Karen I. Mohr

Mesoscale Convective Systems Defined by Their 85-GHz Ice Scattering Signature: Size and Intensity Comparison over Tropical Oceans and Continents

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 min...

Defining Mesoscale Convective Systems by Their 85-GHz Ice-Scattering Signatures

Abstract Mesoseale 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 sy...

Interannual, monthly, and regional variability in the wet season diurnal cycle of precipitation in sub-Saharan Africa

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 1...

Attributing Tropical Cyclogenesis to Equatorial Waves in the Western North Pacific

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 ...

A comparison of WSR-88D reflectivities, SSM/I brightness temperatures, and lightning for mesoscale convective systems in Texas. Part I: Radar reflectivity and lightning

Abstract This is the first part of a two part study. Part I compares radar data from the League City, Texas, WSR-88D and cloud-to-ground (CG) lightning data for a set of eight mesoscale convective systems (MCSs), which occur at various stages of development along the upper Texas gulf coast. Vertical profiles of radar reflectivity (VPRR) as well as plan views and vertical cross sections are constructed to characterize the structure and relative strength of each MCS. The VPRR are also compared with similar profiles from tropical oceanic MCSs. The data show that in all the majority of negative CG lightning flashes are located near high-reflectivity convective cores (>35 dBZ) in the mixed-phase region (0°C ≤T≥ −20°C). Growing or mature MCSs typically had larger negative flash counts and higher percentages of negative lightning (≥80%) associated with convective core than MCSs at later stages of their life cycle. Comparison of the median VPRR for the various MCSs showed that although each case had high-reflecti...

An Analysis of the Environments of Intense Convective Systems in West Africa in 2003

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 met...

Characteristics of Precipitation Features and Annual Rainfall during the TRMM Era in the Central Andes

AbstractThe 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 moi...

A Comparison of Tropical Mesoscale Convective Systems in El Nino and La Nina

Abstract Tropical mesoscale convective systems (MCSs) were identified from satellite data using the 85-GHz ice-scattering channel for a La Nina year, and these MCSs were compared with MCSs identified in the same manner for an El Nino 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 Nino and La Nina, 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 Nino year than in the La Nina 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 relatio...

The Interannual Stability of Cumulative Frequency Distributions for Convective System Size and Intensity

Thecharacteristicsofconvective systempopulationsin WestAfricaandthewesternPacifictropical cyclone basinwereanalyzedtoinvestigatewhetherinterannualvariabilityinconvectiveactivityintropicalcontinental and oceanic environmentsis drivenby variations in the numberof events duringthe wet season or by favoring large and/or intense convective systems. Convective systems were defined from Tropical Rainfall Measuring Mission (TRMM) data as a cluster of pixels with an 85-GHz polarization-corrected brightness temperature below 255 K and with an area of at least 64 km 2 . The study database consisted of convective systems in West Africa from May to September 1998‐2007, and in the western Pacific from May to November 1998‐2007. Annualcumulativefrequencydistributionsforsystemminimumbrightnesstemperatureandsystemareawere constructed for both regions. For both regions, there were no statistically significant differences between the annualcurvesforsystemminimumbrightnesstemperature.Thereweretwogroupsofsystemareacurves,split by the TRMM altitude boost in 2001. Within each set, there was no statistically significant interannual variability. Subsetting the database revealed some sensitivity in distribution shape to the size of the sampling area, the length of the sample period, and the climate zone. From a regional perspective, the stability of the cumulative frequency distributions implied that the probability that a convective system would attain a particular size or intensity does not change interannually. Variability in the number of convective events appeared to be more important in determining whether a year is either wetter or drier than normal.