particularly critical if current projections for global urban growth are accurate. The goal of this paper is to provide a concise review of recent (1990-present) studies related to how the urban environment affects precipitation. In addition to providing a synopsis of current work, recent findings are placed in context with historical investigations such as Metropolitan Meteorological Experiment (METROMEX) studies. Both observational and modeling studies of urban- induced rainfall are discussed. Additionally, a discussion of the relative roles of urban dynamic and microphysical (e.g., aerosol) processes is presented. The paper closes with a set of recommendations for what observations and capa- bilities are needed in the future to advance our understanding of the processes.

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A Review of Current Investigations of Urban-Induced Rainfall and Recommendations for the Future

The relationship between water vapor path W and surface precipitation rate P over tropical oceanic regions is analyzed using 4 yr of gridded daily SSM/I satellite microwave radiometer data. A tight monthly mean relationship P (mm day21) 5 exp[11.4(r 2 0.522)] for all tropical ocean regions and seasons is found between P and a column-relative humidity r obtained by dividing W by the corresponding saturation water vapor path. A similar relation, albeit with more scatter, also holds at daily time scales, and can be interpreted as a moisture adjustment time scale of 12 h for convective rainfall to affect humidity anomalies on 300-km space scales. Cross-spectral analysis shows statistically significant covariability of actual and r-predicted precipitation at all frequencies, with negligible phase lag. The correlation of actual and r-predicted precipitation exceeds 0.5 on intraseasonal and longer time scales. The SSM/I retrievals of W and P are found to be skillful even at daily time scales when compared with in situ radiosonde and radar-derived area-averaged precipitation data from Kwajalein Island, but the microwave estimates of daily P scatter considerably about the radar estimates (which are considered to be more reliable). Using the radar-derived precipitation in combination with microwave-derived W yields a daily r‐P relationship at Kwajalein similar to that derived solely from microwave measurements, but with somewhat less P associated with the highest values of r. This emphasizes that the absolute calibration of the r‐P relationship is somewhat dependent on the datasets used to derive r and especially P. Nevertheless, the results provide a useful constraint on conceptual models and parameterizations of tropical deep convection.

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Relationships between Water Vapor Path and Precipitation over the Tropical Oceans

Horizontal temperature gradients are small in the tropical atmosphere, as a consequence of the smallness of the Coriolis parameter near the equator. This provides a strong constraint on both large-scale fluid dynamics and diabatic processes. This work is a step toward the construction of a balanced dynamical theory for the tropical circulation that is based on this constraint, and in which the diabatic processes are explicit and interactive. The authors first derive the basic fluid-dynamical scaling under the weak temperature gradient (WTG) approximation in a shallow water system with a fixed mass source representing an externally imposed heating. This derivation follows an earlier similar one by Held and Hoskins, but extends the analysis to the nonlinear case (though on an f plane), examines the resulting system in more detail, and presents a solution for an axisymmetric ‘‘top-hat’’ forcing. The system is truly balanced, having no gravity waves, but is different from other balance models in that the heating is included a priori in the scaling. The WTG scaling is then applied to a linear moist model in which the convective heating is controlled by a moisture variable that is advected by the flow. This moist model is derived from the Quasi-equilibrium Tropical Circulation Model (QTCM) equations of Neelin and Zeng but can be viewed as somewhat more general. A number of additional approximations are made in order to consider balanced dynamical modes, apparently not studied previously, which owe their existence to interactions of the moisture and flow fields. A particularly interesting mode arises on an f plane with a constant background moisture gradient. In the limit of low frequency and zero meridional wavenumber this mode has a dispersion relation mathematically identical to that of a barotropic Rossby wave, though the phase speed is eastward (for moisture decreasing poleward in the background state) and the propagation mechanism is quite different. This mode also has significant positive growth rate for low wavenumbers. The addition of the b effect complicates matters. For typical parameters, when b is included the direction of phase propagation is ambiguous, and the growth rate reduced, as the effects of the background gradients in moisture and planetary vorticity appear to cancel to a large degree. Possible relevance to intraseasonal variability and easterly wave dynamics is briefly discussed.

/pdf/the-weak-temperature-gradient-approximation-and-balanced-2t6b2zru3h.pdf

The Weak Temperature Gradient Approximation and Balanced Tropical Moisture Waves

Land cover changes (LCCs) play an important role in the climate system. Research over recent decades highlights the impacts of these changes on atmospheric temperature, humidity, cloud cover, circulation, and precipitation. These impacts range from the local- and regional-scale to sub-continental and global-scale. It has been found that the impacts of regional-scale LCC in one area may also be manifested in other parts of the world as a climatic teleconnection. In light of these findings, this article provides an overview and synthesis of some of the most notable types of LCC and their impacts on climate. These LCC types include agriculture, deforestation and afforestation, desertification, and urbanization. In addition, this article provides a discussion on challenges to, and future research directions in, assessing the climatic impacts of LCC.

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Land cover changes and their biogeophysical effects on climate

A numerical study of the air-mass transformation over the Japan Sea in winter

A significant 4-5-day-period variation was observed in surface and aerological data obtained at the equator, 156E during November 1992. The time-height structure of the variation is determined by use of spectral and composite analysis. The 4-5-day period variation can be basically explained by westward-propagating mixed Rossby-gravity wave disturbances. However, the structure of the circulation was significantly distorted by the presence of a tropical depression. Further, two distinctive events of extremely dry air in the lower troposphere were observed. The vertical profiles of specific humidity as well as the equivalent potential temperature had distinct minima at about the 3 km level. The trajectory analysis revealed that the dry-air parcels were brought from a subtropical area by tropical disturbances, especially by the intense meridional flow associated with 4-5-day period disturbances. These events of dry-air intrusion are detected in the precipitable water data derived from SSM/I satellite imageries and the zonal scale of the intrusion is estimated to be about 1000 km. Convection was strongly suppressed over the dry air, and the latter was maintained to about 4 days due to very weak vertical mixing.

4-5-Day-Period Variation and Low-Level Dry Air Observed in the Equatorial Western Pacific during the TOGA-COARE IOP

A case study is made of an exceptionally heavy rainstorm that hit the northwestern coastal area of Kyushu, Japan, on 23 July 1982. The 5hr rainfall accumulation was as high as 412mm at the city of Nagasaki. The precipitation occurred along the warm front associated with a medium scale disturbance that developed along the Baiu front. The atmosphere in the pre-storm period was very moist through virtually the entire troposphere and its static stability was conditionally unstable with a lifted index of -1.5*, thus exhibiting characteristics more similar to the tropical atmosphere than the typical pre-storm environment over the midwestern United States in spring seasons. Prior to the development of heavy precipitation over the Nagasaki area, an intensive rainband was propagating with a speed of 40km hr-1 eastsoutheastward over northwestern Kyushu. Its structure is found to be different from the structure of typical tropical squall clusters in that the heaviest rainfalll occurred in the middle of the rainband without a well defined leading edge. A dramatic change occurred in the rainband when its southern tip reached the Nagasaki area; it stopped propagating and stayed there for the next 5hr. Its line structure changed to a blob structure at the meso-* scale (*100km). The timing of the precipitation development at Nagasaki coincided with the peak period of the incursion of very moist air associated with the southwesterly low-level jet. During the period of heavy precipitation, a new cloud cluster formed over the sea 300km west of Nagasaki. It kept developing and propagating eastward with a speed of 60km hr-1, and eventually merged with the rainstorm over Nagasaki. These observations strongly suggest that the Nagasaki rainstorm was trapped orographically. Several other heavy precipitation events are cited which also occurred in association with medium scale disturbances along the Baiu front and in which the precipitation developed over the coast areas upwind of the low-level

A Case Study of a Heavy Precipitation Event along the Baiu Front in Northern Kyushu, 23 July 1982: Nagasaki Heavy Rainfall

The behavior of cumulus convection in a prevailing wind with vertical shear is studied by integrating a set of dynamic equations numerically. The motion is considered under the solenoidal condition in a vertical two dimensional plane. Aside from the eddy exchange, the pseudo-adiabatic process is assumed in which the motion is moist adiabatic in saturated ascending air and dry adiabatic in the remaining air. The comparison between both cases, with and without vertical shear, is made concerning the time dependent evolution of convections and their energy conversion. In a prevailing wind field with vertical shear, the axial symmetry of the convection is destroyed and the axis of the convection cell tilts downwind with height. This results in the interaction between the convective motion and the prevailing wind which transforms the kinetic energy of the convection into that of the prevailing wind. Therefore, vertical wind shear tends to suppress the development of the convection in the vertical plane parallel to the wind. Moreover, the discrepancy which develops between the central axes of updraft and the warm regions of the convection decreases the conversion of potential to kinetic energyone which contributes to the development of the convection.

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Cumulus Convection in the Atmosphere with Vertical Wind Shear : Numerical Experiment

Diurnal variations of cloudiness over East Asia and the western Pacific are investigated for the warm season of 1987, based on GMS-3 IR data at every three hours. The domain chosen for the present analysis is between latitude 50°N and 20°S and longitude 90°E and 160°E. The main results obtained are: (1) Diurnal variations of cloudiness result from superposition of large diurnal-cycle and small semidiurnal-cycle variations. Amplitude and phase of the diurnal-cycle variation are much different over land and ocean. Semidiurnal-cycle variations over land and ocean are almost in phase with maxima at 0300 ∼ 0500 LT and 1500 ∼ 1700 LT, and have similar amounts of amplitude. (2) A systematic phase delay of diurnal-cycle variation appears to the east of the Tibetan Plateau. A maximum of cloudiness appears near dusk over the Tibetan Plateau and at midnight in the Sichuan Basin. Phase speed of the variations is almost the same as in the eastward phase propagation of the diurnal-cycle variation of precipitation frequency east of the Rocky Mountains in the United States. Eastward movement of cloud clusters generated over the Tibetan Plateau, as well as locally induced convections, should be taken into account to understand the behavior of diurnal variation of cloudiness to the east of the Plateau. (3) The eastward phase delay of the diurnal-cycle variation to the east of the Tibetan Plateau is obscured during mid-summer. This may be caused by locally induced active convections intensified near dusk to the east of the Plateau, and seasonal disappearance of upper westerly flow passing over the Plateau.

Tomio Asai

Papers

A numerical study of the air-mass transformation over the Japan Sea in winter

4-5-Day-Period Variation and Low-Level Dry Air Observed in the Equatorial Western Pacific during the TOGA-COARE IOP

A Case Study of a Heavy Precipitation Event along the Baiu Front in Northern Kyushu, 23 July 1982: Nagasaki Heavy Rainfall

Cumulus Convection in the Atmosphere with Vertical Wind Shear : Numerical Experiment

Diurnal Variability of Cloudiness over East Asia and the Western Pacific Ocean as Revealed by GMS during the Warm Season