 |
| Titel |
High ice water content at low radar reflectivity near deep convection – Part 2: Evaluation of microphysical pathways in updraft parcel simulations |
| VerfasserIn |
A. S. Ackerman, A. M. Fridlind, A. Grandin, F. Dezitter, M. Weber, J. W. Strapp, A. V. Korolev |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 20 ; Nr. 15, no. 20 (2015-10-22), S.11729-11751 |
| Datensatznummer |
250120114
|
| Publikation (Nr.) |
 copernicus.org/acp-15-11729-2015.pdf |
|
|
|
|
|
| Zusammenfassung |
| The aeronautics industry has established that a threat to aircraft
is posed by atmospheric conditions of substantial ice water content
(IWC) where equivalent radar reflectivity (Ze) does
not exceed 20–30 dBZ and supercooled water is not present; these
conditions are
encountered almost exclusively in the vicinity of deep
convection. Part 1 (Fridlind et al., 2015) of this two-part study presents in situ
measurements of such conditions sampled by Airbus in three tropical
regions, commonly near 11 km and −43 °C, and
concludes that the measured ice particle size distributions are
broadly consistent with past literature with profiling radar
measurements of Ze and mean Doppler velocity obtained
within monsoonal deep convection in one of the regions sampled. In
all three regions, the Airbus measurements generally indicate
variable IWC that often exceeds 2 g m-3 with relatively
uniform mass median area-equivalent diameter (MMDeq) of
200–300 μm. Here we use a parcel model with
size-resolved microphysics to investigate microphysical pathways
that could lead to such conditions. Our simulations indicate that
homogeneous freezing of water drops produces a much smaller ice
MMDeq than observed, and occurs only in the absence of
hydrometeor gravitational collection for the conditions
considered. Development of a mass mode of ice aloft that overlaps
with the measurements requires a substantial source of small ice
particles at temperatures of about −10 °C or warmer,
which subsequently grow from water vapor. One conceivable source in
our simulation framework is Hallett–Mossop ice production; another
is abundant concentrations of heterogeneous ice freezing nuclei
acting together with copious shattering of water drops upon
freezing. Regardless of the production mechanism, the dominant mass
modal diameter of vapor-grown ice is reduced as the ice-multiplication source strength increases and as competition for
water vapor increases. Both mass and modal diameter are reduced by
entrainment and by increasing aerosol concentrations. Weaker
updrafts lead to greater mass and larger modal diameters of
vapor-grown ice, the opposite of expectations regarding lofting of
larger ice particles in stronger updrafts. While stronger updrafts
do loft more dense ice particles produced primarily by raindrop
freezing, we find that weaker updrafts allow the warm rain process
to reduce competition for diffusional growth of the less dense ice
expected to persist in convective outflow. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|