# Bibliography - Nadir Jeevanjee

- Chua, Xin Rong, Yi Ming, and Nadir Jeevanjee, August 2019:
**Investigating the Fast Response of Precipitation Intensity and Boundary Layer Temperature to Atmospheric Heating Using a Cloud‐Resolving Model**. *Geophysical Research Letters*, **46(15)**, DOI:10.1029/2019GL082408.

Abstract Coarse‐resolution global climate models cannot explicitly resolve the intensity distribution of tropical precipitation and how it responds to a forcing. We use a cloud‐resolving model to study how imposed atmospheric radiative heating (such as that caused by greenhouse gases or absorbing aerosols) may alter precipitation intensity in the setting of radiative‐convective equilibrium. It is found that the decrease in total precipitation is realized through reducing weak events. The intensity of strong precipitation events is maintained by a cancellation between the moistening of air parcels and weakening of updrafts. A boundary layer energy budget analysis suggests that free‐tropospheric heating raises boundary layer temperatures mainly through a reduction in rain re‐evaporation. This insight leads to a predictive scaling for the surface sensible and latent flux changes. The results imply that cloud microphysical processes play a key role in shaping the temperature and precipitation responses to atmospheric heating.

- Jeevanjee, Nadir, and Stephan Fueglistaler,
*in press*: **On the Cooling-to-Space Approximation**. *Journal of the Atmospheric Sciences*. DOI:10.1175/JAS-D-18-0352.1. August 2019.

Abstract The cooling-to-space (CTS) approximation says that the radiative cooling of an atmospheric layer is dominated by that layer’s emission to space, while radiative exchange with layers above and below largely cancel. Though the CTS approximation has been demonstrated empirically and is thus fairly well-accepted, a theoretical justification is lacking. Furthermore, the intuition behind the CTS approximation cannot be universally valid, as the CTS approximation fails in the case of pure radiative equilibrium.
Motivated by this, we investigate the CTS approximation in detail. We frame the CTS approximation in terms of a novel decomposition of radiative flux divergence, which better captures the cancellation of exchange terms. We also derive validity criteria for the CTS approximation, using simple analytical theory. We apply these criteria in the context of both gray gas pure radiative equilibrium (PRE) as well as radiative-convective equilibrium (RCE), to understand how the CTS approximation arises and why it fails in PRE. When applied to realistic gases in RCE, these criteria predict that the CTS approximation should hold well for H2O but less so for CO2, a conclusion we verify with line-by-line radiative transfer calculations. Along the way we also discuss the well-known ‘τ = 1 law’, and its dependence on the choice of vertical coordinate.

- Lecoanet, D, and Nadir Jeevanjee,
*in press*: **Entrainment in Resolved, Dry Thermals**. *Journal of the Atmospheric Sciences*. DOI:10.1175/JAS-D-18-0320.1. September 2019.

Abstract Entrainment in cumulus convection remains ill-understood and difficult to quantify. For instance, entrainment is widely believed to be a fundamentally turbulent process, even though Turner (1957) pointed out that dry thermals entrain primarily because of buoyancy (via a dynamical constraint requiring an increase in radius r). Furthermore, entrainment has been postulated to obey a 1/r scaling, but this scaling has not been firmly established.
Here, we study the classic case of dry thermals in a neutrally stratified environment using fully resolved direct numerical simulation. We combine this with a thermal tracking algorithm which defines a control volume for the thermal at each time, allowing us to directly measure entrainment. We vary the Reynolds number Re of our thermals between laminar (Re ≈ 600) and turbulent (Re ≈ 6000) regimes, finding only a 20% variation in entrainment rate ɛ, supporting the claim that turbulence is not necessary for entrainment. We also directly verify the postulated ε ~ 1/r scaling law.

- Seeley, J T., and Nadir Jeevanjee, et al., January 2019:
**Formation of Tropical Anvil Clouds by Slow Evaporation**. *Geophysical Research Letters*, **46(1)**, DOI:10.1029/2018GL080747.

Abstract Tropical anvil clouds play a large role in the Earth's radiation balance, but their effect on global warming is uncertain. The conventional paradigm for these clouds attributes their existence to the rapidly declining convective mass flux below the tropopause, which implies a large source of detraining cloudy air there. Here we test this paradigm by manipulating the sources and sinks of cloudy air in cloud‐resolving simulations. We find that anvils form in our simulations because of the long lifetime of upper‐tropospheric cloud condensates, not because of an enhanced source of cloudy air below the tropopause. We further show that cloud lifetimes are long in the cold upper troposphere because the saturation specific humidity is much smaller there than the condensed water loading of cloudy updrafts, which causes evaporative cloud decay to act very slowly. Our results highlight the need for novel cloud‐fraction schemes that align with this decay‐centric framework for anvil clouds.

- Seeley, J T., Nadir Jeevanjee, and D M Romps, February 2019:
**FAT or FiTT: Are Anvil Clouds or the Tropopause Temperature Invariant?** *Geophysical Research Letters*, **46(3)**, DOI:10.1029/2018GL080096.

Abstract The Fixed Anvil Temperature (FAT) hypothesis proposes that upper tropospheric cloud fraction peaks at a special isotherm that is independent of surface temperature. It has been argued that a FAT should result from simple ingredients: Clausius‐Clapeyron, longwave emission from water vapor, and tropospheric energy and mass balance. Here the first cloud‐resolving simulations of radiative‐convective equilibrium designed to contain only these basic ingredients are presented. This setup does not produce a FAT: the anvil temperature varies by about 40% of the surface temperature range. However, the tropopause temperature varies by only 4% of the surface temperature range, which supports the existence of a Fixed Tropopause Temperature (FiTT). In full‐complexity radiative‐convective equilibrium simulations, the spread in anvil temperature is smaller by about a factor of 2, but the tropopause temperature remains more invariant than the anvil temperature by an order of magnitude. In other words, our simulations have a FiTT, not a FAT.

- Anber, Usama, Nadir Jeevanjee, Lucas Harris, and Isaac M Held, July 2018:
**Sensitivity of Radiative‐Convection Equilibrium to Divergence Damping in GFDL‐FV3 Based Cloud‐Resolving Model Simulations**. *Journal of Advances in Modeling Earth Systems*, **10(7)**, DOI:10.1029/2017MS001225.

Abstract Using a non‐hydrostatic model based on a version of GFDL's FV3 dynamical core at a cloud‐resolving resolution in radiative‐convective equilibrium (RCE) configuration, the sensitivity of the mean RCE climate to the magnitude and scale‐selectivity of the divergence damping is explored. Divergence damping is used to reduce small‐scale noise in more realistic configurations of this model. This sensitivity is tied to the strength (and width) of the convective updrafts, which decreases (increases) with increased damping and acts to organize the convection, dramatically drying out the troposphere and increasing the outgoing longwave radiation.
Increased damping also results in a much‐broadened precipitation probability distribution and larger extreme values, as well as reduction in cloud fraction, which correspondingly decreases the magnitude of shortwave and longwave cloud radiative effects. Solutions exhibit a monotonic dependence on the strength of the damping and asymptotically converge to the inviscid limit. While the potential dependence of RCE simulations on resolution and microphysical assumptions are generally appreciated, these results highlight the potential significance of the choice of sub‐grid numerical diffusion in the dynamical core.

- Jeevanjee, Nadir, and D M Romps, November 2018:
**Mean precipitation change from a deepening troposphere**. *Proceedings of the National Academy of Sciences*, **115(45)**, DOI:10.1073/pnas.1720683115.

Abstract Global climate models robustly predict that global mean precipitation should increase at roughly 2–3% K−1, but the origin of these values is not well understood. Here we develop a simple theory to help explain these values. This theory combines the well-known radiative constraint on precipitation, which says that condensation heating from precipitation is balanced by the net radiative cooling of the free troposphere, with an invariance of radiative cooling profiles when expressed in temperature coordinates. These two constraints yield a picture in which mean precipitation is controlled primarily by the depth of the troposphere, when measured in temperature coordinates. We develop this theory in idealized simulations of radiative–convective equilibrium and also demonstrate its applicability to global climate models.

- Tarshish, Nathaniel, and Nadir Jeevanjee, et al., September 2018:
**Buoyant Motion of a Turbulent Thermal**. *Journal of the Atmospheric Sciences*, **75(9)**, DOI:10.1175/JAS-D-17-0371.1.

Abstract By introducing an equivalence between magnetostatics and the equations governing buoyant motion, we derive analytical expressions for the acceleration of isolated density anomalies (thermals). In particular, we investigate buoyant acceleration, defined as the sum of the Archimedean buoyancy B and an associated perturbation pressure gradient. For the case of a uniform spherical thermal, the anomaly fluid accelerates at 2B/3, extending the textbook result for the induced mass of a solid sphere to the case of a fluid sphere. For a more general ellipsoidal thermal, we show that the buoyant acceleration is a simple analytical function of the ellipsoid’s aspect ratio. The relevance of these idealized uniform-density results to turbulent thermals is explored by analyzing direct numerical simulations of thermals at a Reynolds number (Re) of 6300. We find that our results fully characterize a thermal’s initial motion over a distance comparable to its length. Beyond this buoyancy-dominated regime, a thermal develops an ellipsoidal vortex circulation and begins to entrain environmental fluid. Our analytical expressions do not describe the total acceleration of this mature thermal, but they still accurately relate the buoyant acceleration to the thermal’s mean Archimedean buoyancy and aspect ratio. Thus, our analytical formulas provide a simple and direct means of estimating the buoyant acceleration of turbulent thermals.

- Jeevanjee, Nadir, P Hassanzadeh, and S A Hill, et al., August 2017:
**A perspective on climate model hierarchies**. *Journal of Advances in Modeling Earth Systems*, **9(4)**, DOI:10.1002/2017MS001038.

Abstract To understand Earth's climate, climate modelers employ a hierarchy of climate models spanning a wide spectrum of complexity and comprehensiveness. This essay, inspired by the World Climate Research Programme's recent ‘Model Hierarchies Workshop’, attempts to survey and synthesize some of the current thinking on climate model hierarchies, especially as presented at the workshop. We give a few formal descriptions of the hierarchy, and survey the various ways it is used to generate, test, and confirm hypotheses. We also discuss some of the pitfalls of contemporary climate modeling, and how the ‘elegance’ advocated for by Held [2005] has (and has not) been used to address them. We conclude with a survey of current activity in hierarchical modeling, and offer suggestions for its continued fruitful development.

- Jeevanjee, Nadir, October 2017:
**Vertical velocity in the gray zone**. *Journal of Advances in Modeling Earth Systems*, **9(6)**, DOI:10.1002/2017MS001059.

Abstract We describe how convective vertical velocities wc vary in the ‘gray zone’ of horizontal resolution, using both hydrostatic and non-hydrostatic versions of GFDL's FV3 dynamical core, as well as analytical solutions to the equations of motion. We derive a simple criterion (based on parcel geometries) for a model to resolve convection, and find that O(100m) resolution can be required for convergence of wc. We also find, both numerically and analytically, that hydrostatic systems over-estimate wc, by a factor of 2 - 3 in the convection-resolving regime. This over-estimation is simply understood in terms of the ‘effective buoyancy pressure’ of Jeevanjee and Romps (2015, 2016).

Direct link to page: http://www.gfdl.noaa.gov/bibliography/results.php?author=5996