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Karen I. Mohr

Abstract

Convective systems in sub-Saharan Africa were defined from measurements by the Tropical Rainfall Measuring Mission satellite Microwave Imager at 85 GHz for four wet seasons, May–September 1998–2001. By applying a convective–stratiform discrimination algorithm to each convective system, the pixels within were designated as convective or stratiform cloud, life cycle ages assigned, and rainfall rates calculated. The years 1998 and 1999 were wetter than the long-term (1898–2000) mean, while 2000 and 2001 were drier. The wetter years had about 10% more convective systems than the drier years, but the size and intensity distributions for the wetter and drier years were virtually identical.

The wet season diurnal cycle of precipitation in the study area varied regionally, intraseasonally, and interannually. Analysis of precipitation versus time revealed different diurnal cycles for each of the three 10° zones south of the Sahara Desert. The diurnal cycle was bimodal north of 10°N and unimodal south of 10°N. The bimodal diurnal cycle was more pronounced north of 15°N. Diurnal cycles in each zone exhibited regional and seasonal variability of about 10% per four-hour time block. In wetter years the regional mean diurnal cycle was unimodal, but in drier years it was bimodal. The variability of the diurnal cycle appeared to be primarily influenced by variability in the frequency and life cycle of organized convective systems and thus the physical and dynamical factors responsible for their development.

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Stefanie M. Herrmann and Karen I. Mohr

Abstract

A classification of rainfall seasonality regimes in Africa was derived from gridded rainfall and land surface temperature products. By adapting a method that goes back to Walter and Lieth’s approach of presenting climatic diagrams, relationships between estimated rainfall and temperature were used to determine the presence and pattern of humid, arid, and dry months. The temporal sequence of humid, arid, and dry months defined nonseasonal as well as single-, dual-, and multiple-wet-season regimes with one or more rainfall peaks per wet season. The use of gridded products resulted in a detailed, spatially continuous classification for the entire African continent at two different spatial resolutions, which compared well to local-scale studies based on station data. With its focus on rainfall patterns at fine spatial scales, this classification is complementary to coarser and more genetic classifications based on atmospheric driving forces. An analysis of the stability of the resulting seasonality regimes shows areas of relatively high year-to-year stability in the single-wet-season regimes and areas of lower year-to-year stability in the dual- and multiple-wet-season regimes as well as in transition zones.

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Stephen D. Nicholls and Karen I. Mohr

Abstract

The intense surface heating over arid land surfaces produces dry well-mixed layers (WML) via dry convection. These layers are characterized by nearly constant potential temperature and low, nearly constant water vapor mixing ratio. To further the study of dry WMLs, we created a detection methodology and supporting software to automate the identification and characterization of dry WMLs from multiple data sources including rawinsondes, remote sensing platforms, and model products. The software is a modular code written in Python, an open-source language. Radiosondes from a network of synoptic stations in North Africa were used to develop and test the WML detection process. The detection involves an iterative decision tree that ingests a vertical profile from an input data file, performs a quality check for sufficient data density, and then searches upward through the column for successive points where the simultaneous changes in water vapor mixing ratio and potential temperature are less than the specified maxima. If points in the vertical profile meet the dry WML identification criteria, statistics are generated detailing the characteristics of each layer in the profile. At the end of the vertical profile analysis, there is an option to plot analyzed profiles in a variety of file formats. Initial results show that the detection methodology can be successfully applied across a wide variety of input data and North African environments and for all seasons. It is sensitive enough to identify dry WMLs from other types of isentropic phenomena such as subsidence layers and distinguish the current day’s dry WML from previous days.

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Karen I. Mohr, Daniel Slayback, and Karina Yager

Abstract

The central Andes extends from 7° to 21°S, with its eastern boundary defined by elevation (1000 m and greater) and its western boundary by the coastline. The authors used a combination of surface observations, reanalysis, and the University of Utah Tropical Rainfall Measuring Mission (TRMM) precipitation features (PF) database to understand the characteristics of convective systems and associated rainfall in the central Andes during the TRMM era, 1998–2012. Compared to other dry (West Africa), mountainous (Himalayas), and dynamically linked (Amazon) regions in the tropics, the central Andes PF population was distinct from these other regions, with small and weak PFs dominating its cumulative distribution functions and annual rainfall totals. No more than 10% of PFs in the central Andes met any of the thresholds used to identify and define deep convection (minimum IR cloud-top temperatures, minimum 85-GHz brightness temperature, maximum height of the 40-dBZ echo). For most of the PFs, available moisture was limited (<35 mm) and instability low (<500 J kg−1). The central Andes represents a largely stable, dry to arid environment, limiting system development and organization. Hence, primarily short-duration events (<60 min) characterized by shallow convection and light to light–moderate rainfall rates (0.5–4.0 mm h−1) were found.

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Stephen D. Nicholls and Karen I. Mohr

Abstract

The local- and regional-scale environments associated with intense convective systems in West Africa during 2003 were diagnosed from soundings, operational analysis, and space-based datasets. Convective system cases were identified from the Tropical Rainfall Measuring Mission (TRMM) microwave imagery and classified by the system minimum 85-GHz brightness temperature and the estimated elapsed time of propagation from terrain greater than 500 m. The speed of the midlevel jet, the magnitude of the low-level shear, and the surface equivalent potential temperature θe were greater for the intense cases compared to the nonintense cases, although the differences between the means tended to be small: less than 3 K for surface θe and less than 2 × 10−3 s−1 for low-level wind shear. Hypothesis testing of a series of commonly used intensity prediction metrics resulted in significant results only for low-level metrics such as convective available potential energy and not for any of the mid- or upper-level metrics such as the 700-hPa θe. None of the environmental variables or intensity metrics by themselves or in combination appeared to be reliable direct predictors of intensity. In the regional-scale analysis, the majority of intense convective systems occurred in the surface baroclinic zone where surface θe exceeded 344 K and the 700-hPa zonal wind speeds were less than −6 m s−1. Fewer intense cases compared to nonintense cases were associated with African easterly wave troughs. Fewer than 25% of these cases occurred in environments with detectable Saharan dust loads, and the results for intense and nonintense cases were similar. Although the discrimination between the intense and nonintense environments was narrow, the results were robust and consistent with the seasonal movement of the West African monsoon, regional differences in topography, and African easterly wave energetics.

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Karen I. Mohr and Edward J. Zipser

Abstract

This study used the 85-GHz ice scattering signature to describe the size, intensity, and geographic distribution of mesoscale convective systems (MCSs) between 35°N and 35°S for January, April, July, and October 1993. An MCS was defined as an area below 250 K of at least 2000 km2, with an enclosed minimum brightness temperature below 225 K. The geographic distribution of MCSs identified by these criteria was consistent with large-scale seasonal dynamics. There was no significant relationship (R2 ≈ 0.05) between the size and intensity for the MCSs in the study database. Tropical South America, tropical Africa, and the oceanic warm pool had the greatest number of MCSs. Equatorial regions such as tropical Africa had the smallest median areas. The subtropical oceans had the largest median areas, about 20% greater than other regions. MCSs in the continental regions tended to have colder minimum brightness temperatures than MCSs in the oceanic regions. The sub-tropical oceans had the warmest median minimum brightness temperatures, and tropical Africa had the coldest. Sunrise/sunset stratification of the data provided additional insight into land–water differences. MCSs were 35% more frequent over the oceans at sunrise than at sunset and 60% more frequent over tropical continents at sunset than at sunrise. Except over the subtropical oceans, MCSs tended to be larger at sunrise than at sunset. Continental MCSs tended to be colder at sunset than sunrise and colder than oceanic MCSs, particularly at sunset. The minimum brightness temperatures of oceanic MCSs tended to be only marginally colder at sunrise than at sunset. In general, continental MCSs appeared to be smaller and more intense than oceanic MCSs, and the largest and the most intense MCSs occurred more frequently in the subtropics.

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Karen I. Mohr and Edward J. Zipser

Mesoscale convective systems are composed of numerous deep convective cells with varying amounts of large, convectively produced ice particles aloft. The magnitude of the 85-GHz brightness temperature depression resulting from scattering by large ice is believed to be related to the convective intensity and to the magnitude of the convective fluxes through a deep layer. The 85-GHz ice-scattering signature can be used to map the distribution of organized mesoscale regions of convectively produced large ice particles. The purpose of this article is to demonstrate the usefulness of the 85-GHz ice-scattering signature for describing the frequency, convective intensity, and geographic distribution of mesoscale convective systems.

Objective criteria were developed to identify mesoscale convective systems from raw data from January, April, July, and October 1993. To minimize the effects of background contamination and to ensure that bounded areas contained convective elements, a “mesoscale convective system” was defined as an area bounded by 250 K of at least 2000 km2 of 85 GHz, with a minimum brightness temperature ≤ 225 K. Mesoscale convective systems extracted from the raw data were sorted and plotted by their areas and by their minimum brightness temperatures. Four area and brightness temperature classes were used to account for a spectrum of organized convection ranging from small to very large and from less organized to highly organized. The populations of mesoscale convective systems by this study's definition were consistent with infrared-based climatologies and large-scale seasonal dynamics. Land/water differences were highlighted by the plots of minimum brightness temperature. Most of the intense mesoscale convective systems were located on or near land and seemed to occur most frequently in particular areas in North America, South America, Africa, and India.

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Jody L. Zolman, Edward J. Zipser, and Karen I. Mohr

Abstract

Tropical mesoscale convective systems (MCSs) were identified from satellite data using the 85-GHz ice-scattering channel for a La Niña year, and these MCSs were compared with MCSs identified in the same manner for an El Niño year in previous work by Mohr and Zipser. The number, size, and intensity of the MCSs were examined for differences between the years in 18 different regions. There are well-documented patterns of anomalous precipitation related to El Niño and La Niña, and, in general, the MCS distributions between the two years tend to follow these patterns. There were more MCSs in the central Pacific and eastern Pacific in the El Niño year than in the La Niña year, and there were fewer MCSs in the “Maritime Continent.” The area distributions and median intensities of MCSs were found to be similar in each region during the two years. In contrast, the number and total area of MCSs in a region changed between the years. The changes between the years in MCS distributions showed a strong relationship to differences in an independent estimate of rainfall for the two years.

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Carl J. Schreck III, John Molinari, and Karen I. Mohr

Abstract

Tropical cyclogenesis is attributed to an equatorial wave when the filtered rainfall anomaly exceeds a threshold value at the genesis location. It is argued that 0 mm day−1 (simply requiring a positive anomaly) is too small a threshold because unrelated noise can produce a positive anomaly. A threshold of 6 mm day−1 is too large because two-thirds of storms would have no precursor disturbance. Between these extremes, consistent results are found for a range of thresholds from 2 to 4 mm day−1.

Roughly twice as many tropical cyclones are attributed to tropical depression (TD)-type disturbances as to equatorial Rossby waves, mixed Rossby–gravity waves, or Kelvin waves. The influence of the Madden–Julian oscillation (MJO) is even smaller. The use of variables such as vorticity and vertical wind shear in other studies gives a larger contribution for the MJO. It is suggested that its direct influence on the rainfall in forming tropical cyclones is less than for other variables.

The impacts of tropical cyclone–related precipitation anomalies are also presented. Tropical cyclones can contribute more than 20% of the warm-season rainfall and 50% of its total variance. The influence of tropical cyclones on the equatorial wave spectrum is generally small. The exception occurs in shorter-wavelength westward-propagating waves, for which tropical cyclones represent up to 27% of the variance. Tropical cyclones also significantly contaminate wave-filtered rainfall anomalies in their immediate vicinity. To mitigate this effect, the tropical cyclone–related anomalies were removed before filtering in this study.

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Charles J. Alonge, Karen I. Mohr, and Wei-Kuo Tao

Abstract

The West African Sahel lies between the wet, humid equatorial zone of Africa to the south and the Sahara Desert to the north. This topography results in a strong north–south precipitation gradient. A coupled land–atmosphere (cloud resolving) model and observed data from the Hydrological Atmospheric Pilot Experiment in the Sahel were used to simulate both wet and dry soil moisture regimes. There are two case studies—one characterized by convective precipitation, the other by fair weather. In both of the case studies, evapotranspiration from the tiger bush land cover was noticeably larger in the wet soil moisture regime. The increase in latent heat flux was the key factor in creating a boundary layer that was more favorable to late-afternoon deep convection in the wet regime. Differences in boundary layer growth and development between the case studies suggested a more important role for the land surface in fair weather environments versus convective precipitation environments.

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