Paul E. Ciesielski

Trimodal characteristics of tropical convection

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.

Structure and Properties of Madden–Julian Oscillations Deduced from DYNAMO Sounding Arrays

AbstractThe kinematic and thermodynamic characteristics of the October and November 2011 Madden–Julian oscillations (MJOs) that occurred over the Indian Ocean during Dynamics of the MJO (DYNAMO) are investigated. Analyses are presented 1) for two primary sounding arrays, where results are independent of model parameterizations, and 2) on larger scales, including the Indian Ocean, using operational and reanalysis data.Mean precipitation during DYNAMO was characterized by maxima in two east–west bands north and south of the equator. This pattern alternated between two bands during the inactive phase of the MJOs and a single rainfall maximum on the equator during the active phases. Precipitation over the northern sounding array (NSA), where the MJO signal was strongest, was significantly modulated by the MJOs, while the southern array experienced more frequent, briefer episodes of rainfall mostly related to ITCZ convection. Over the NSA the MJOs were characterized by gradual moistening of the low to midtropo...

Tropical Inversions near the 0°C Level

Abstract Soundings taken from the tropical western Pacific warm pool region during TOGA COARE reveal the common occurrence of temperature and moisture perturbations near the 0°C level. The perturbations frequently are characterized by shallow layers of increased stability (or occasionally temperature inversions) and reversals or inflections in the vertical profile of specific humidity. Similar temperature and moisture inversions have been observed elsewhere in the Tropics and midlatitudes but have not received much attention. Isothermal layers are known to exist just below the melting level in stratiform rain regions; however, not all stable layers observed over the warm pool are confined to precipitation systems. The perturbation in the specific humidity profile accounts for the often-observed double-peak structure in the apparent moisture sink Q2 in tie Tropics. Stratification of the data based on relative humidity criteria indicates that the stable layers near the 0°C level generally fall into two main...

Corrected TOGA COARE Sounding Humidity Data: Impact on Diagnosed Properties of Convection and Climate over the Warm Pool

This study reports on the humidity corrections in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) upper-air sounding dataset and their impact on diagnosed properties of convection and climate over the warm pool. During COARE, sounding data were collected from 29 sites with Vaisala-manufactured systems and 13 sites with VIZ-manufactured systems. A recent publication has documented the characteristics of the humidity errors at the Vaisala sites and a procedure to correct them. This study extends that work by describing the nature of the VIZ humidity errors and their correction scheme. The corrections, which are largest in lower-tropospheric levels, generally increase the moisture in the Vaisala sondes and decrease it in the VIZ sondes. Use of the corrected humidity data gives a much different perspective on the characteristics of convection during COARE. For example, application of a simple cloud model shows that the peak in convective mass flux shifts from about 88N with the uncorrected data to just south of the equator with corrected data, which agrees better with the diagnosed vertical motion and observed rainfall. Also, with uncorrected data the difference in mean convective available potential energy (CAPE) between Vaisala and VIZ sites is over 700 J kg21; with the correction, both CAPEs are around ;1300 J kg21, which is consistent with a generally uniform warm pool SST field. These results suggest that the intensity and location of convection would differ significantly in model simulations with humidity-corrected data, and that the difficulties which the reanalysis products had in reproducing the observed rainfall during COARE may be due to the sonde humidity biases. The humidity-corrected data appear to have a beneficial impact on budget-derived estimates of rainfall and radiative heating rate, such that revised estimates show better agreement with those from independent sources.

The NAME 2004 Field Campaign and Modeling Strategy

The North American Monsoon Experiment (NAME) is an internationally coordinated process study aimed at determining the sources and limits of predictability of warm-season precipitation over North America. The scientific objectives of NAME are to promote a better understanding and more realistic simulation of warm-season convective processes in complex terrain, intraseasonal variability of the monsoon, and the response of the warm-season atmospheric circulation and precipitation patterns to slowly varying, potentially predictable surface boundary conditions. During the summer of 2004, the NAME community implemented an international (United States, Mexico, Central America), multiagency (NOAA, NASA, NSF, USDA) field experiment called NAME 2004. This article presents early results from the NAME 2004 campaign and describes how the NAME modeling community will leverage the NAME 2004 data to accelerate improvements in warm-season precipitation forecasts for North America.

Rainfall and Radiative Heating Rates from TOGA COARE Atmospheric Budgets

Abstract Atmospheric heat and moisture budgets are used to determine rainfall and radiative heating rates over the western Pacific warm pool during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Results are compared to independent estimates of these quantities from the other sources. Using the COARE bulk flux algorithm to estimate surface evaporation over the intensive flux array (IFA), the IFA moisture budget-derived average rainfall for the 120-day intensive observing period (IOP) is 8.2 mm day−1. This value agrees closely with recent estimates from satellites and the ocean salinity budget. For a smaller area within the IFA containing the rain-mapping domain of the TOGA and Massachusetts Institute of Technology 5-cm radars, the atmospheric budget for the 101-day radar deployment yields 6.8 mm day−1, slightly greater than the independent radar rain rate estimate of 5.4 mm day−1. Comparison of budget-derived rainfall with National Centers for Environmental ...

Quality-Controlled Upper-Air Sounding Dataset for DYNAMO/CINDY/AMIE: Development and Corrections

AbstractThe upper-air sounding network for Dynamics of the Madden–Julian Oscillation (DYNAMO) has provided an unprecedented set of observations for studying the MJO over the Indian Ocean, where coupling of this oscillation with deep convection first occurs. With 72 rawinsonde sites and dropsonde data from 13 aircraft missions, the sounding network covers the tropics from eastern Africa to the western Pacific. In total nearly 26 000 soundings were collected from this network during the experiment’s 6-month extended observing period (from October 2011 to March 2012). Slightly more than half of the soundings, collected from 33 sites, are at high vertical resolution. Rigorous post–field phase processing of the sonde data included several levels of quality checks and a variety of corrections that address a number of issues (e.g., daytime dry bias, baseline surface data errors, ship deck heating effects, and artificial dry spikes in slow-ascent soundings).Because of the importance of an accurate description of ...

Multigrid Methods for Elliptic Problems: A Review

Abstract Multigrid methods solve a large class of problems very efficiently. They work by approximating a problem on multiple overlapping grids with widely varying mesh sizes and cycling between thew approximations, using relaxation to reduce the error on the scale of each grid. Problems solved by multigrid methods include general elliptic partial differential equations, nonlinear and eigenvalue problems, and systems of equations from fluid dynamics. The efficiency is optimal: the computational work is proportional to the number of unknowns. This paper reviews the basic concepts and techniques of multigrid methods, concentrating on their role as fast solvers for elliptic boundary-value problems. Analysis of simple relaxation schemes for the Poisson problem shows that their slow convergence is due to smooth error components; approximating these components on a coarser grid leads to a simple multigrid Poisson solver. We review the principal elements of multigrid methods for more general problems, including ...

Potential Vorticity Modeling of the ITCZ and the Hadley Circulation

Abstract A simple zonally symmetric balanced model of the Hadley circulation is presented. The model is based on potential vorticity arguments and consists of a predictive equation for the potential pseudodensity and an invertibility principle to diagnose the associated balanced wind and mass fields. When the theory is formulated in the potential latitude coordinate, the meridional advection is implicit in the coordinate transformation, which makes the prediction equation for potential pseudodensity analytically solvable. For convective heating patterns that simulate the ITCZ, the model produces upper and lower tropospheric potential vorticity anomalies of opposite sign. The associated winds are easterly at low levels and westerly aloft, except between the equator and the ITCZ, where there are low-level westerlies and upper-level easterlies. Since the potential vorticity anomalies develop within a background state that has potential vorticity increasing to the north, reversed poleward gradients of potenti...

Multiscale Variability of the Atmospheric Mixed Layer over the Western Pacific Warm Pool

Abstract Sounding data from Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) have provided a first opportunity to document the variability of the atmospheric mixed layer over the western Pacific warm pool on timescales ranging from diurnal to intraseasonal. Six-hourly sounding data from four sites—the atoll Kapingamarangi and R/Vs Moana Wave, Shiyan 3, and Xiangyanghong 5—are used to determine the mixed layer depth and its thermodynamic properties. Almost three-quarters of the soundings at these four sites exhibited well-mixed structures: nearly constant profiles of potential temperature and specific humidity capped by a ∼150-m-deep entrainment zone. The majority of the remaining soundings were modified by precipitation and their associated downdrafts. It is estimated that approximately 40%–50% of the total soundings in COARE were influenced by precipitation downdrafts. The mean mixed layer depth at the four sites was 512 m with large variations on multiple timesc...