The behavior and various controls of diurnal variability in tropical-subtropical rainfall are investigated using Tropical Rainfall Measuring Mission (TRMM) precipitation measurements retrieved from: (1) TRMM Microwave Imager (TMI), (2) Precipitation Radar (PR), and (3) TMI/PR Combined, standard level 2 algorithms for the 1998 annual cycle. Results show that the diurnal variability characteristics of precipitation are consistent for all three algorithms, providing assurance that TRMM retrievals are providing 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, various important exceptions are found. 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, co-existing with the LE-EM maximum, at latitudes dominated by large scale convergence and deep convection. Analogously, there is a preponderance for an LE-EM maximum over land, co-existing with the stronger MLA maximum, although it is not evident that this secondary continental feature is closely associated with the large scale circulation. The ocean results clearly indicate that rainfall diurnal variability associated with large scale convection is an integral part of the atmospheric general circulation.

https://ntrs.nasa.gov/api/citations/20050156637/downloads/20050156637.pdf

Mechanisms for Diurnal Variability of Global Tropical Rainfall Observed from Trmm

The International H2O Project (IHOP_2002) included four complementary research components: quantitative precipitation forecasting, convection initiation, atmospheric boundary layer processes, and instrumentation. This special issue introductory paper will review the current state of knowledge on surface-forced convection initiation and then describe some of the outstanding issues in convection initiation that partially motivated IHOP_2002. Subsequent papers in this special issue will illustrate the value of combining varied and complementary datasets to study convection initiation in order to address the outstanding issues discussed in this paper and new questions that arose from IHOP_2002 observations. The review will focus on convection initiation by boundaries that are prevalent in the U.S. southern Great Plains. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or convergence in Doppler velo...

A Review of Convection Initiation and Motivation for IHOP_2002

A coupled convection-resolving mesoscale atmosphere–land surface model (LSM) is used to investigate land surface–planetary boundary layer (PBL) interactions responsible for the initiation of deep, moist convection over the southern Great Plains of the United States on 19 June 1998. A high-resolution land data assimilation system provides initial conditions to the LSM, facilitating examination of soil moisture effects on forecasts of deep convection. During the late morning and early afternoon, the southwestern portion of a simulated southwest–northeast (SW–NE)-oriented surface water vapor gradient zone evolves into an intense dryline, unlike the northeastern portion, which remains relatively weak. Despite these regional differences, midafternoon convection initiation occurs within a ∼100-km-wide region of enhanced PBL depth along much of the SW–NE extent of the water vapor gradient zone. The afternoon PBL depth maximum results from a midmorning-to-early afternoon surface sensible heat flux maximu...

A study of convection initiation in a mesoscale model using high-resolution land surface initial conditions

The authors give an overview of the diurnal cycle of the low troposphere during 2006 at two different sites, Niamey (Niger) and Nangatchori (Benin). This study is partly based on the first observations of UHF wind profilers ever made in West Africa in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. Also used are the radiosoundings made in Niamey and ground station observations at Nangatchori, which allow for the study of the impact of the dynamics on the water vapor cycle and the turbulence observed at the ground. Profiler measurements revealed a very consistent year-round nocturnal low-level jet maximal around 0500 UTC and centered at 400-m above the ground, with wind speed around 15 m s−1. This jet comes either from the northeast during the dry season or from the southwest during the wet season, in relation with the position of the intertropical discontinuity. The radiosoundings made in Niamey highlight both the role of the nocturnal jet in bringing water vapor fro...

Observation of the Diurnal Cycle in the Low Troposphere of West Africa

A nonhydrostatic, three-dimensional version of the Colorado State University Regional Atmospheric Modeling System (CSU-RAMS) is used to deduce the processes responsible for the formation of drylines and the subsequent initiation of deep, moist dryline convection. A range of cumuliform cloud types are explicitly simulated along drylines on 15, 16, and 26 May 1991 in accordance with observations. In the simulations, narrow convergence bands along the dryline provide the lift to initiate deep moist convection. The thermally direct secondary convective boundary layer (CBL) circulations along the dryline are frontogenetic and solenoidally forced. Maximum updrafts reach 5 m s−1 and the bands are 3–9 km wide and 10–100 km or more in length. The updrafts penetrate and are decelerated by the overlying stable air above the CBL, reaching depths of about 2000 m in the cases studied. Moisture convergence along the mesoscale updraft bands destabilizes the local sounding to deep convection, while simultaneously...

/pdf/convective-initiation-at-the-dryline-a-modeling-study-1eq1g9suqn.pdf

Convective Initiation at the Dryline: A Modeling Study

The formation and diurnal evolution of the dryline during fair weather have been investigated through the use of a two-dimensional mesoscale model that includes condensation/evaporation, budget equations of surface energy and moisture field, as well as turbulence and radiation parameterizations.A moderately strong, vertical wind shear was introduced on a sloping terrain, where the soil is very dry on the west side but moist on the east. Initially, a weak easterly geostrophic wind exists to the east but a weak westerly geostrophic wind to the west of the dryline. During daytime, deepening of the mixed layer due to vertical mixing, especially on the west side, forces the dryline to advance eastward. The westerly wind on the west side is maintained by downward transport of westerly momentum due to strong vertical mixing; the easterly wind on the east side is strengthened due to the inland sea-breeze circulation. Therefore, the resulting low-level convergence sustains a strong moisture gradient along...

Ching-Chi Wu

Papers

Formation and Diurnal Variation of the Dryline