Stipo Sentić

Convective response to changes in the thermodynamic environment in idealized weak temperature gradient simulations

We investigate the response of convection to idealized perturbations in the thermodynamic environment in simulations which parameterize the large-scale circulations using the weak temperature gradient (WTG) approximation. The perturbations include a combination of modifying the environmental moisture and atmospheric stability via imposing anomalies in reference moisture and temperature profiles. We find that changes in atmospheric stability strongly influence the character of convection by drastically modifying the vertical motion profile, whereas changes to atmospheric moisture modulate the intensity of precipitation produced by the convection, but do not qualitatively change the shape of the vertical motion profile. An important question is how does horizontal moisture advection into the domain affect convection? We test several different parameterizations of this process; these include lateral entrainment by circulations induced by enforcing WTG, a moisture relaxation which parameterizes the advection of moisture by large-scale nondivergent circulations, and control simulations in which both of these mechanisms are turned off so horizontal advection is assumed negligible compared to vertical advection. Interestingly, the most significant differences resulting from the choice of horizontal moisture advection scheme appear in environmental conditions which suppress–rather than support–the development of deep tropical convection. In this case, lateral entrainment related to WTG circulations is the only parameterization which results in extreme drying of the troposphere in environments which suppress convection. Consequently, this is the only parameterization which permits multiple equilibria—dry or precipitating steady states—in convection.

Diagnosing DYNAMO convection with weak temperature gradient simulations

Determining relationships between convective and environmental diagnostics can improve our understanding of mechanisms controlling tropical convection, and consequently, result in better representations of convection in coarsely resolved models. We identify important diagnostic relationships in observations taken during the Dynamics of the Madden-Julian Oscillation (DYNAMO) campaign and perform weak temperature gradient (WTG) simulations of DYNAMO convection to determine if the observed relationships are reproduced in our model. We find that the WTG approximation models local changes in the diagnostics used in the study—precipitation rate, atmospheric stability, moisture, and gross moist stability (GMS)—and reproduces diagnostic relationships suggested in previous studies; an increase in precipitation rate is correlated with increased atmospheric moisture content, which, in turn, is correlated with greater atmospheric stability. Large-scale atmospheric stability—changes of which might be related to balanced dynamics, we speculate—seems to be a candidate for a convective controlling mechanism. Observed and modeled interactions of local convection with the large-scale environment—quantified by the GMS—are in agreement with the theory of Inoue and Back (2015b); the GMS increases from small, positive or negative, values during developing convection and further increases for decaying convection past a critical GMS found at peak precipitation rates, atmospheric stability, and moisture content. Understanding the link between the critical GMS and the diagnostics—still a standing problem—could further our understanding of interactions between local convection and the large-scale environment.

The role of radiation in organizing convection in weak temperature gradient simulations

Using a cloud system resolving model with the large scale parameterized by the weak temperature gradient approximation, we investigated the influence of interactive versus noninteractive radiation on the characteristics of convection and convective organization. The characteristics of convecting environments are insensitive to whether radiation is interactive compared to when it is not. This is not the case for nonconvecting environments; interactive radiative cooling profiles show strong cooling at the top of the boundary layer which induces a boundary layer circulation that ultimately exports moist entropy (or analogously moist static energy) from dry domains. This upgradient transport is associated with a negative gross moist stability, and it is analogous to boundary layer circulations in radiative convective equilibrium simulations of convective self-aggregation. This only occurs when radiation cools interactively. Whether radiation is static or interactive also affects the existence of multiple equilibria-steady states which either support precipitating convection or which remain completely dry depending on the initial moisture profile. Interactive radiation drastically increases the range of parameters which permit multiple equilibria compared to static radiation; this is consistent with the observation that self-aggregation in radiative-convective equilibrium simulations is more readily attained with interactive radiation. However, the existence of multiple equilibria in absence of interactive radiation suggests that other mechanisms may result in organization.

Climate Change Meets Urban Environment

The effects of global warming permeate to local scales in numerous ways, and at times the adverse effects of global change are amplified by urban anthropogenic activities. These local climate influences, however, have not received due attention as current climate discourse mainly focuses on global scales. In this paper, a brief overview is presented on how urban areas bear the brunt of global climate change, in particular, how such climatic signals as sea level rise, desertification, adjustment of hydrological cycle and enhanced cloud cover can have significant repercussions on local climate, thus raising human health and national security concerns. The reduction of diurnal temperature range (DTR) with global warming and its further amplification with urbanization are used as examples to illustrate local impacts of climate change. The possible amplification of urban heat island may even lead to local meteorological regime shifts, which have an important bearing on sustainability of cities. Meteorological variables are related to air pollution, and the relationship between particulate matter and meteorological variables in Phoenix area is used to illustrate possible relationships between human health and climate change.

Idealized modeling of convective organization with changing sea surface temperatures using multiple equilibria in weak temperature gradient simulations

The weak temperature gradient (WTG) approximation is a method of parameterizing the influences of the large-scale on local convection in limited domain simulations. WTG simulations exhibit multiple equilibria in precipitation; depending on the initial moisture content, simulations can precipitate or remain dry for otherwise identical boundary conditions. We use a hypothesized analogy between multiple equilibria in precipitation in WTG simulations, and dry and moist regions of organized convection to study tropical convective organization. We find that the range of wind speeds that support multiple equilibria depends on sea surface temperature (SST). Compared to the present SST, low SSTs support a narrower range of multiple equilibria at higher wind speeds. In contrast, high SSTs exhibit a narrower range of multiple equilibria at low wind speeds. This suggests that at high SSTs, organized convection might occur with lower surface forcing. In order to characterize convection at different SSTs, we analyze the change in relationships between precipitation rate, atmospheric stability, moisture content, and the large-scale transport of moist entropy and moisture with increasing SSTs. We find an increase in large-scale export of moisture and moist entropy from dry simulations with increasing SST, which is consistent with a strengthening of the up-gradient transport of moisture from dry regions to moist regions in organized convection. Furthermore, the changes in diagnostic relationships with SST are consistent with more intense convection in precipitating regions of organized convection for higher SSTs.