An idealized, three‐dimensional, 1 km horizontal grid spacing numerical simulation of a rapidly intensifying tropical cyclone is used to extend basic knowledge on the role of mean and eddy momentum transfer on the dynamics of the intensification process. Examination of terms in the tangential and radial velocity tendency equations provides an improved quantitative understanding of the dynamics of the spin‐up process within the inner‐core boundary layer and eyewall regions of the system‐scale vortex. Unbalanced and non‐axisymmetric processes are prominent features of the rapid spin‐up process. In particular, the wind asymmetries, associated in part with the asymmetric deep convection, make a substantive contribution (∼30%) to the maximum wind speed inside the radius of this maximum. The analysis provides a novel explanation for inflow jets sandwiching the upper‐tropospheric outflow layer which are frequently found in numerical model simulations. In addition, it provides an opportunity to assess the applicability of generalized Ekman balance during rapid vortex spin‐up. The maximum tangential wind occurs within and near the top of the frictional inflow layer and as much as 10 km inside the maximum gradient wind. Spin‐up in the friction layer is accompanied by supergradient winds that exceed the gradient wind by up to 20%. Overall, the results affirm prior work pointing to significant limitations of a purely axisymmetric balance description, for example, gradient balance/Ekman balance, when applied to a rapidly intensifying tropical cyclone.

The dependence of tropical cyclone intensification rate on the sea-surface temperature (SST) is examined in the prototype problem for tropical cyclone intensification on an fplane using a three-dimensional, non-hydrostatic numerical model. The effects of changing the SST are compared with those of changing the latitude examined in a recent article. It is found that the dependence of intensification rate on latitude is largest when the SST is marginal for tropical cyclone intensification (26 • C) and reduces in significance as the SST is increased. Further, at a given latitude, intensification begins earlier and the rate of intensification increases with increasing SST, on account of a significant increase of surface moisture fluxes from the warmer ocean. These higher fluxes result in higher values of near-surface moisture and equivalent potential temperature, leading to a larger radial gradient of diabatic heating rate in the low to middle troposphere above the boundary layer. This larger radial gradient leads to a stronger overturning circulation, which in turn leads to a stronger radial import of absolute angular momentum surfaces and therefore more rapid spin-up. These arguments invoke the classical axisymmetric spin-up mechanism. Non-axisymmetric issues are touched upon briefly.

Abstract. The interaction between radiation and clouds represents a source of uncertainty in numerical weather prediction (NWP) due to both intrinsic problems of one-dimensional radiation schemes and poor representation of clouds. The underlying question addressed in this study is how large the NWP radiative bias is for shallow cumulus clouds and how it scales with various input parameters of radiation schemes, such as solar zenith angle, surface albedo, cloud cover and liquid water path. A set of radiative transfer calculations was carried out for a realistically evolving shallow cumulus cloud field stemming from a large-eddy simulation (LES). The benchmark experiments were performed on the highly resolved LES cloud scenes (25 m grid spacing) using a three-dimensional Monte Carlo radiation model. An absence of middle and high clouds is assumed above the shallow cumulus cloud layer. In order to imitate the poor representation of shallow cumulus in NWP models, cloud optical properties were horizontally averaged over the cloudy part of the boxes with dimensions comparable to NWP horizontal grid spacing (several kilometers), and the common δ-Eddington two-stream method with maximum-random overlap assumption for partial cloudiness was applied (denoted as the “1-D” experiment). The bias of the 1-D experiment relative to the benchmark was investigated in the solar and thermal parts of the spectrum, examining the vertical profile of heating rate within the cloud layer and the net surface flux. It is found that, during daytime and nighttime, the destabilization of the cloud layer in the benchmark experiment is artificially enhanced by an overestimation of the cooling at cloud top and an overestimation of the warming at cloud bottom in the 1-D experiment (a bias of about −15 K d−1 is observed locally for stratocumulus scenarios). This destabilization, driven by the thermal radiation, is maximized during nighttime, since during daytime the solar radiation has a stabilizing tendency. The daytime bias at the surface is governed by the solar fluxes, where the 1-D solar net flux overestimates (underestimates) the corresponding benchmark at low (high) Sun. The overestimation at low Sun (bias up to 80 % over land and ocean) is largest at intermediate cloud cover, while the underestimation at high Sun (bias up to −40 % over land and ocean) peaks at larger cloud cover (80 % and beyond). At nighttime, the 1-D experiment overestimates the amount of benchmark surface cooling with the maximal bias of about 50 % peaked at intermediate cloud cover. Moreover, an additional experiment was carried out by running the Monte Carlo radiation model in the independent column mode on cloud scenes preserving their LES structure (denoted as the “ICA” experiment). The ICA is clearly more accurate than the 1-D experiment (with respect to the same benchmark). This highlights the importance of an improved representation of clouds even at the resolution of today's regional (limited-area) numerical models, which needs to be considered if NWP radiative biases are to be efficiently reduced. All in all, this paper provides a systematic documentation of NWP radiative biases, which is a necessary first step towards an improved treatment of radiation–cloud interaction in atmospheric models.

Low clouds over tropical oceans reflect a great proportion of solar radiation back to space and thereby cool the Earth, yet this phenomenon has been poorly simulated in several previous generations of climate models. The principal aim of the present study is to employ satellite observations to evaluate the representation of marine tropical low clouds and their radiative effect at the top of the atmosphere in a subset of latest climate models participating in CMIP6. We strive for regime-oriented model validation and hence introduce a qualitative approach to discriminate stratocumulus (Sc) from shallow cumulus (Cu). The novel Sc-Cu categorization has a conceptual advantage of being based on cloud properties, rather than relying on a model response to a cloud-controlling factor. We find that CMIP6 models underestimate low-cloud cover in both Sc- and Cu-regions of tropical oceans. A more detailed investigation of cloud biases reveals that most CMIP6 models underestimate the relative frequency of occurrence (RFO) of Sc and overestimate RFO of Cu. We further demonstrate that tropical low cloudiness in CMIP6 models remains too bright. The regime-oriented validation represents the basis for improving parameterizations of physical processes that determine the cloud cover and radiative impact of Sc and Cu, which are still misrepresented in current climate models.

Motivated in part by a potential application to modelling tropical cyclones in the Australian region, mean radiosonde soundings are determined for the three northern Australian stations, Willis Island, Darwin and Weipa, during the core months of the cyclone season (December–February). More than 8500 individual soundings are examined in 30‐year datasets for Willis Island and Darwin (1980–2010) and a 15‐year dataset for Weipa (1998–2013). These soundings are stratified into three groups according to the low‐level wind direction (monsoon regime, easterly flow regime and the rest). The mean soundings for the monsoon regime (low‐level winds in the sector west to north) are compared at the three stations and diurnal differences are investigated at stations with two soundings per day. The mean monsoon Willis Island sounding is compared also with the Dunion moist tropical (MT) sounding, which is frequently used as an environmental sounding in the numerical modelling of tropical cyclones. The Willis Island sounding is 1–3 °C warmer and somewhat drier than the Dunion MT sounding through the entire troposphere, although the relative humidity differences are relatively small (less than 5% at most observed levels). Idealized numerical simulations of tropical cyclone evolution are performed to assess the implications of using one thermodynamic sounding or another for tropical cyclones in the Australian region. The simulations highlight the importance of not only the environmental sounding for the intensification of model storms, but also the sea surface temperature combined with the sounding.