Identification of snowfall microphysical processes from Eulerian vertical gradients of polarimetric radar variables

International audiencePolarimetric radar systems are commonly used to study the microphysics of precipitation. While they offer continuous measurements with a large spatial coverage, retrieving information about the microphysical processes that govern the evolution of snowfall from the polarimetric signal is challenging. The present study develops a new method, called process identification based on vertical gradient signs (PIVSs), to spatially identify the occurrence of the main microphysical processes (aggregation and riming, crystal growth by vapor deposition and sublimation) in snowfall from dual-polarization Doppler radar scans. We first derive an analytical framework to assess in which meteorological conditions the local vertical gradients of radar variables reliably inform about microphysical processes. In such conditions, we then identify regions dominated by (i) vapor deposition, (ii) aggregation and riming and (iii) snowflake sublimation and possibly snowflake breakup, based on the sign of the local vertical gradients of the reflectivity ZH and the differential reflectivity ZDR. The method is then applied to data from two frontal snowfall events, namely one in coastal Adélie Land, Antarctica, and one in the Taebaek Mountains in South Korea. The validity of the method is assessed by comparing its outcome with snowflake observations, using a multi-angle snowflake camera, and with the output of a hydrometeor classification, based on polarimetric radar signal. The application of the method further makes it possible to better characterize and understand how snowfall forms, grows and decays in two different geographical and meteorological contexts. In particular, we are able to automatically derive and discuss the altitude and thickness of the layers where each process prevails for both case studies. We infer some microphysical characteristics in terms of radar variables from statistical analysis of the method output (e.g., ZH and ZDR distribution for each process). We, finally, highlight the potential for extensive application to cold precipitation events in different meteorological contexts

Distance between ROV and sea ice from ROV survey ARTofMELT2023/1_22-7 on 2023-06-04, survey 1

The distance between a remotely operated vehicle (ROV) and the sea-ice underside was measured by a single-beam upward-looking acoustic sonar altimeter (Tritech PA500) attached to the ROV during the ARTofMELT2023 expedition in May and June 2023. Sea-ice draft was derived by subtracting the distance to the sea-ice underside from the ROV depth, uncorrected for ROV attitude (pitch, roll). An offset between the depth reference (ROV bumper bars) and the altimeter of 0.105 m is accounted for in the presented data

Distance between ROV and sea ice from ROV survey ARTofMELT2023/1_22-6 on 2023-06-02, survey 3

The distance between a remotely operated vehicle (ROV) and the sea-ice underside was measured by a single-beam upward-looking acoustic sonar altimeter (Tritech PA500) attached to the ROV during the ARTofMELT2023 expedition in May and June 2023. Sea-ice draft was derived by subtracting the distance to the sea-ice underside from the ROV depth, uncorrected for ROV attitude (pitch, roll). An offset between the depth reference (ROV bumper bars) and the altimeter of 0.105 m is accounted for in the presented data

pH values from ROV survey ARTofMELT2023/1_20-2 on 2023-05-19, survey 5

pH values were obtained using a SBE18 pH sensor (Seabird) mounted on the remotely operated vehicle (ROV) during the ARTofMELT2023 expedition in May and June 2023. The values were derived from the sensor voltages using the same calibration during the entire expedition

Distance between ROV and sea ice from ROV survey ARTofMELT2023/1_20-3 on 2023-05-20, survey 2

The distance between a remotely operated vehicle (ROV) and the sea-ice underside was measured by a single-beam upward-looking acoustic sonar altimeter (Tritech PA500) attached to the ROV during the ARTofMELT2023 expedition in May and June 2023. Sea-ice draft was derived by subtracting the distance to the sea-ice underside from the ROV depth, uncorrected for ROV attitude (pitch, roll). An offset between the depth reference (ROV bumper bars) and the altimeter of 0.105 m is accounted for in the presented data

pH values from ROV survey ARTofMELT2023/1_20-3 on 2023-05-20, survey 1

pH values were obtained using a SBE18 pH sensor (Seabird) mounted on the remotely operated vehicle (ROV) during the ARTofMELT2023 expedition in May and June 2023. The values were derived from the sensor voltages using the same calibration during the entire expedition

Attention points from ROV survey ARTofMELT2023/1_23-9 on 2023-06-06, survey 4

Attention points as logged by the operators in the recording software Spot.On for remotely operated vehicle (ROV) surveys during the ARTofMELT2023 expedition in May and June 2023

Chlorophyll, FDOM and backscatter from ROV survey ARTofMELT2023/1_22-5 on 2023-06-01, survey 2

Fluorometric data on chlorophyll a concentration, Fluorescent Dissolved Organic Matter (FDOM) concentration, and optical backscatter were measured by a triplet fluorometer (ECO-Puck BBFL2SSC, Wetlabs) attached to a remotely operated vehicle (ROV) during the ARTofMELT2023 expedition in May and June 2023. Data use manufacturer calibration

Attention points from ROV survey ARTofMELT2023/1_20-1 on 2023-05-18, survey 1

Attention points as logged by the operators in the recording software Spot.On for remotely operated vehicle (ROV) surveys during the ARTofMELT2023 expedition in May and June 2023