Abstract
Abstract. This study presents the evaluation of a technique to estimate cloud condensed water content (CWC) in tropical convection from airborne cloud radar reflectivity factors at 94 GHz and in situ measurements of particle size distributions (PSDs) and aspect ratios of ice crystal populations. The approach is to calculate from each 5 s mean PSD and flight-level reflectivity the variability of all possible solutions of m(D) relationships fulfilling the condition that the simulated radar reflectivity factor (T-matrix method) matches the measured radar reflectivity factor. For the reflectivity simulations, ice crystals were approximated as oblate spheroids, without using a priori assumptions on the mass–size relationship of ice crystals. The CWC calculations demonstrate that individual CWC values are in the range ±32 % of the retrieved average CWC value over all CWC solutions for the chosen 5 s time intervals. In addition, during the airborne field campaign performed out of Darwin in 2014, as part of the international High Altitude Ice Crystals/High Ice Water Content (HAIC/HIWC) projects, CWCs were measured independently with the new IKP-2 (isokinetic evaporator probe) instrument along with simultaneous particle imagery and radar reflectivity. Retrieved CWCs from the T-matrix radar reflectivity simulations are on average 16 % higher than the direct CWCIKP measurements. The differences between the CWCIKP and averaged retrieved CWCs are found to be primarily a function of the total number concentration of ice crystals. Consequently, a correction term is applied (as a function of total number concentration) that significantly improves the retrieved CWC. After correction, the retrieved CWCs have a median relative error with respect to measured values of only −1 %. Uncertainties in the measurements of total concentration of hydrometeors are investigated in order to calculate their contribution to the relative error of calculated CWC with respect to measured CWCIKP. It is shown that an overestimation of the concentration by about +50 % increases the relative errors of retrieved CWCs by only +29 %, while possible shattering, which impacts only the concentration of small hydrometeors, increases the relative error by about +4 %. Moreover, all cloud events with encountered graupel particles were studied and compared to events without observed graupel particles. Overall, graupel particles seem to have the largest impact on high crystal number-concentration conditions and show relative errors in retrieved CWCs that are higher than for events without graupel particles.
Highlights
Clouds play an important role within the hydrological cycle, radiative transfer, and heat balance of the Earth
When no direct simultaneous measurements of condensed water content (CWC) are available, Z–CWC relationships are established by using constant mass–size relationships (for CWC calculations from particle size distribution (PSD) measurements) and most of the time these studies use m(D) coefficients suggested by Brown and Francis (1995)
The section of this paper presents the dataset of the first High Altitude Ice Crystals (HAIC)/High Ice Water Content (HIWC) airborne campaign and associated data processing
Summary
Clouds play an important role within the hydrological cycle, radiative transfer, and heat balance of the Earth. The methodology applied in this study to simulate 94 GHz radar reflectivity factors is based on assumptions about individual cloud particle properties. Even though a mass–size relationship for ice crystals with constant coefficients has been utilized, the study of Hogan et al (2011) claimed minimal errors between measured and simulated radar reflectivity factors, even smaller than the calibration uncertainty of the cloud radar. When no direct simultaneous measurements of CWC are available, Z–CWC (and Z–CWC–T ) relationships are established by using constant mass–size relationships (for CWC calculations from particle size distribution (PSD) measurements) and most of the time these studies use m(D) coefficients suggested by Brown and Francis (1995). The study ends with a discussion and conclusion section
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