Search Results

You are looking at 1 - 8 of 8 items for :

  • Author or Editor: Samson Hagos x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
Samson Hagos

Abstract

Rotated EOF analyses are used to study the composition and variability of large-scale tropical diabatic heating profiles estimated from eight field campaigns. The results show that the profiles are composed of a pair of building blocks. These are the stratiform heating with peak heating near 400 hPa and a cooling peak near 700 hPa and the convective heating with a heating maximum near 700 hPa. Variations in the contributions of these building blocks account for the evolution of the large-scale heating profile. Instantaneous top-heavy (bottom-heavy) large-scale heating profiles associated with excess of stratiform (convective) heating evolve toward a stationary mean profile due to exponential decay of the excess stratiform (convective) heating.

Full access
Chidong Zhang
and
Samson M. Hagos

Abstract

Tropical diabatic heating profiles estimated using sounding data from eight field campaigns were diagnosed to document their common and prevailing structure and variability that are relevant to the large-scale circulation. The first two modes of a rotated empirical orthogonal function analysis—one deep, one shallow—explain 85% of the total variance of all data combined. These two modes were used to describe the heating evolution, which led to three composited heating profiles that are considered as prevailing large-scale heating structures. They are, respectively, shallow, bottom heavy (peak near 700 hPa); deep, middle heavy (peak near 400 hPa); and stratiform-like, top heavy (heating peak near 400 hPa and cooling peak near 700 hPa). The amplitudes and occurrence frequencies of the shallow, bottom-heavy heating profiles are comparable to those of the stratiform-like, top-heavy ones. The sequence of the most probable heating evolution is deep tropospheric cooling to bottom-heavy heating, to middle heavy heating, to stratiform-like heating, then back to deep tropospheric cooling. This heating transition appears to occur on different time scales. Each of the prevailing heating structures is interpreted as being composed of particular fractional populations of various types of precipitating cloud systems, which are viewed as the building blocks for the mean. A linear balanced model forced by the three prevailing heating profiles produces rich vertical structures in the circulation with multiple overturning cells, whose corresponding moisture convergence and surface wind fields are very sensitive to the heating structures.

Full access
Samson Hagos
,
L. Ruby Leung
, and
Jimy Dudhia

Abstract

To identify the main thermodynamic processes that sustain the Madden–Julian oscillation (MJO), an eddy available potential energy budget analysis is performed on a regional model simulation with moisture constrained by observations. The model realistically simulates the two MJO episodes observed during the winter of 2007/08. Analysis of these two cases shows that instabilities and damping associated with variations in diabatic heating and energy transport work in concert to provide the MJO with its observed characteristics. The results are used to construct a simplified paradigm of MJO thermodynamics.

Furthermore, the effect of moisture nudging on the simulation is analyzed to identify the limitations of the model cumulus parameterization. Without moisture nudging, the parameterization fails to provide adequate low-level (upper level) moistening during the early (late) stage of the MJO active phase. The moistening plays a critical role in providing stratiform heating variability that is an important source of eddy available potential energy for the model MJO.

Full access
Samson Hagos
,
Zhe Feng
,
Sally McFarlane
, and
L. Ruby Leung

Abstract

By applying a cloud-tracking algorithm to tropical convective systems in a regional high-resolution model simulation, this study documents the environmental conditions before and after convective systems are initiated over ocean and land by following them during their lifetime. The comparative roles of various mechanisms of convection–environment interaction on the longevity of convective systems are quantified. The statistics of lifetime, maximum area, and propagation speed of the simulated deep convection agree well with geostationary satellite observations.

Among the environmental variables considered, lifetime of convective systems is found to be most related to midtropospheric moisture before as well as after the initiation of convection. Over ocean, convective systems enhance surface fluxes through the associated cooling and drying of the boundary layer as well as increased wind gusts. This process appears to play a minor positive role in the longevity of systems. For systems of equal lifetime, those over land tend to be more intense than those over ocean especially during the early stages of their life cycle. Both over ocean and land, convection is found to transport momentum vertically to increase low-level shear and decrease upper-level shear, but no discernible effect of shear on the lifetime of the convective systems is found.

Full access
Jingyi Chen
,
Samson Hagos
,
Zhe Feng
,
Jerome D. Fast
, and
Heng Xiao

Abstract

Some of the climate research puzzles relate to a limited understanding of the critical factors governing the life cycle of cumulus clouds. These factors force the initiation and the various mixing processes during cloud life cycles. To shed some light into these processes, we tracked the life cycle of thousands of individual shallow cumulus clouds in a large-eddy simulation during the Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems field campaign in the U.S. southern Great Plains. Concurrent evolution of clouds is tracked and their respective neighboring clouds are examined. Results show that the clouds initially smaller than neighboring clouds can grow larger than the neighboring clouds by a factor of 2 within 20% of their lifetime. Two groups of the tracked clouds with growing and decaying neighboring clouds, respectively, show distinct characteristics in their life cycles. Clouds with growing neighboring clouds form above regions with larger surface heterogeneity, whereas clouds with decaying neighboring clouds are associated with less heterogeneous surfaces. Also, those with decaying neighboring clouds experience larger instability and a more humid boundary layer, indicating evaporation below the cloud base is likely occurring before those clouds are formed. Larger instability leads to higher vertical velocity and convergence within the cloud, which causes stronger surrounding downdrafts and water vapor removal in the surrounding area. The latter appears to be the reason for the decaying neighboring clouds. Understanding those processes provide insights into how cloud–cloud interactions modulate the evolution of cloud population and into how this evolution can be represented in future cumulus parameterizations.

Free access
Jingyi Chen
,
Samson Hagos
,
Heng Xiao
,
Jerome Fast
, and
Zhe Feng

Abstract

This study uses semi-idealized simulations to investigate multiscale processes induced by the heterogeneity of soil moisture observed during the 2016 Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems (HI-SCALE) field campaign. The semi-idealized simulations have realistic land heterogeneity, but large-scale winds are removed. Analysis on isentropic coordinates enables the tracking of circulation that transports energy vertically and facilitates the identification of the primary convective processes induced by realistic land heterogeneity. The isentropes associated with upward motion are found to connect the ground characterized by high latent heat flux to cloud bases directly over the ground with high sensible heat flux, while isentropes associated with downward motion connect precipitation to the ground characterized by high sensible heat fluxes. The mixing of dry, warm parcels ascending from the ground with high sensible heat fluxes and moist parcels from high latent heat regions leads to cloud formation. This new mechanism explains how soil moisture heterogeneity provides the key ingredients such as buoyancy and moisture for shallow cloud formation. We also found that the submesoscale dominates upward energy transport in the boundary layer, while mesoscale circulations contribute to vertical energy transport above the boundary layer. Our novel method better illustrates and elucidates the nature of land atmospheric interactions under irregular and realistic soil moisture patterns.

Significance Statement

Models that resolve boundary layer turbulence and clouds have been used extensively to understand processes controlling land–atmosphere interactions, but many of their configurations and computational expense limit the use of variable land properties. This study aims to understand how heterogeneous land properties over multiple spatial scales affect energy redistribution by moist convection. Using a more realistic land representation and isentropic analyses, we found that high sensible heat flux regions are associated with relatively higher vertical velocity near the surface, and the high latent heat flux regions are associated with relatively higher moist energy. The mixing of parcels rising from these two regions results in the formation of shallow clouds.

Free access
Katelyn A. Barber
,
Casey D. Burleyson
,
Zhe Feng
, and
Samson M. Hagos

Abstract

In this study, a pair of convection-permitting (2-km grid spacing), month-long, wet-season Weather Research and Forecasting (WRF) Model simulations with and without the eddy-diffusivity mass-flux (EDMF) scheme are performed for a portion of the Green Ocean Amazon (GoAmazon) 2014/15 field campaign period. EDMF produces an ensemble of subgrid-scale convective plumes that evolve in response to the boundary layer meteorological conditions and can develop into shallow clouds. The objective of this study is to determine how different treatments of shallow cumulus clouds (i.e., with and without EDMF) impact the total cloud population and precipitation across the Amazonian rain forest, with emphasis on impacts on the likelihood of shallow-to-deep convection transitions. Results indicate that the large-scale synoptic conditions in the EDMF and control simulations are nearly identical; however, on the local scale their rainfall patterns diverge drastically and the biases decrease in EDMF. The EDMF scheme significantly increases the frequency of shallow clouds, but the frequencies of deep clouds are similar between the simulations. Deep convective clouds are tracked using a cloud-tracking algorithm to examine the impact of shallow cumulus on the surrounding ambient environment where deep convective clouds initiate. Results suggest that a rapid increase of low-level cloudiness acts to cool and moisten the low to midtroposphere during the day, favoring the transition to deep convection.

Open access
Chidong Zhang
,
Jian Ling
,
Samson Hagos
,
Wei-Kuo Tao
,
Steve Lang
,
Yukari N. Takayabu
,
Shoichi Shige
,
Masaki Katsumata
,
William S. Olson
, and
Tristan L’Ecuyer

Abstract

Four Tropical Rainfall Measuring Mission (TRMM) datasets of latent heating were diagnosed for signals in the Madden–Julian oscillation (MJO). In all four datasets, vertical structures of latent heating are dominated by two components—one deep with its peak above the melting level and one shallow with its peak below. Profiles of the two components are nearly ubiquitous in longitude, allowing a separation of the vertical and zonal/temporal variations when the latitudinal dependence is not considered. All four datasets exhibit robust MJO spectral signals in the deep component as eastward propagating spectral peaks centered at a period of 50 days and zonal wavenumber 1, well distinguished from lower- and higher-frequency power and much stronger than the corresponding westward power. The shallow component shows similar but slightly less robust MJO spectral peaks. MJO signals were further extracted from a combination of bandpass (30–90 day) filtered deep and shallow components. Largest amplitudes of both deep and shallow components of the MJO are confined to the Indian and western Pacific Oceans. There is a local minimum in the deep components over the Maritime Continent. The shallow components of the MJO differ substantially among the four TRMM datasets in their detailed zonal distributions in the Eastern Hemisphere. In composites of the heating evolution through the life cycle of the MJO, the shallow components lead the deep ones in some datasets and at certain longitudes. In many respects, the four TRMM datasets agree well in their deep components, but not in their shallow components and in the phase relations between the deep and shallow components. These results indicate that caution must be exercised in applications of these latent heating data.

Full access